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Light-induced effects in glycine aqueous solution studied by Fourier transform infrared-emission spectroscopy and ultraviolet-visible spectroscopy

Abstract Although amino acids are insensitive to visible light, as is generally accepted, we show that particular light-matter interaction can break this obviousness. Using sensitive (FT-IR)-technique in a combination of a broadband visible light source, we registered emission spectra of glycine in the range 2500-500 cm−1. Sensitivity of the infrared emission spectrum to the exciting power –induced changes in the glycine structure was demonstrated experimentally. Vibrational spectra of glycine displayed the prominent spectral features of CH2, COO-, COOH, NH+3 groups in the “fingerprint region”. Simultaneous appearance of ionised COO- and unionised COOH forms of glycine in solution at neutral pH clearly indicated that visible light induces the partial protonation of COO- groups; if so, visible light irradiation should lead to occurrence of dimers or dimeric hydrogen - bonded structures. Spectroscopic and microscopic evidence of visible light-mediated formation of aggregates and nucleus in aqueous solution was presented. Electronic absorption/emission spectra of glycine in aqueous solution were primarily characterized in the near ultraviolet-visible region (240-600 nm). Negligible absorption near 270 nm was observed for a 1.0 M solution and dramatically enhanced with its “aging”. Moreover, an extension of the absorption edge into the region above 400 nm could be seen. Due to the visible light irradiation, we observed modification of electronic structure or occurrence of additional species causing changes in absorption of glycine amino acid. For “aged” solution, it was shown that excitation spectra corresponding to the different emission wavelengths were entirely different, at that each excitation-spectral band had a characteristic emission band. Communicated by Ramaswamy H. Sarma