Intramolecular Hydrogen Bonding In N-(2-Amino-2-Deoxy-β-D-Glucopyranoside)-N'-Carbamoyl-L-Dipeptidylesters: An Infrared And 1H Nmr Study

Abstract

ABSTRACT Ten N-(2-amino-2-deoxy-β-D-glucopyranoside)-N'-carbamoyl-L-dipeptidylesters with different amino acid sequences in the dipeptide unit were studied by means of IR and 1H NMR spectroscopy. In the IR spectra three bands at 3453, 3420 and 3390 cm-1 were observed which could be assigned to the free NH, the intramolecularly hydrogen bonded NH species forming five-membered, C5, and seven-membered, C7, rings, respectively. Comparing the NH band positions which correspond to the C7 rings of the Gly-Xaa and the Xaa-Gly dipeptidylesters, the signals of the Xaa-Gly sequence were shifted by 10 cm-1 to lower wave numbers indicating stronger hydrogen bonds. The temperature effect dv/dT was an order of magnitude larger for the C7 associates than for C5 showing the highest enthalpy of the C7 hydrogen bond. The 1H NMR spectra give three separate signals for the NH groups. The temperature coefficient ▵δ/▵T was the largest for N-1-H indicating the formation of less stable hydrogen bonds (C7). The solvent induced changes of the chemical shift of the NH signals was lowest for the N-3-H signal. Obviously the deshielding properties on this function do not vary in dependence of the solvent polarity. The hydrogen/deuterium exchange rate was lowest for the N-6-H proton indicating the lower accessibility of this proton. Combining the results of both spectroscopic methods it can be concluded that the N-1-H forms only C7 rings whereas N-6-H can participate in C5 and C7 intramolecular hydrogen bonds. The strength of the formed C7 associates depends on the amino acid sequence in the dipeptide residue.