We report model compound data for primary amide hydrogen exchange in three protected amino acids: Nacetyl-Asn-iV0-methylamide (Asn'), /V-acetyl-Gln-7Va-methylamide (Gin'), and 7V-acetyl-Gly-NH2 (Gly'). The solvent exchange rates of the individual E and Z protons have been measured for each of these amino acids in H20 by employing double-resonance NMR techniques. The corresponding acidand base-catalyzed rate constants (M-1 s-1) are kHE = 897, kuz = 806, k0HE = 9.39 X 107, and k0liz = 2.2 X 107 for Asn' at 25 °C; = 1134, kHz = 1095, koliE = 5.52 X 107, and koiiz = 1.36 X 107 for Gin' at 22 °C; and kHE = 327, kHz = 333, koiiE = 13.48 X 107, and kouz = 7.87 X 107 for Gly' at 25 °C. Our model compound data provide a basis for evaluating the primary amide hydrogen exchange behavior of these amino acids in peptides and proteins. The acid-catalyzed exchange in these model primary amides appears to take place predominantly through N-protonation.Essential to conformational and dynamic analyses based on hydrogen exchange kinetics is the estimation of the exchange rates of specific labile hydrogens in "completely solvated" environments.2-6 For proteins and peptides, such estimates have been obtained from studies of model peptides7 and indoles,8,9 but only inadequate data are available for the primary amide components of these biomolecules.10 Here we have employed the NMR double-resonance method4-6,8,9,11 to measure the individual exchange rates of the E and Z carboxamide hydrogens of Nacetyl-Asn-(Va-methylamide (Asn'), ¿V-acetyl-Gln-A'a-methylamide (Gin'), and TV-acetyl-Gly-amide (Gly'), which serve as models for Asn, Gin, and Gly NH2 residues.12 NMR methods have been employed to measure the exchange rates of secondary amides or Trp indole NH hydrogens of peptides4-6,13-19 and proteins.20,21(1) (a) University of Alabama in Birmingham, (b) Leukemia Society of America Scholar, 1982-1987.
