MEASUREMENT OF INTRINSIC EXCHANGE-RATES OF AMIDE PROTONS IN A N-15-LABELED PEPTIDE

We have used a modified version of a previously proposed technique, ME XICO [Gemmecker et al. (1993) J. Am. Chem. Sec., 115, 11620], and impr oved data analysis procedures in order to measure rapid hydrogen excha nge (HX) rates of amide protons in peptides labeled only with N-15. Th , requirement of C-13-/N-15-labeled material has been circumvented by adjusting conditions so that NOE effects associated with amide protons can be neglected (i.e., omega(0) tau(c) similar to 1). The technique was applied to an unstructured N-15 labeled 12-residue peptide to meas ure intrinsic HX rates, which are the essential reference for examinin g protein and peptide structure and dynamics through deceleration of H X rates. The method provided accurate HX rates from 0.5 to 50 s(-1) un der the conditions used. The measured rates were in good agreement wit h those predicted using correction factors determined by Englander and co-workers [Bai et al. (1993) Proteins, 17, 75], with the largest dev iations from the predicted rates found for residues close to the N-ter minus. The exchange rates were found to exhibit significant sensitivit y to the concentration of salt in the sample.