Hyper-Raman spectroscopy of non-proteinogenic amino acids

Anal Sci. 2024 Dec 13. doi: 10.1007/s44211-024-00698-1. Online ahead of print.

Abstract

We report 532-nm and 1064-nm excited hyper-Raman (HR) spectra of representative non-proteinogenic amino acids, including α-, β-, and γ-amino acids. Different from the common 20 proteinogenic amino acids, natural non-proteinogenic amino acids cannot be incorporated into proteins during translation, while they are indispensable as intermediates in many processes like biosynthesis and neurotransmitters. In 532-nm excited HR spectra, the COO symmetric stretching bands are commonly intense, and the NH3+ bands are clearly observable. In addition, based on the reported IR and Raman study, we found that some HR bands are IR-active but Raman-inactive. In contrast, HR signals with the 1064-nm excitation are much weaker than the 532-nm excitation. Nevertheless, we observed the COO scissoring band unexpectedly, much stronger than other bands with the 1064-nm excitation. Our results suggest that the electronic resonance effect plays a role in enabling us to detect HR signals in the UV region readily. We expect that this study provides a supplementary reference for HR spectroscopy of natural amino acids.

Keywords: Electronic resonance effect; Hyper-Raman spectroscopy; Non-proteinogenic amino acid; Nonlinear Raman spectroscopy.