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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 pK_a 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 pK_a 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 –NH3⁺ to –NH₂ group and protonation of the –COO⁻ to –COOH group, changing the pK_a values.

Keywords

Attenuated total reflection–Fourier transform infrared spectroscopy ATR-FTIR Near-infrared radiation L-phenylalanine Amino acid pH Acid–base equilibrium Dehydration Hydrogen bond Protonation Aggregation

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Influence of Near-Infrared Radiation on the p K a Values of L-Phenylalanine

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