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

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.