Application to Biomolecules: glucose [1]
Saccharide plays important roles for life, e.g., it can be a nutrient (mono- and disaccharide), modification of soluble and membrane proteins (oligosaccharide), and a material having mechanical strength (polysaccharide). We applied HR spectroscopy to glucose in phosphate-buffered saline (PBS) in order to examine what kind of the information was obtained.
Large and unique HR bands were observed from 1500-1650 cm^-1 (a). The HR spectral pattern was completely different from Raman spectra excited with 532 nm (b) or 266 nm (c), and infrared absorption spectrum (d). These bands are likely the combination and/or overtone modes rather than the fundamental modes. The isotope effect was seen upon the substitution of a carbon atom of glucose to ¹³C (e), while not by ¹⁸O substitution of phosphate (f). These unique HR bands were also observed in buffers containing phosphate, carbonate, and Tris but not in H₂O. The unique feature of the HR spectrum suggests its uniqueness.


HR study on trifluoroethanol as model compound of sugar [2]
While the glucose/PBS is a simple solution in biology, it is not in chemistry. It is because glucose consists of 24 atoms and PBS contains phosphate ions and NaCl. To simplify the object, we chose trifluoroethanol (TFE) that involves the CHOH group as a model compound of sugar.
The HR spectrum of 50 mM TFE aqueous solution at pH 13.2 was found to be totally different from that of neat TFE. Also, the spectral pattern was different from the Raman spectrum. The observed HR bands of TFE at pH 13.2 was not ascribed to TFE but another species.
By analyzing the laser power dependence, concentration dependence, and isotope effect, we concluded that the unique HR bands originated from the TFE radical anions. We explained the reason of the selective detection of the HR signals of the radical anions considering the one- and two-photon double resonance effect that occurs only in HR process. The double resonance effect enhances the signals of minor species much larger than the resonance effect in Raman spectroscopy. This study demonstrates the uniqueness of HR spectroscopy.

The HR spectrum of TFE at pH 13.2 resembles that of glucose. It is possible to consider the contribution of the CHOH group. The detailed analysis of the origin of the unique HR bands of sugar is now underway.

[1] S.Maity, H.Hiramatsu, J. Raman. Spectrosc. 2022, 53, 1845-1847.
[2] K.-C.Chien, S.Maity, H.Hiramatsu, Phys. Chem. Chem. Phys. 2023, 25, 27949-27952.