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Long-range correlations between aliphatic 13C nuclei in protein MAS NMR spectroscopy.

Solid-state NMR is a powerful technique for the investigation of complex biological systems such as membrane proteins and amyloid fibrils. In magic-angle spinning (MAS) NMR, structural information is obtained via the reintroduction of anisotropic interactions.[1] In particular, a large number of homonuclear dipolar recoupling schemes have been developed and applied to record correlation spectra and measure internuclear distances in peptides and proteins.[1] While many recoupling techniques efficiently transfer polarization between directly bonded 13C nuclei, their effectiveness can be reduced significantly when distant 13C spins with weak dipolar couplings are involved. This limitation is generally imposed both by the experimental constraints that must be maintained during long mixing periods and by the inherent complexities of multiple-spin systems such as dipolar truncation,[2, 3] that is the attenuation of weak dipolar couplings by stronger dipolar couplings in the recoupled dipolar Hamiltonian. As a consequence, spin diffusion techniques,[4–8] which circumvent some of these limitations,[9] have been widely utilized to estimate long-range homonuclear structural constraints in protein solid-state NMR studies.[10, 11] In this communication we present an experimental approach that provides highly sensitive long-range correlations between aliphatic 13C nuclei through the combination of isotope dilution and efficient polarization transfer via a band selective radio frequency dipolar recoupling (BASE RFDR) scheme. We demonstrate this method with a sample of PI3-SH3 (the SH3 domain of the p85α subunit of phosphatidylinositol 3 kinase) in amyloid fibril form.