The 532-nm-excited hyper-Raman spectroscopy of globular protein and aromatic amino acids

Abstract

Hyper-Raman (HR) spectroscopy reveals unique information about molecular structures, because of its different selection rule, compared with that of the infrared (IR) and Raman spectroscopy. The application of HR spectroscopy on biomolecules' research has been expected for a long time. Herein, we report the HR spectra of the amino acids tryptophan (Trp), tyrosine (Tyr), histidine, and bovine serum albumin (BSA) in aqueous solution, obtained at an excitation wavelength of 532 nm. The HR bands of BSA were mostly assigned to those of the aromatic amino acids. The predominance of the aromatic amino acids was explained by considering the resonance effect of the excitation beam to the electronic absorption around 266 nm, the double frequency of 532 nm (e.g. L-a of Trp, L-b of Tyr). Besides the Raman bands observed in the 266-nm-excited ultraviolet resonance Raman spectrum, some of the IR-active bands of Tyr were also detected. Hence, the HR spectroscopy serves as a new tool toward the investigation of proteins as well as of biomolecules and more complicated biological structures with the 532-nm excitation.