Uncovering the vibrational modes of zwitterion glycine in aqueous solution

Vibrational spectroscopy is widely employed to probe and characterise chemical, biological and biomedical samples. Glycine solutions are relevant in a variety of biological and chemical systems, yet the reported experimental vibrational wavenumbers of the glycine zwitterion, which is the dominant species in aqueous solution, are inconsistent and incomplete. This study presents a procedure that obtained a complete set of vibrational frequencies for the glycine zwitterion in aqueous solution, apart from the two lowest wavenumber modes which are available from a previous THz study. Vibrational spectra were measured using IR and Raman spectroscopy, to obtain both IR and Raman-active modes for a range of different glycine solution concentrations using four different instruments. Insight from a literature survey of density functional theory calculations in implicit and explicit water was used to guide the deconvolution of the experimental spectra into vibrational modes, giving 22 out of 24 vibrational wavenumbers with a standard error of less than 3 cm⁻¹. This thorough analysis of the glycine vibrational spectra has enabled missing and erroneous wavenumbers in literature to be identified, and the systematic procedure for determining vibrational modes will pave the way for deeper quantitative analysis of glycine systems, and serve as a benchmark for computational method development.