Conformational changes of L-phenylalanine induced by near infrared radiation. ATR-FTIR studies

In our previous work the influence of water evaporation on Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectra of l-phenylalanine (l-phe) in a function of pH (Olsztynska et al. Appl. Spectrosc. 60(9):1040, (14)) was studied. The presence of symmetric dimers of hydrogen-bonded amino acid was observed when simultaneously CO2 − ionised and COOH unionised forms of the amino acid appear in the solution (near pK 1). It is suggested that Near Infrared (NIR) radiation may induce partial protonation of CO2 − groups at a neutral pH and formation the same type of dimers. The aim of this work was to study this hypothesis. Therefore, ATR-FTIR spectra of l-phe aqueous solution before and after NIR radiation (15 min., 700–2,000 nm) were obtained as a continuation of our earlier studies. Spectral characteristic bands of l-phe were described. The vibrational spectroscopic study of l-phe showed that it undergoes photochemical reactions under NIR exposure. It has been found that the irradiation process indeed induces a protonation of polar groups of l-phe at neutral pH what leads to forming of neutral forms and as a consequence hydrogen bonded dimers –C=O···HOOC–. Moreover, hydrophobic interactions strongly increase, what favours aggregation of l-phe molecules. The phenomenon is probably due to modifications of water structure around l-phe molecules. Intra- and intermolecular hydrogen bonds weaken which could favour aggregation and protonation of polar groups what induces also formation of symmetrical hydrogen bonds between protonated and deprotonated carboxylic groups.