EXCHANGE KINETICS OF INDIVIDUAL AMIDE PROTONS IN N-15-LABELED HELICALPEPTIDES MEASURED BY ISOTOPE-EDITED NMR

Amide proton exchange measured by one-dimensional N-15-edited proton N MR has been used to probe helical structure in an alanine-based peptid e. This study is the first report of individual peptide NH exchange ra tes determined in a simple, repeating sequence peptide whose helical s tructure can be predicted by helix-coil theory. Measured protection fa ctors directly demonstrate that the ends of the helix are frayed. The protection factors are compared to the Lifson-Roig theory, modified to include N-capping, using known values for helix propensities and N-ca p propensities. Base-catalyzed exchange rates are shown to measure the extent of hydrogen bonding of the peptide NHs, and the results are fi tted by a simple model in which hydrogen bonding of the peptide NH gro up provides protection and no exchange occurs from the hydrogen-bonded state. Protection from acid-catalyzed exchange correlates with hydrog en bonding by both the NH and CO groups of a peptide unit: the data ar e fitted by a model in which exchange occurs only when both hydrogen b onds formed by a peptide unit are broken. This result indicates that a cid-catalyzed exchange occurs by the O-protonation mechanism, in agree ment with earlier work [Perrin & Arrhenius (1982) J. Am. Chem. Soc. 10 4, 6693-6696; Perrin et al. (1984) J. Am. Chem. Soc. 106, 2749-2753;Tu chsen & Woodward (1985) J. Mol. Biol. 185, 421-430].