Water evaporation analysis of L-phenylalanine from initial aqueous solutions to powder state by vibrational spectroscopy

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-Rama 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 -CO2- groups, and the -NH3+ and -NH2 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 H2O molecules.