Conformational Analysis of Linear Peptides. 3. Temperature Dependence of NH Chemical Shifts in Chloroform

Article abstract of DOI:10.1021/ja00543a025

The following conclusions concerning NH chemical shifts for peptides in chloroform are based on numerous observations on small model peptides reported here and also on data in the literature. (1) The temperature dependence of chemical chift for an amide NH proton exposed to solvent in chloroform solutions is 0.0024 +/- 0.0005 ppm K^-1. (2) Small temperature dependencies can also be observed if the NH group is shielded from solvent and remains shielded over the temperature range of the 1H NMR measurements. (3) Larger temperature dependencies are observed if the NH group is shielded from solvent initially but becomes exposed with increasing temperature, a situation which holds when intermolecular self-association is significant or when intermolecularly hydrogen-bonded conformations unfold as the temperature is increased.Previously proposed conformations based on the presumption that NH chemical shifts display in chloroform solution the same behavior as in strongly polar solvents are herein reexamined.