13 C n.m.r. study of l‐alanine peptides

The di‐, tri, and tetrapeptides of l‐alanine have been studied in aqueous solution by ¹³C n.m.r. spectroscopy at 25 and 50 MHz. By using selectively ¹³C enriched analogs containing either 90% ¹³C methyl or carbonyl carbons and measurements as a function of pH, assignment of the chemical shifts of the peptides has been made. T₁ and NOE measurements of the peptides in their cationic, anionic, and zwitterionic states have been recorded as a function of concentration. The results show considerable segmental motion along the backbone carbons of the peptides, with only small changes occurring in the dynamic motions of the peptides as their charge states are altered. The lack of concentration dependence of the chemical shift and T₁ values, as well as the similarity of T₁ values for individual peptides in the three charge states, indicate that the peptides do not self‐associate in aqueous solution.