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 det ermination of the population fractions of hydrogen bonding states in p eptides. However, in such works the determination of the limiting chem ical shift for each hydrogen bonding state is quite problematic. In ou r 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 tempe ratures. In the present study, the method was improved, and was applie d to conformational studies of N-acetyl-L-proline N-methylamide (APM) in CDCl3 solution. The population fractions obtained led to thermodyna mic quantities of APM for both intra- and intermolecular hydrogen bond ings, and also to the complete interpretation of the temperature depen dence of the amide proton resonance.