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Charge-transfer chemistry of azithromycin, the antibiotic used worldwide to treat the coronavirus disease (COVID-19). Part II: Complexation with several π-acceptors (PA, CLA, CHL)

Finding a vaccine or cure for the coronavirus disease (COVID-19) responsible for the worldwide pandemic and its economic, medical, and psychological burdens is one of the most pressing issues presently facing the global community. One of the current treatment protocols involves the antibiotic azithromycin (AZM) alone or in combination with other compounds. Obtaining additional insight into the charge-transfer (CT) chemistry of this antibiotic could help researchers and clinicians to improve such treatment protocols. Toward this aim, we investigated the CT interactions between AZM and three π-acceptors: picric acid (PA), chloranilic acid (CLA), and chloranil (CHL) in MeOH solvent. AZM formed colored products at a 1:1 stoichiometry with the acceptors through intermolecular hydrogen bonding. An n → π* interaction was also proposed for the AZM-CHL CT product. The synthesized CT products had markedly different morphologies from the free reactants, exhibiting a semi-crystalline structure composed of spherical particles with diameters ranging from 50 to 90 nm.


Introduction
The concept of charge-transfer (CT) or donor-acceptor interactions was first introduced by Robert Mulliken in 1969 based on his studies of Lewis acid-base chemistry. Thereafter, Roy Foster widely discussed and disseminated the concept of CT interactions [1][2][3][4][5][6]. A great deal of research has been dedicated to CT interactions due to their wide range of applications in the fields of chemistry, biology, physics, biochemistry, medicine, pharmacology, material science, and industrial technology [7][8][9][10][11][12][13][14][15][16]. Specifically, in the pharmacology and biochemistry fields, CT interactions contribute to the study of antimicrobial, antitumorigenic, and anti-inflammatory agents, binding mechanisms of pharmaceutical receptors, the thermodynamics and pharmacodynamics of clinical candidate compounds, DNA binding, enzymatic reactions, drug delivery, and quantitively characterizing pharmaceuticals [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. In the fields of material engineering and technology, CT interactions facilitate the development and optimization of solar energy storage devices, organic solar cells, organic semiconductors, electrical conductors, biosensors, optoelectronics, non-linear optical materials, optical communication, photocatalysts, dendrimers, and several other magnetic, optical, and electrical technologies .This long list of diverse applications has motivated extensive research efforts focusing on the physical and chemical properties of CT interactions including their spectral, photophysical, kinetic, thermodynamic, and crystallographic properties in addition to the factors that affect the yield of these products such as solvent, temperature, and concentration . Azithromycin (AZM) (C 38 H 72 N 2 O 12 , 748.98 g mol −1 , Fig. 1a) is the donor used in this work. Due to the wide range of its physiological effects along with its high level of safety, AZM is used to treat many infectious diseases (viral and bacterial in the respiratory tract, genitourinary, and enteric regions) including but not limited to pneumonia, influenza, chlamydia, typhoid fever, and malaria [130][131][132][133]. Recently, AZM has been incorporated into the treatment protocols for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) disease (COVID-19) responsible for the current worldwide pandemic. This syndrome originated in Wuhan, China in December 2019 and quickly spread around the globe [134], causing the World Health Organization (WHO) to declare the COVID-19 outbreak a pandemic in early March 2020 [135]. Now, the mostly pressing issue facing the worldwide community is finding a means to treat or prevent COVID-19 [136]. One of the treatment protocols reported in literature includes AZM in combination with hydroxychloroquine or chloroquine as a short-term course [137][138][139][140][141][142]. The acceptors used in this study are picric acid (PA), chloranilic acid (CLA), and chloranil (CHL) (Fig. 1b). PA is a strong organic acid and the most frequently used acceptor investigated in CT interactions [23,32,44,53,105,116,117,125,. CLA and CHL are interesting from a biological perspective as benzoquinone derivatives. The CLA molecule has two chloro groups and two carbonyl groups, whereas the CHL molecule has four chloro groups and two carbonyl groups. These groups act as electron-withdrawing systems that decrease the electron density of the aromatic rings of CLA and CHL, thereby rendering them robust electron-accepting systems [35,149,[164][165][166][167][168]. The purpose of this work was to investigate the CT interaction of AZM with three πacceptors (PA, CLA, and CHL) in MeOH solvent.

Chemicals and characterization techniques
All chemicals used in this work (donor, acceptors, solvents) were procured from international chemical sources: Fluka (Germany), BDH (United Kingdom), and Sigma-Aldrich (USA). AZM, PA, CLA, and CHL were obtained at the highest purity available (≥ 98%). Throughout all experiments, dust masks, eye shields, and gloves served as personal protective equipment. A Bruker High-Performance Digital FT-NMR Spectrometer (DRX-250) was used to collect the 1 H and 13 C NMR spectra at 400 MHz with dimethylsulfoxide (DMSO-d 6 ) as the solvent and tetramethylsilane (TMS) as the internal reference. A Shimadzu Fourier Transform Infrared Spectrophotometer (IR Tracer-100) was used to collect the FT-IR spectra in transmission mode from 4000 to 400 cm −1 . The HACH LANGE GmbH UV/VIS Spectrophotometer (DR6000 Benchtop) was used to record the UV-visible spectra and spectrophotometric measurements in absorption mode from 800 to 200 nm. Elemental analyses of C, H, and N (in %) were performed on a Perkin-Elmer CHN Microanalyzer (Model PE 2400 series II). A Malvern Panalytical X-ray Powder Diffractometer (X'Pert 3 MRD) was used to collect the XRD spectra over the diffraction angle (2θ) 5°to 80°at a radiation wavelength (λ) of 0.154056 nm. High-magnification images and EDX data were collected using the JEOL Scanning Electron Microscope (SEM; JSM-639OLA) coupled with an Energy Dispersive X-ray Spectrometer (EDXRF; JED-2300) and the JEOL Transmission Electron Microscope (TEM; JEM-1200EX II).

Synthesis of the solid CT products
The AZM donor was mixed with each acceptor (PA, CLA, or CHL) separately at a molar ratio of 1:1 (donor to acceptor) in pure, analyticalgrade MeOH solvent at room temperature. The three mixtures were heated to 60°C, followed by intense stirring for 20 min during which colored precipitates formed in each reaction system. The volume of each system was reduced by half using a water bath, and the samples were then left at room temperature for 24 h to complete the precipitation process. The resulting colored precipitates were sceptered from the solvent by filtration using Whatman 42 grade filter paper, rinsed well with MeOH several times, and recrystallized in MeOH to increase the purity of the products. After being dried in a vacuum over anhydrous CaCl 2 , the final colored CT products shown in Fig. 2 were obtained. AZM formed a canary yellow-colored product with PA, a violet-colored product with CLA, and a brownish-red-colored product with CHL. 1 H and 13 C NMR, FT-IR spectroscopies and elemental analyses were carried out to characterize the acquired solid CT products.

Stoichiometry of the CT interaction
The stoichiometry of the CT interaction between the AZM donor and the acceptors (PA, CLA, and CHL) was determined using three methods: elemental analysis of the solid CT products, and the spectrophotometric titration method [169] and Job's continuous variation method [170] Fig. 1a. Structure of the AZM molecule. Fig. 1b Structures of the π-acceptors used in this work.  The microanalytical results suggested that AZM interacted with all acceptors at a molar ratio of 1:1 to form a complex with the general formula: [(AZM)(acceptor)].

SEM-EDX data
The SEM-EDX technique was used to observe the elemental compositions of the solid products and to visualize their surface morphology and the shape of the microstructure. Figs. 3, 4, 5, and 6 contain the SEM images and EDX spectra of the free AZM, PA, CLA, and the solid CLA product, respectively. Fig. S1, S2, and S3 contain the SEM images and EDX spectra of the free CHL, the solid PA product, and solid CHL product, respectively. The elemental compositions indicated by EDX analysis of the PA, CLA, and CHL products aligned with those obtained from the elemental analysis. The presence of oxygen and carbon elements was corroborated for all products and chlorine for the CLA and CHL products by their EDX spectra. The SEM images of the free reactants revealed that AZM consisted of small pieces with undefined shapes and features. The PA particles were cubic, the CLA particles had a crocodile skin-like structure, and the CHL particles clumped into polygonal agglomerates. The morphology of the free reactants changed upon complexation. In the PA and CHL products, the surface of the particles became smoother than the free reactants. The particles of the PA product were no longer cubic but instead formed into agglomerates. In the CLA product, the rough, crocodile skin-like surface that characterized the free CLA particles was replaced by small, smooth pieces of different shapes and sizes.

TEM imaging and XRD results
TEM was used to determine the size and shape of the particles from the solid CT products. The instrument used an accelerating voltage of 100 kV, and the TEM images were collected at presents the XRD patterns of the solid products scanned from a Braggʼs diffraction angle (2θ) of 5°to 80°. The PA product generated a strong, sharp, intense Bragg diffraction line at angle 2θ 17.05°and a group of medium-strong intensity lines in the range of 18°to 30°. The CLA product gave one sharp, intense, very strong Bragg diffraction line located a 2θ of 15.295°and a medium-strong line at 22.325°. The CHL product displayed one strong, sharp, intense diffraction line at a 2θ of 15.164°and a group of medium intensity lines between 18°a nd 30°. These XRD profiles suggested that the solid CT products possess semi-crystalline structures.
The IR spectral data in (cm −1 ) for the free PA molecule were: 3100 ν (
The observed 13 C NMR spectra of the free AZM molecule and the CT products contained the following chemical shifts (δ) for the different types of carbon moieties (in ppm):  The free AZM molecule produced 33 protons in the range of δ = 0.58-5.61 ppm. This region was crowded with singlet, doublet, and triplet signals from -CH, -CH 2 , and -CH 3 protons. AZM also produced 37 carbon resonances located in the δ = 9.36-177.66 ppm range. All of these proton and carbon resonances were also present in the 1 H and 13 C NMR spectra of the PA, CLA, and CHL products. The six protons of the tertiary amino group attached to carbon number C 4'' [N(CH 3 ) 2 ] of the AZM molecule exhibited considerable up-field shifts from 2.99 ppm for the free AZM molecule to 2.68 for its product with the PA acceptor, to 2.74 for its product with the CLA acceptor, and to 2.60 for its product with the CHL acceptor. This up-field shift suggested that the nitrogen atom in this group participated in the CT complexation with the acceptors. The free AZM molecule exhibited a singlet at 4.33 ppm due to the five protons from the (OH) groups. This signal was still observed in all of the products representing small shifts from the analogous signal in the free AZM spectrum. It was located at 4.2 ppm for the PA and CHL products, and at 4.11 for the CLA product. The characteristic signal from the OH proton that resonated at δ~9.95 ppm in the spectrum of the free PA molecule up-field shifted to 8.59 ppm in the spectrum of the PA product upon complexation, suggesting that the OH group of PA participated in the formation of the CT product. The characteristic signal from the two OH protons that resonated at δ~9.15 ppm in the spectrum of the free CLA molecule was observed in two different positions in the spectrum of the CLA product. The signal centered at 9.84 ppm was assigned to the non-hydrogen-bonded OH of the CLA moiety, while the signal centered at 10.08 was attributed to the hydrogen-bonded OH. Several changes in the values of the 13 C chemical shifts in the AZM molecule were observed after its complexation with the acceptors. In the 13 C NMR spectra of the products, the 41, 43, and 39 resonant carbon signals in the spectra of the PA, CLA, and CHL products, respectively, were in agreement with the structures proposed for these products. Based on the foregoing elemental and spectral results, the structures of the CT products were formulated as illustrated in Fig. 13. Solutions of free AZM, free acceptors, and the resultant soluble CT products were scanned in the UV-visible region, as seen in Fig. 14. Several conclusions were drawn from these data: i). The solution of free AZM is MeOH solvent is colorless, displaying no measurable absorption band in the UV-visible region and tailing in the visible region with no peak maximum. ii). All of the acceptors solubilized in MeOH absorbed in the visible region. The PA acceptor had a strong, wide absorption band ranging from 300 nm to 450 nm with a λ max at 353 nm. The CLA acceptor displayed a weak, wide absorption band ranging from 440 nm to 600 nm with a λ max at 523 nm. The CHL acceptor displayed a strong, wide absorption band ranging from 330 nm to 400 nm with a λ max at 360 nm. iii). The formation of a CT complex changed the electronic spectrum of the donor, acceptor, or both. These spectral changes included bathochromic shifts: increases in the intensity of the absorption band of the donor, acceptor, or both and hypsochromic shifts: the appearance of a new, intense, broad band in the UV-visible region where the uncomplexed donor or acceptor absorbs. The interactions between AZM and both PA and CLA were characterized by bathochromic shifts. While AZM didn't absorb in the visible region, the absorption band that characterized the free PA and CLA molecules considerably increased in intensity after complexation with the AZM molecule. iv). A hypsochromic shift was observed when AZM was complexed with CHL. The interaction between AZM and CHL resulted in the formation of a new, intense, very broad and strong band that appeared at a much longer wavelength than was present for the free CHL. This new band had a λ max of 412 nm.

Stoichiometry
The composition of AZM and the acceptors in solution was determined using the spectrophotometric titration method and Job's continuous variation method, and the obtained plots are given in Fig. 15a, 15b. Both methods suggested that the AZM molecule interacted with the PA, CLA, and CHL acceptors at a 1:1 ratio. Therefore, the general composition of the products was [(AZM)(acceptor)].

Spectroscopic parameters
Several spectroscopic parameters were evaluated for the PA, CLA, and CHL products using the 1:1 Benesi-Hildebrand plots at 298 K (Fig. 16), and equations described elsewhere [186][187][188][189][190][191][192][193][194][195]. These parameters are the transition dipole moment (μ), the formation constant (K CT ), the standard free energy (ΔG°), the energy of interaction (E CT ), the oscillator strength (f), the molar extinction coefficient (ε max ), the ionization potential (I P ), and the resonance energy (R N ). Table 1 lists the spectroscopic parameters of the PA, CLA, and CHL products. All ΔG°values were negative, indicating that the interactions between the acceptors and AZM were exothermic, spontaneous, and reasonably stable [196]. Generally, the three products had high K CT and ε max values. The high K CT suggested that the complex was strongly bound and highly stable [197]. The complex with the PA acceptor showed the highest values of ε max and K CT (Fig. 17).

Conclusions
Obtaining a vaccine or cure for COVID-19 is the most pressing worldwide concern in 2020. One of the treatment protocols for COVID-19 uses AZM alone or in conjugation with other compounds. Providing additional insight into the CT chemistry of AZM by examining its interactions with several π-acceptors may help researchers and physicians to improve the treatment protocols for COVID-19. This work investigated the formation of three CT products arising from the complexation of AZM with the π-acceptors PA, CLA, and CHL in MeOH solvent. The structures and morphologies of the synthesized CT products were fully characterized using several spectroscopic and physicochemical approaches. investigated acceptors. The characteristic shifts in the FT-IR and 1 H NMR spectra suggested that the CT interaction between AZM and the acceptors occurred through intermolecular hydrogen bonding. An n → π* interaction was also proposed for the AZM-CHL CT complex. The morphologies of the free reactants were markedly different than the synthesized CT products with semi-crystalline structures composed of spherical particles with diameters ranging from 50 to 90 nm.

Declaration of Competing Interest
The authors declare that no conflict of interest.  17. Values of the formation constant (K CT ), and the molar extinction coefficient (ε max ) for the PA, CLA and CHL products.