Abstract
The effect of pH on L-phenylalanine (L-phe) before and after exposure to near-infrared (NIR) radiation (15 min, 700-2000 nm) was investigated by attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectroscopy. Characteristic bands of L-phe were described and the p<i>K<sub>a</sub></i> values were retrieved from IR spectra by using an intensity ratio method according to our recent paper (Olsztynska et al., Appl. Spectrosc. 55, 901 (2001)). It has been found that the irradiation process modifies p<i>K<sub>a</sub></i> values of L-phe. The spectroscopic study clearly shows the shift of acid–base equilibrium after exposure to NIR radiation. The phenomenon is due to modification of the water structure. Intra- and intermolecular hydrogen bonds weaken, which could induce conformational changes of the phe molecule. Subsequently, hydrophobic interactions strongly increase. These processes favor aggregation of phe molecules, which leads to deprotonation of the −NH<sub>3</sub><sup>+</sup> to −NH<sub>2</sub> group and protonation of the −COO<sup>−</sup> to −COOH group, changing the p<i>K<sub>a</sub></i> values.
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