Intermolecular Hydrogen Bonds Formed Between Amino Acid Molecules in Aqueous Solution Investigated by Temperature-jump Nanosecond Time-resolved Transient Mid-IR Spectroscopy

Abstract

Carboxyl (COO) vibrational modes of two amino acids histidine and glycine in D2O solution were investigated by temperature-dependent FTIR spectroscopy and temperature-jump nanosecond time-resolved IR difference absorbance spectroscopy. The results show that hydrogen bonds are formed between amino acid molecules as well as between the amino acid molecule and the solvent molecules. The asymmetric vibrational frequency of COO around 1600-1610 cm1 is blue shifted when raising temperature, indicating that the strength of the hydrogen bonds becomes weaker at higher temperature. Two bleaching peaks at 1604 and 1612 cm1 were observed for histidine in response to a temperature jump from 10 C to 20 C. The lower vibrational frequency at 1604 cm1 is assigned to the chain COO group which forms the intermolecular hydrogen bond with {NH3}+ group, while the higher frequency at 1612 cm1 is assigned to the end COO group forming hydrogen bonds with the solvent molecules. This is because that the hydrogen bonds in the former are expected to be stronger than the latter. In addition the intensities of these two bleaching peaks are almost the same. In contrast, only the lower frequency at 1604 cm1 bleaching peak has been observed for glycine. The fact indicates that histidine molecules form a dimer-like intermolecular chain while glycine forms a relatively longer chain in the solution. The rising phase of the IR absorption kinetics in response to the temperature-jump detected at 1604 cm1 for histidine is about 3010 ns, within the resolution limit of our instrument, indicating that breaking or weakening the hydrogen bond is a very fast process.