Structural dynamics of N-propionyl-D-glucosamine probed by infrared spectroscopies and ab initio computations

N-Acylglucosamine is an important component in many oligosaccharides in eukaryotes, where it plays a very important biological role. Located between a glucose ring and an alkyl group of such species is an amide unit (-CONH-), which exhibits an infrared absorption band, mainly due to the C═O stretching, in the region of 1600-1700 cm(-1), similar to the amide-I band found in polypeptides. In this work, vibrational properties of such an "amide-I mode" in N-propionyl-d-glucosamine (GlcNPr) are examined in three typical solvents (water, methanol, and dimethylsulfoxide) by using steady-state infrared and femtosecond infrared dispersed pump-probe spectroscopies. As a result of solute-solvent interactions, multiple structured GlcNPr-solvent clusters are formed in water and methanol but are unlikely in dimethylsulfoxide. The vibrational relaxation rate of the amide-I mode is slightly frequency-dependent, supporting the presence of multiple solvated structures. Further, the amide-I lifetime is significantly shorter in GlcNPr than that in a well-known monopeptide, N-methylacetamide, which can be attributed to the presence of additional downstream vibrational modes caused by the sugar unit. Ab initio molecular dynamics simulations are used to reveal microscopic details of the first solvation shell of GlcNPr. Our results demonstrate that the amide-I mode in glucosamine exhibits both structural and solvent sensitivities that can be used to characterize the three-dimensional arrangement of sugar residues and their structural dynamics in glycopeptides.