IR Fingerprint of the Intermolecular Hydrogen Bond on Amino Acids and Its Relevance to Chaperone Activity of αB-Crystallin.

Intermolecular hydrogen bonds between carboxyl (COO^-) and amino groups are a common weak interaction in proteins. Infrared (IR) spectral assignment of such an intermolecular hydrogen bond provides a fingerprint for studying protein-protein interactions as its absorption frequency is affected by the molecular electrostatic environment. Temperature-dependent FTIR and temperature-jump time-resolved IR absorbance difference spectra of several typical amino acids and those of wild type and single-site mutated αB-crystallin were performed. It was found that the antisymmetric vibrational frequency of the COO^- groups in amino acids decreases from approximately 1626 to 1610 cm^-1 upon the formation of intermolecular hydrogen bonds, which was further supported by DFT calculations, while the IR frequency of the intermolecular hydrogen bonds on the formation of intermolecular hydrogen bonds, which was further supported by DFT calculations, while the IR frequency of the intermolecular hydrogen bonded COO^- groups at the αB-crystallin dimeric interface was also observed around 1610 cm^-1. With this spectral label, the active site of αB-crystallin, a heat shock molecular chaperone against the UV-light-damaged γD-crystallin was investigated. The active site was found to be localized at an arch loop structure connecting the two β-strands locked by intermolecular hydrogen bonds at the dimeric interface. It is the liberated arch loop after breaking of the intermolecular hydrogen bond locks that binds the damaged γD-crystallin, leading to the observed chaperone-like activity of αB-crystallin.