Hyper-Raman spectroscopy of non-proteinogenic amino acids.

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 NH 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.