Intramolecular hydrogen bonding in N-(2-amino-2-deoxy-beta-D-glucopyranoside)-N '-carbamoyl-L-dipeptidylesters: Infrared and H-1 NMR study

Ten N-(2-amino-2-deoxy-beta -D-glucopyranoside)-N'-carbamoyl-L-dipeptidyles ters with different amino acid sequences in the dipeptide unit were studied by means of IR and H-1 NMR spectroscopy. In the IR spectra three bands at 3453, 3420 and 3390 cm(-1) were observed which could be assigned to the fre e NH, the intramolecularly hydrogen bonded NH species forming five-membered , C-5, and seven-membered, C-7, rings, respectively. Comparing the NH band positions which correspond to the C-7 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 temperat ure effect dv/dT was an order of magnitude larger for the C-7 associates th an for C-5 showing the highest enthalpy of the C-7 hydrogen bond. The H-1 N MR spectra give three separate signals for the NH groups. The temperature c oefficient Delta delta/DeltaT was the largest for N-1-H indicating the form ation of less stable hydrogen bonds (C-7) The solvent induced changes of th e chemical shift of the NH signals was lowest for the N-3-H signal. Obvious ly 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. Combini ng the results of both spectroscopic methods it can be concluded that the N -1-H forms only C-7 rings whereas N-6-H can participate in C-5 and C-7 intr amolecular hydrogen bonds. The strength of the formed C-7 associates depend s on the amino acid sequence in the dipeptide residue.