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1 H nuclear magnetic resonance studies of N ‐acetyl‐ L ‐proline N ‐methylamide. Molecular conformations, hydrogen bondings, and thermodynamic quantities in various solvents
Author(s) -
Higashijima Tsutomu,
Tasumi Mitsuo,
Miyazawa Tatsuo
Publication year - 1977
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1977.360160608
Subject(s) - chemistry , hydrogen bond , intramolecular force , intermolecular force , cis–trans isomerism , solvent , enthalpy , chemical shift , molecule , solvent effects , crystallography , stereochemistry , organic chemistry , physics , quantum mechanics
Concentration and temperature dependences of the 1 H nmr spectra of N ‐acetyl‐ L ‐proline N ‐methylamide were observed in various solvents [CCl 4 , CDCl 3 , (CD 3 ) 2 CO, (CD 3 ) 2 SO, H 2 O, and D 2 O]. The fraction of the cis isomer (with respect to the bond between the acetyl carbonyl carbon and prolyl nitrogen atoms) depends greatly on the solvent used; the fraction of the cis isomer is higher in polar solvents than in nonpolar solvents. It depends also on concentration and temperature in nonpolar solvents but not in polar solvents. In nonpolar solvents the trans isomer mostly exists in the γ‐turn structure with an intramolecular hydrogen bond and the cis isomer tends to form molecular aggregates by intermolecular hydrogen bonds. In polar solvents both the cis and trans isomers exist in monomeric forms which interact with solvent molecules. The pH dependences of the N ‐methyl proton resonances indicate that the γ‐turn structure of the trans isomer is present also in aqueous solution, though its population is difficult to determine. Apparent enthalpy and entropy changes for the conversion of the trans isomer to cis isomer are evaluated for various solvents. The results are discussed in terms of the intra‐ and intermolecular hydrogen bondings.