Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectra of l-phenylalanine in aqueous solution were recorded in the pH range 0–13. The vibrational molecular spectroscopic study of this amino-acid, a well-known component of human tissue, showed that l-phenylalanine undergoes structural changes under the influence of pH. Characteristic bands of each ionic species were highlighted and the pKa values were determined from IR measurements by using an intensity ratio method. ATR-FTIR spectroscopy appears to be a powerful tool for obtaining valuable information on the building blocks of proteins and for measuring tissue damage in aqueous environments.

The water evaporation from L-phenylalanine (L-phe) aqueous solutions at different initial pH (0-13) was studied by vibrational spectroscopy. Next, the attenuated total reflection-Fourier transform infrared (ATR-FT-IR) spectra of aqueous solutions were compared to those recorded after drying for 72 h at 21 degrees C at appropriate initial pH values. Micro-Raman results collected after the water evaporation process are also presented and interpreted. Between pH = 2.5 and 8.76 a white non-transparent gel was observed, possibly due to the presence of the NaCl salt. The significant differences of the band intensities of L-phe functional groups noticed at pH near pK(a) values indicate the structural changes of L-phe molecules due to dimer formation (hydrogen bonds between the -COOH and -CO(2)(-) groups, and the -NH(3)(+) and -NH(2) groups). The presence of the hydrophobic interactions leads to the aggregation of L-phe molecules, most probably via phe-phe stacking as well as complexes of phe with Na(+) ions, HCl, or H(2)O molecules.