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B-CePs as cross-linking probes for the investigation of RNA higher-order structure

Abstract Elucidating the structure of RNA and RNA ensembles is essential to understand biological functions. In this work, we explored the previously uncharted reactivity of bis-chloropiperidines (B-CePs) towards RNA. We characterized at the molecular level the different adducts induced by the fast reacting compound B-CeP 1 with RNA. Following an approach based on solution thermal melting coupled with ESI mass spectrometry (STHEM-ESI), we proved the ability of B-CePs to induce inter-molecular cross-links between guanines in double stranded RNA. These results open the possibility of using B-CePs as structural probes for investigating higher-order structures, such as the kissing loop complex established by the dimerization initiation site (DIS) of the HIV-1 genome. We confirmed the potential of B-CePs to reveal the identity of RNA structures involved in long-range interactions, expecting to benefit the characterization of samples that are not readily amenable to traditional high-resolution techniques, and thus promoting the elucidation of pertinent RNA systems associated with old and new diseases.


INTRODUCTION
Substantial progress in RNA research is profoundly changing our understanding of the biological significance of this biopolymer, while driving technological applications and scientific innovation (1,2). The RNA revolution has highlighted the importance of a myriad of non-coding RNAs (ncRNAs) that play pivotal roles in essential biological processes in all living organisms (3). Besides, the increasing threat to human health due to old and new RNA viruses, coupled with their highly disrupting economic and social consequences, make the challenge of probing and blocking key viral RNAs even more urgent (4). The structures adopted by RNA sequences are essential to carry out their intended functions and are critical for binding partners such as other nucleic acids, proteins and small molecule ligands. However, methods and tools employed to characterize the interactions of RNA at the molecular level are still limited as compared to those available for proteins. An established approach involves the utilization of small molecules with well-defined activity as chemical probes to accurately and reliably investigate specific structures (1). In the context of RNA structural studies, widely used methods for structure-2 Nucleic Acids Research, 2021 probing experiments at single nucleotide-resolution employ chemicals reacting with folded RNA in a quantitative and structurally informative way, helping to elucidate mechanistic and phenotypic questions (5).
Bis-3-chloropiperidines (B-CePs) are an emerging class of alkylating agents with excellent reactivity toward nucleic acids (6)(7)(8)(9). B-CePs react with DNA leading to depurination and strand cleavage at guanines (dG) (6)(7)(8)(9). The analysis of a large library of B-CePs showed that a high reactivity toward DNA is detrimental to their development as cytotoxic agents (10,11), but might be instead ideal to support possible applications as chemical probes for structural studies. To test this hypothesis, we selected compound 1 (1,3-bis(5-chloro-3,3-dimethylpiperidin-1-yl)propane, Figure 1A) as a possible prototype of B-CePs structural probe by virtue of its high reactivity with nucleophiles (6,8). In this report, we specifically explored its activity toward structured RNA by using both single-and double-stranded substrates ( Figure 1B). Electrophoretic and mass spectrometric analysis were employed in concert to elucidate molecular mechanism of the reaction of B-CeP 1 with this type of biopolymer. We implemented an approach that coupled solution thermal melting with electrospray ionization (STHEM-ESI) (12,13) mass spectrometry to probe rapidly and unambiguously the presence of inter-strand conjugates in folded RNA structures.
This approach offered us the ability to clarify the identity of the stable bi-functional adducts between adjacent guanines in paired RNA strands. We finally evaluated the potential of B-CeP 1 as a possible cross-linking probe by investigating the spatial arrangement of the kissing complex formed by the dimerization initiation site (DIS) domain of the HIV-1 genome ( Figure 1C), which represents an excellent example of a highly structured RNA of great biological and pharmacological relevance.

RNA substrates
Stock solutions of individual dinucleotides (i.e. ApA, CpC, GpG, UpU) were 2 -ACE deprotected as suggested by the manufacturer (GE Healthcare Dharmacon, CO, USA), dried and then dissolved in nuclease-free water. Synthetic oligoribonucleotides were purchased in lyophilized form from commercial sources. The RNA oligonucleotides used for gel electrophoresis analysis were chemically synthesized by Metabion International AG (Martinsried, Germany), whereas those used for mass spectrometric analysis were purchased from Integrated DNA Technologies (IDT, Coralville, IA, USA). All oligonucleotides were used as received with no further purification. All names, sequences and secondary structures are reported in Figure 1. Each substrate possessed specific features explained in the text. RNA duplex such as dsRNA was obtained by annealing equimolar amounts of the complementary strands RNA1 and RNA2 in 1× BPE buffer (BPE 10×: NaH 2 PO 4 ·2H 2 O 2 mM, Na 2 HPO 4 ·12H 2 O 6 mM, Na 2 EDTA·2 H 2 O 1 mM, pH 7.4). The mixture was denatured at 95 • C for 5 min, and then left to cool to room temperature to obtain the desired RNA duplex ( Figure 1B). The DIS kissing-loop (KL) dimer was prepared by heating to 95 • C for 5 min the RNA solution and then quickly cooling on ice to allow for proper folding of the RNA hairpins and obtain the kissing-loop conformer (KL, Figure 1C) (14,15).

Chemical reagents
Bis-3-chloropiperidine 1 ( Figure 1A) was synthesized in house as previously described (8). Chlorambucil (Chl) was purchased from Sigma-Aldrich (Milan, Italy). Aliquots of chemical probes were freshly prepared by diluting an 8 mM DMSO stock in MilliQ water and were instantly reacted with the RNA substrate to avoid the typical quenching effects of the aqueous environment. All the other chemical reagents, including salts and solvents, were purchased from Sigma-Aldrich (Milan, Italy).

B-CeP 1 hydroxylation kinetic
The formation of B-CeP 1 reactive and hydrolyzed species upon incubation at 37 • C in 1× BPE buffer (pH 7.4) was followed in time by ESI-MS. A fresh solution of test compound was prepared at the concentration of 80 M and incubated at 37 • C. Samples were taken after 5 min, 15 min, 30 min, 1 h, 2 h and 24 h, diluted 1:10 with methanol and analyzed on a Thermo Fisher Scientific (West Palm Beach, CA, USA) LTQ-Orbitrap Velos mass spectrometer. The analyses were performed in nanoflow mode by using quartz emitters produced in house by using a Sutter Instruments Co. (Novato, CA, USA) P2000 laser pipette puller. Up to 5 l samples were typically loaded onto each emitter by using a gel-loader pipette tip. A stainless steel wire was inserted in the back-end of the emitter to supply an ionizing voltage around 1 kV. Typical source temperature was 200 • C, whereas desolvation voltage was in the range of 40-50 V. Data were processed by using Xcalibur 2.1 software (Thermo Scientific). We calculated the percentage of each species over total compound in each sample to obtain relative percentages of B-CeP 1 reacted species, whose chemical structures are schematized in panel B of Supplementary Figure S1.

Probing reactions
Based on the final aim of the experiment, different reaction conditions were explored, including different substrates, probe to substrate ratios and incubation times. Case by case reaction conditions are detailed within the text. Typical reaction mixtures consisted of 20 l of a final 2 M solution substrate, properly folded when necessary, in 1× BPE buffer. RNA solutions were added with compound 1 or Chlorambucil as reference substance at final concentrations ranging from 5 to 50 M and incubated at 37 • C for different times, depending on the purpose of the experiment.

Gel electrophoretic analysis
An initial assessment of probing reactions results was achieved by submitting reaction mixtures to sequencing polyacrylamide gel electrophoresis (PAGE) analysis. Appropriate RNA strand was 5 -radioactively labeled by using T4 polynucleotyde kinase (Ambion, Applied Biosystems, TX, USA) in the presence of [␥ 32 P] ATP (3000 Ci/mmol, PerkinElmer, MA, USA) at 37 • C for 1 h, following manufacturer's instructions. The kinase activity was inactivated by heating the reaction mixture at 95 • C for 5 min, followed by two phenol extractions to remove the enzyme. Reaction mixtures were prepared by mixing a final 2 M solution of 5 -radiolabeled RNA substrate in 1× BPE buffer with compound 1 at final concentrations of either 5 or 50 M. Reactions mixtures were incubated at 37 • C for 1, 2, 4, 7, 15 and 24 h, as indicated in the text. Samples were dried in a vacuum centrifuge, re-suspended in 5 l of denaturing gel loading buffer (10 mM Tris·HCl, 80% formamide, 0.025% bromophenol blue), and then loaded onto a 20% denaturing polyacrylamide gel (7 M urea) in 1× TBE buffer (Tris·HCl 89 mM, borate 89 mM, EDTA 2 mM). Acrylamide/bis-acrylamide (19:1) 40% solution was purchased from VWR International PBI Srl (Milan, Italy). 5radiolabeled oligonucleotide products were detected by using a STORM B40 Phosphorimager (GE Healthcare, Italy).

Mass spectrometric analysis
Samples prepared in 1× BPE were buffer-exchanged by performing ethanol precipitation in the presence of 1 M ammonium acetate. In case of reaction mixtures, this treatment served also to achieve reaction quenching. Samples were redissolved and diluted in 150 mM ammonium acetate (pH adjusted to 7.0) to achieve a final 2 M concentration of total RNA. All samples were analyzed by direct infusion electrospray ionization (ESI) on a Thermo Fisher Scientific (West Palm Beach, CA, USA) LTQ-Orbitrap Velos mass spectrometer. The analyses were performed in nanoflow mode as above described for the 'B-CeP 1 hydroxylation ki-netic'. Source temperature and desolvation conditions were adjusted by closely monitoring the incidence of ammonium adducts and water clusters. Typical source temperature was 200 • C, whereas desolvation voltage was in the range of 40-50 V, enabling the detection of intact RNA and RNA adducts. All experiments were performed in negative ion mode. The instrument was calibrated by using a 0.5 mg/ml solution of CsI in 50/50 water/methanol, which provided a typical 2 ppm mass accuracy. Data were processed by using Xcalibur 2.1 software (Thermo Scientific). Melting experiments were performed by using a home-built temperaturecontrolled nanospray source (described in the next paragraph) that allowed us to perform thermal melting of any sample solution contained in the nanospray emitter. All experiments were performed in negative ion mode.

Home-built temperature-controlled nanospray source
The temperature-controlled heater for nanospray analysis was built from a brass block, which was cut in two halves and carved to obtain a channel that fit snugly around a typical quartz emitter (see Supplementary Figure S4). The halves have through holes to accommodate a thermocouple probe (Omega, PR-21-1) and two 25W heater cartridges (Tempco, HDC19100). The channel allows for a stainlesssteel wire to be inserted from the back end of the emitter to provide the necessary spray voltage. The wire is insulated from the block by a PTFE sleeve to avoid voltage discharge. When the two halves are reassembled, the entire block can be encased in a water jacket made of silicone material. The temperature is set by a programmable controller (Laird, TC-XX-PR-59) unit with a precision of ±0.1 • C at 1 Hz. The water jacket is connected to a temperature-controlled circulator, which can be used to maintain temperatures below ambient and to expedite cooling. The apparatus is capable of raising the temperature from 4 • C to 25 • C in <1 min. The fact that the large mass of the block requires some time to reach the set temperature is more than compensated by long-term temperature stability afforded by the apparatus. This feature is a particularly beneficial for studying slow nucleic acid interactions, which can take minutes to hours.

B-CeP 1 reacts fast with nucleophiles
B-CePs reactivity lies in the fast formation of the reactive aziridinium ion (N+) that is readily attacked by nucleophiles (6,8). In aqueous solution, aziridinium ions react with water to produce several species comprising mono-(OH) and di-hydroxylated (2 OH) products (6,10). It is therefore possible to assess the kinetics of B-CePs reactivity by analyzing the species obtained with water in the absence of competing nucleic acids (10). To do so, a solution of 1 was incubated at 37 • C in BPE buffer from 5 min to 24 h. Small aliquots were taken at different incubation times and directly analyzed by high resolution electrospray ionization mass spectrometry (ESI-MS). Representative spectra and the structure of all identified species are thoroughly reported in Supplementary Figure S1 (see Supporting Information), while graphs in Figure 2 show the relative percentage of B-CeP 1 species at different incubation time.
The results at 5 min displayed the immediate formation of the reactive electrophilic aziridinium ion (species 1 N+ , turquoise in Figure 2). Subsequent attack by the nucleophilic water was revealed by the appearance of the 3-hydroxy substituted species (species 1 OH , mint green in Figure 2). After 15 min incubation, the intact compound 1 was no longer observed, whereas the formation of aziridinium ion on the second 3-chloropiperidine moiety (species 1 N+OH , orange in Figure 2) could be readily noted. The fact that all reactive intermediates were completely converted into water-reacted 1 2OH species (peachcolored in Figure 2) after 2 h incubation provided a measure of the rather high rate of reaction of compound 1 in aqueous environment. No further changes could be noted in the sample incubated up to 24 h. The kinetics of reactant consumption observed here were consistent with earlier observations (6,8), which supported its possible utilization as a fast-acting chemical probe.

B-CeP 1 reacts with guanines in RNA
With the aim to gain information on the base-specific reactivity with RNA, we treated compound 1 with an equimolar combination of homo-dinucleotides, namely ApA (green), CpC (blue), GpG (red), UpU (black). A representative ESI-MS spectrum obtained after 2 h reaction is provided in Figure 3A, whereas that of an untreated control mixture is in Supporting Information, Supplementary Figure S2. Characteristic covalent adducts were readily detected, thus demonstrating the ability of B-CePs to alkylate RNA substrates. More specifically, the products corresponded to the monofunctional addition of compound 1 to GpG, ApA, and CpC (species a, b and c in Figure 3A and Table 1).
In contrast, the mass of species d was consistent with a putative bifunctional adduct of GpG with compound 1 bridging across adjacent guanines. At the same time, no adduct whatsoever could be observed for UpU, probably due to the significantly lower nucleophilic character of the corresponding type of nucleobase. These data allowed us to quantify the yields of the various alkylating reactions by using the unreacted UpU as a putative internal standard. A safe estimate of probe reactivity should also take in account the expected variation of ionization efficiency between reactants and products. In ESI-MS experiments, the intensity of signals afforded by nucleic acid samples is susceptible to their overall hydrophobicity and gas-phase proton affinity, which are established by their base composition (16,17). This effect can be readily appreciated by examining the spectrum of the untreated homo-dinucleotide mixture (Supporting Information, Supplementary Figure S2), in which different signal intensities were recorded for the various species in spite of their equimolar concentration in solution. The effects of chemical modifications may affect the ionization characteristics of the species in an unpredictable way, but they are likely to change substantially for adducts of small and singly-charged substrates such as the dinucleotides employed in the experiment. The intensities observed for each dinucleotide immediately before addition of compound 1 and after 2 h incubation were normalized against that of UpU, thus providing the means for obtaining an accurate measure of the percentage of the initial material consumed during the reaction. As shown in Figure 3B, the results afforded a G >> A ≈ C scale of relative nucleobase-reactivity. Considering that the formation of bifunctional alkylation products must necessarily involve the intramolecular closing of an initial mono-functional precursor, the lower overall rate of formation of such product exhibited by adenine and cytosine could explain the absence of cross-linked products noted for the corresponding dinucleotides under the selected experimental conditions. These results reveal the preferential attack of B-CeP 1 to guanines, leading to both mono-and bi-functional adducts.

Fast reaction of B-CeP 1 with single-stranded RNA yields stable adducts
In light of the encouraging results obtained from the dimeric oligoribonucleotides, we evaluated the reactivity of bis-3-chloropiperidine 1 towards single-stranded RNA by using a combination of gel electrophoresis and mass spectrometry analyses. As a model system, we utilized a 16-mer RNA construct (named RNA1, Figure 1B), which replicated a DNA oligonucleotide described in previous studies (6). Its sequence contained a stretch of five adjacent guanine ribonucleotides (G), which were expected to constitute multiple nucleophilic centers susceptible to alkylation. Compound 1 was reacted with RNA1 according to the experimental conditions described previously for the DNA analog (6), i.e. by mixing 2 M of nucleic acid substrate with either 5 or 50 M compound in BPE buffer at 37 • C for incubation times up to 24 h. A radioactively labeled version of RNA1 was also prepared to enable product analysis by high-resolution polyacrylamide gel electrophoresis (PAGE). The detection of numerous discrete bands with  lower mobilities than that of the untreated control was attributed to covalent B-CeP 1 adducts ( Figure 4A). Increasing the amount of probe in solution increased both abundance and stoichiometry of adducts in the reaction mixture, to the point in which the initial substrate could be no longer detected at the higher concentration (i.e. 50 M). The reactivity of B-CeP 1 was compared with that of the nitrogen mustard chlorambucil, a potent and effective DNA alkylating agent (18). In clear contrast with the former, the latter was unable to induce RNA adducts even at the highest concentration employed in the experiment (i.e. 100 M) after 24 h incubation (lane Chl in Figure 4A). Interestingly, the fact that no band migrating faster than the control was ever de-tected indicated that the B-CeP 1 products were very stable and unable of producing cleavage of the biopolymer chain. These exquisite features provided a striking contrast with the reactivity exhibited by 1 toward DNA, which produced extensive backbone cleavage after only 1 h incubation under otherwise identical conditions (6).
Prompted by this observation, we performed ESI-MS analysis of reaction mixtures to achieve the characterization of RNA alkylation products at the molecular level. The representative data in Figure 4B were obtained from a sample containing 2 M of RNA1 and 5 M of compound 1, which was incubated for 2 h at 37 • C. Only the region containing the 4-charge state is shown for the sake of clar- Table 1. Nomenclature, nucleobases involved, symbols, representative chemical structure, experimental and calculated masses of species detected in the spectrum reported in Figure 3A ity. The base peak labeled [RNA1-4H] 4− was readily recognized as the unreacted substrate, whereas additional signals were assigned to covalent RNA adducts by matching experimental masses with values calculated from the RNA1 sequence and the structure of compound 1 (see Supporting Information, Supplementary Table S1). More specifically, the RNA1+1 B species displaying an incremental mass of 262.24 u over that of unmodified RNA1 was readily assigned to putative bi-functional adducts formed by reaction of both 3-chloropiperidine moieties with the RNA strand (indicated by in Figure 4B, inset). In contrast, the RNA1+1 M species with a 280.24 u increment was diagnostic for mono-functional adducts in which only one of the 3-chloropiperidines had reacted with the substrate, whereas the other had been hydrolyzed to 3-hydroxyl by the aqueous environment (indicated by in Figure 4B, inset). Analyses performed as a function of concentration and time corroborated the observations afforded by the gel electrophoresis experiments by revealing the identity of the species with lower mobility. The increasing amounts of products with masses matching different combinations of mono and bifunctional adducts (labelled , , in Supplementary Figure S3, Supporting Information) con-firmed the dose-dependence of the reaction between B-CeP 1 and RNA1. The coexistence of free RNA1 and the alkylation products with two equivalent of probe indicates that the second alkylation events occurred before saturation of a first reaction site was complete. The absence of a predominant product, which might be clearly favoured over the others, thus suggested that the various guanines in the construct possessed similar susceptibility to alkylation. This finding was consistent with the absence of any stable secondary structure that may prevent access by the reagent. The fact that no variations of the alkylation pattern were observed for incubation times greater than 2 h confirmed the relatively fast rate of reaction observed with GpG in the dinucleotide mixture. In this direction, the time-dependence of the B-CeP 1 reaction with RNA1 ( Figure 4A) matched very closely that of hydrolysis in the aqueous buffer ( Figure  2). The majority of alkylating probes have an unfavourable balance between hydrolysis and reaction with the intended target, which lead to anemic yields and call for greater reagent concentrations (19). The comparable reaction rates afforded by B-CeP 1 would be expected to enable a fairer competition, which should afford better probing reaction yields.

B-CePs can effectively cross-link double-stranded RNA
We next evaluated the potential of B-CeP 1 as a putative structural probe by reacting it with different structured RNAs. The first RNA assembly consisted of a doublestranded construct (dsRNA, Figure 1B), which was obtained by annealing RNA1 with its complementary strand RNA2. The reaction conditions employed in these experiments replicated those identified earlier for the singlestranded RNA1. Analysis by denaturing PAGE revealed the formation of a series of alkylation products of radiolabeled RNA1, which displayed the usual dose-and time-dependent trends ( Figure 5A). These bands matched very closely those produced by the reaction with individual single-stranded RNA1 ( Figure 4A). The absence of radioactive tracer on the complementary RNA2 prevented its detection in these experiments. However, alkylation adducts were observed when the dsRNA construct included the radioactively labeled RNA2 ( Figure 5B). In both cases, a new product with much lower mobility could be clearly detected at the higher reagent concentrations and longer reaction times (indicated by red arrows in Figure 5), which was absent in the reaction mixture of individual single-stranded RNA1 ( Figure 4A). The fact that no band was detected for intact dsRNA in the control lane containing untreated samples (i.e. C ds lanes in Figure 5) confirmed that the denaturing conditions employed in the analysis were definitively capable of inducing complete dissociation of the annealed strands. Therefore, the slow-moving band could be plausibly assigned to bifunctional conjugated products of dsRNA, in which inter-strand cross-linking by B-CeP 1 prevented the expected dissociation. The rate of reaction with the double-stranded construct was comparable to that observed with the single-stranded one. Complete consumption of the initial material and formation of putative inter-strand conjugates were already observed after only 1 h incubation with 50 M of B-CeP 1. By comparison, treatment with the reference probe chlorambucil induced no significant amounts of adducts upon more extended incubation with higher reagent concentration, as exemplified by the lane containing the products of 24 h incubation with 100 M probe ( Figure 5). Under these conditions, the inefficient reactivity of chlorambucil could be explained by a combination of possible kinetics and structural factors. In particular, the high yield of hydrolysis products exhibited by typical nitrogen mustards in aqueous environments would be expected to favor hydroxylated over alkylated products. Upon formation of an initial mono-functional product, the competing pathways involving the second 2-chloroethyl function could thus lead almost exclusively to 'dangling' hydroxylated products rather than 'closed' bifunctional conjugates across opposite strands (16,19). Moreover, it should be noted that the structure of the dsRNA construct is characterized by the presence of a unique GC step at the 10-11 position of the RNA1 sequence ( Figure 1B). These nucleotides are basepaired with a complementary CG sequence in RNA2. The arrangement dictated by the Watson-Crick rules places G10 of the former in close spatial proximity with G6 of the latter. Owing to the presence of only one G in the entire RNA2 sequence, a similar arrangement would not be possible anywhere else in the double-stranded structure. Therefore, the two guanines on opposite strands were considered as the most likely sites to be bridged by B-CeP 1 in the interstrand conjugate. This hypothesis was further supported by the predominant reactivity of B-CeP 1 with guanine, which minimized any possible involvement of other types of nucleotides in the cross-linking reactions. Furthermore, the steric situation of guanine in a typical helical structure leaves the N7 position (the most nucleophilic function in the nucleotide) exposed to the solvent and readily accessible to any alkylating agent. Differently from chlorambucil, which was unable to produce significant bifunctional conjugates across opposite strands ( Figure 5), the detection of stable inter-strand conjugates for B-CeP 1 indicated that the spacing between 3-chloropiperidine functions was clearly appropriate to cover the distance between susceptible N7 positions.

STHEM-ESI analysis of inter-strand RNA cross-links
The identity of the products obtained from the doublestranded substrate was further investigated by ESI-MS analysis. Figure 6A provides the spectrum obtained from a representative reaction mixture containing 2 M dsRNA and 5 M B-CeP 1, which was incubated for 2 h at 37 • C. These conditions enabled the detection of abundant products with different stoichiometries including up to three equivalents of B-CeP 1 adduct. Based on the respective masses, the various signals could be immediately assigned to the initial unreacted substrate (indicated with in Figure 6A) and 'dangling' monofunctional products in which only one of the 3-chloropiperidine functions had reacted with dsRNA (labelled , see inset in Figure 6A). In contrast, putative intra-and inter-strand bifunctional products, labelled respectively and , shared the same elemental composition and thus could not be unambiguously distinguished according to mass (see inset in Figure 6A). The fact that the selected experimental conditions were unequivocally non-denaturing, as demonstrated by the detection of intact duplex construct, precluded the possibility of differentiating the bifunctional conjugates by inducing strand dissociation. This usually desirable feature prevented us from verifying the information obtained from the same type of sample by denaturing PAGE.
Over the years, different approaches based on gas-phase activation have been employed to induce strand dissociation in cross-linked samples, which have met variable levels of success as a function of system complexity and overall stability of binding between complementary strands (19)(20)(21). In this report, we explored an approach that coupled solution thermal melting with electrospray ionization mass spectrometry (STHEM-ESI) to achieve strand dissociation directly in the sample solution rather than in the gas phase. The home-built apparatus employed in these experiments consisted of a copper block surrounding the nanospray emitter, which could be heated under computer control in 0.2 • C increments (see Supporting Information, Supplementary Figure S4, for additional details) (12,13). This arrangement allowed us to spray any melting product directly into the ion source for analysis.
Preliminary experiments were conducted on untreated dsRNA control by raising the emitter temperature from 25 to 95 • C (Supporting Information, Supplementary Figure  S5). The results indicated that the duplex was fully dissociated in individual strands at the higher end of the temperature range. When these conditions were applied to the reaction mixture of dsRNA with B-CeP 1, we observed the complete dissociation of any unreacted dsRNA remaining in the sample, as well as a variety of mono-functional and intrastrand bifunctional products ( Figure 6B). In contrast, a series of inter-strand conjugated products were observed in a still intact duplex form, owing to the presence of a bridging B-CeP 1 adduct that had effectively prevented strand dissociation. The mono-functional products of the RNA1 strand displayed multiple stoichiometries, consistent with the susceptibility of any of the guanines in the sequence to B-CeP 1 alkylation. In the context of the dsRNA, no equivalent mono-functional product could be detected for RNA2, thus indicating that the only G present in this sequence was fully involved in the inter-strand conjugate with RNA1. In fact, classic tandem mass spectrometry experiments (MS/MS) (16,22,23) performed at 25 • C confirmed that the bifunctional adducts, including both intra-and inter-strand products, occurred in the dsRNA only between two Gs (see Supporting Information, Supplementary Figure S6). Unlike the dsRNA construct, the single-stranded RNA2 afforded the direct identification of both mono-and bi-functional alkylation adducts produced by compound 1 (see Supplementary Figure S7, Supporting Information). Based on the presence of a single G in position 6 of this construct, the latter could be readily attributed to bridging reactions involving the contiguous A5 and C7 nucleotides placed on either side, which would be consistent with the spacing between the reactive functions of B-CeP 1. However, when Gs are advantageously located in a dsRNA context to form interstrand cross-links, they prevail over the other nucleotides to react with the probe. Moreover, it should be noted that the stability of the bond between B-CeP 1 and RNA was verified by analyzing a sample of the RNA1 reaction mixture held at 95 • C, which reported no variations of the alkylation pattern. Even at the higher temperatures tested in the study, no evidence of possible cleavage of the covalent backbone was ever observed.

B-CeP compounds as probes for RNA higher-order structure
The experiments performed on both single-and doublestranded substrates afforded valuable insights into the structural determinants of B-CeP 1 activity. The feature that stood out the most was the ability of this compound to effi- ciently alkylate not only guanines that are solvent-exposed in single-stranded substrates, but also those involved in base-pairing interactions in helical structures. Such feature could be particularly appealing in the identification of regions involved in tertiary interactions, such as those established between secondary structures folded by distal sequences of the same strand, or quaternary contacts between discrete domains present in separate strands. To test these capabilities, we selected a system mimicking the kissing loop (KL) complex formed by two copies of the dimerization initiation site (DIS) of the HIV-1 genome (24). The quaternary interaction that characterizes this system is stabilized by the annealing of complementary palindromic sequences located on the single-stranded loop of each individual hairpin ( Figure 1C). Any mutation in this highly conserved region has been shown to reduce viral infectivity by disrupting both genome dimerization and packaging in vivo, thus making this domain a very desirable target for the development of new antiviral strategies (24)(25)(26)(27)(28).
The desired KL complex employed in these experiments was prepared from a synthetic oligonucleotide by following an established heat-refolding procedure (29). Complex formation was verified by ESI-MS analysis under nondenaturing conditions (Supporting Information, Supplementary Figure S8). The probing reaction was then accomplished by adding 5 M of B-CeP 1 to 2 M KL RNA. STHEM-ESI analysis was performed at both 25 and 95 • C to enable the discrimination between intra-and intermolecular crosslinks, as described above. At 25 • C, the representative spectrum in Figure 7A revealed the presence of both unreacted and alkylated species, which were assigned according to the observed masses. Among them, a series of Nucleic Acids Research, 2021 11 mono-and bi-functional alkylated products, which include both intra-and inter-strand, could be immediately recognized with a range of stoichiometries up to three equivalents of B-CeP 1. In contrast, the higher temperature induced the melting of the quaternary interaction between hairpins. All KL species present in the spectrum ( Figure 7B) contained inter-molecular cross-links that bridged across the annealed palindromic sequences and prevented complex dissociation.
A close examination of the structure of the KL construct showed that the duplex formed by the base-paired palindromes contained only one site in which two guanines (i.e., the G 12 of each hairpin, Figure 1C) were placed in proper position to enable inter-molecular cross-linking. In contrast, at least two different sites comprising G 2 -G 23 and G 4 -G 19 of each hairpin ( Figure 1C) were available to support the formation of intra-molecular cross-links that bridged across the opposite strands of the stem structure. The actual presence of these intra-molecular conjugates was confirmed by the detection of corresponding signals in the low m/z range of the spectrum obtained at 95 • C (Supporting Information, Supplementary Figure S9). The formation of numerous mono-functional adducts could be explained by the initial alkylation of any of the guanines in the construct, followed by failure to produce a second alkylation event because of the absence of another guanine with spatial reach, or the lost competition with the hydrolysis reaction in the aqueous solution. Indeed, each alkylation product observed in these experiments could be clearly explained on the basis of a well-defined structural context, thus indicating that B-CeP 1 could serve as a valid structural probe.

CONCLUSIONS
Bis-chloropiperidine 1 is an alkylating agent capable of reacting at relatively high rate with guanine nucleotides present in any type of nucleic acid substrate. In the case of RNA, B-CeP 1 can lead to stable mono-and bi-functional adducts without inducing the depurination and backbone cleavage observed in DNA (6)(7)(8)(9). The results obtained from dinucleotide and oligonucleotide models provided a clear understanding of the reactivity toward RNA and enabled the characterization of typical alkylation products. The utilization of a double-stranded substrate afforded valuable insights into the structural determinants of B-CeP 1 activity, which revealed the uncommon ability to form bifunctional adducts bridging across guanines in opposite strands. This feat is typically hindered by the rather constrained steric context of helical structures and the need to match the well-defined distance between susceptible N7 positions in the opposite strands. The experiments involving the KL complex demonstrated that, aided by STHEM-ESI analysis, this capability could be leveraged to identify quaternary interactions established by complementary sequences located in different structural domains. In a series of earlier reports, the merits of combining footprinting and crosslinking reagents with MS detection to probe the structure of RNA molecules have been explored (30,31). The information on solvent accessibility and spatial contiguity afforded by such reagents could be translated into valid spatial constraints to support full-fledged molecular modeling operations (32), and allowed to generate all-atom structures for the feline immunodeficiency virus (FIV) frameshifting pseudoknot and the HIV-1 genome packaging signal ( -RNA), which had not been previously solved (33,34). In this context, B-CeP 1 will become an excellent addition to the available toolkit of bifunctional crosslinkers employed for the characterization of long-range tertiary and quaternary interactions in complex RNA systems. Their observed preference for purine nucleotides could readily complement the selectivity of psoralens and related crosslinkers for pyrimidines (35,36), which has recently brought the spotlight on these types of reagents for their ability to support transcriptome-wide structural investigations (37)(38)(39). However, while the rather low yields afforded by the latter necessitate detection strategies based on strand amplification, such as Next-Generation Sequencing (40), the crosslinking efficiency afforded by the former has been shown sufficient to support the greater demands of direct MS-analysis, which must be carried out exclusively on genuine samples, not their amplification copies. Furthermore, while B-CePs conjugates were shown to be stable during the entire analytical workflow, those generated by psoralens and other photoactivated reagents tend to be prone to unwanted instability and, in some cases, can be readily reversed by UV light exposure (40). These favorable characteristics are shared with mechlorethamine and related nitrogen mustards, which are also prominent members of the crosslinking toolkit (16,19). Moreover, B-CePs produce inter-strand crosslinks only on RNA-RNA duplexes, thus making these reagents an excellent choice for applications aimed at elucidating the structure-function relationships in biologically relevant RNA systems and assessing the druggability of RNAs associated with diseases (1,(41)(42)(43). The recent SARS-CoV-2 pandemic has keenly highlighted the relevance of RNA viruses to human health and underscored the importance of structural characterization to expedite the functional elucidation of viral genomes (4,44,45). Therefore, alternative experimental approaches based on structural probes such as B-CeP 1 will be expected to play an ever-expanding role in complementing traditional high resolution techniques.