Van der Waals' induced 13C NMR shifts in crystalline amino acids and peptides.

Abstract

Recent reports in the literature of solid-state 13C nuclear magnetic resonance (NMR) spectra of crystalline L-alanine [Naito, A., Ganapathy, S., Akasaka, K., and McDonnell, C. A. (1981) J. Chem. Phys. 74, 3190-3197] and L-leucine [Frey, M. H. and Opella, S. J. (1980) J. Chem. Soc. Chem. Commun., 474-475], recorded with cross-polarization and magic-angle spinning (CP-MAS), show downfield resonance shifts of several parts per million in their side-chain methyl groups, relative to their resonance positions in aqueous solution. Similar findings are reported here for crystalline aliphatic amino acids and L-alanine peptides, including tetra(L-alanine), which show similar, specific downfield shifts in their side-chain methyl resonances. Coupled with X-ray crystallographic data of these compounds, and previous gas and solution-phase 13C NMR studies, the CP-MAS 13C NMR data indicate that these downfield shifts are a result of van der Waals' interactions. This group have reported similar van der Waals' induced shifts of the same magnitude for 13C resonances of the side-chain methyl groups of 13C-enriched tetra(L-alanine) upon binding to high-affinity Fab' fragments of heterogeneous sheep anti-[poly(L-alanine)] antibodies in aqueous solution [Geller, S., Wei, S. C., Shkuda, G. K., Marcus, D. M., and Brewer, C. F. (1980) Biochemistry 19, 3614-3623]. The above findings show that van der Waals' induced 13C NMR shifts of similar magnitudes can be detected in specific antibody-hapten complexes and the side chains of crystalline aliphatic amino acids and peptides. The results also indicate that water possesses relatively little attractive van der Waals' interactions with aliphatic molecules.