Internal motions of aromatic amino acid side chains. Selective excitation nuclear relaxation and scalar coupling constant analysis on a model system

The model compound N-acetyl-L-phenylalanine methyl ester was used to investigate the motional behavior of aromatic amino acid residues in peptides and proteins. A combined analysis of proton scalar coupling constants and selective excitation spin-lattice relaxation rates yields a self-consistent insight into (i) relaxation mechanisms, (ii) overall molecular motion, and (iii) local mobilities along the side chain. At the operative frequency of 270 MHz, the amino acid derivative tumbling fulfilled the condition ωo2τc2 ≪1. The proton relaxation pathway was dominated by the intramolecular dipole-dipole relaxation mechanism. Extensive internal motions of the side chain were found. © 1979 American Chemical Society.