Kinetics of the exchange of individual amide protons in the basic pancreatic trypsin inhibitor

Abstract

The kinetics of the exchange with solvent deuterons was studied for 16 individual amide protons in the basic pancreatic trypsin inhibitor. Nuclear magnetic resonance was used to measure the proton exchange rates over the p 2H range from 0·1 to 10·9 at temperatures between 10°C and 60°C. Considerably different absolute rates of exchange were observed for individual protons, and the p 2H and temperature dependences were also found to vary. For all the amide protons the overall rate is governed by acid/base catalyzed exchange, i.e. the rate is proportional to [H30]2 and [OHe]y. x and y were found to vary between 0·5 and 1·5 for individual protons, which contrasts with the behaviour of solvent-accessible amide protons in model peptides, where x = y = 1. The p 2H minima of the exchange rates are also shifted compared to the model peptides. The observed exchange kinetics cannot be explained by classical models proposed for interpretation of global amide proton exchange rates. A satisfactory interpretation results from a new multi-state model for globular proteins ( Wüthrich & Wagner, 1978). This is the second paper in a series of three nuclear magnetic resonance studies of the amide protons in the basic pancreatic trypsin inhibitor. A detailed description of the multi-state model will be presented in a forthcoming paper.