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Influence of Near-Infrared Radiation on the pKa Values of L-Phenylalanine

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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 pKa 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 pKa 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 pKa values.

Keywords: ACID-BASE EQUILIBRIUM; AGGREGATION; AMINO ACID; ATR-FTIR; ATTENUATED TOTAL REFLECTION-FOURIER TRANSFORM INFRARED SPECTROSCOPY; DEHYDRATION; HYDROGEN BOND; L-PHENYLALANINE; NEAR-INFRARED RADIATION; PH; PKA; PROTONATION

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/000370206777670701

Affiliations: 1: Department of Biomedical Engineering and Instrumentation, Wroclaw University of Technology, Smoluchowskiego 19, 50-370 Wroclaw, Poland 2: Institute of Physics, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370. Wroclaw, Poland 3: Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), UMR 8516 - CNRS, Bat. C5, Université des Sciences et Technologies de Lille, 59 655 Villeneuve d'Ascq, France

Publication date: June 1, 2006

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