CONFORMATIONAL DISTRIBUTIONS OF N-ACETYL-L-PROLINE N-METHYLAMIDE IN CDCL3 SOLUTION STUDIED BY NMR AND IR-SPECTRA

The amide proton NMR chemical shifts have been widely used for the determination of the population fractions of hydrogen bonding states in peptides. However, in such works the determination of the limiting chemical shift for each hydrogen bonding state is quite problematic. In our previous paper, we derived the expression for the limiting chemical shift as a function of IR band intensity and chemical shift, and then determined the values of the limiting chemical shifts at various temperatures. In the present study, the method was improved, and was applied to conformational studies of N-acetyl-L-proline N-methylamide (APM) in CDCl3 solution. The population fractions obtained led to thermodynamic quantities of APM for both intra- and intermolecular hydrogen bondings, and also to the complete interpretation of the temperature dependence of the amide proton resonance.