Structural behavior of neutral, protonated, and deprotonated l-valine in aqueous solutions: a combined study using chirality sensitive (VCD) and non sensitive (IR and Raman) vibrational spectroscopies and quantum chemical calculations

The aim of this study is to better understand the conformational preference of l-valine [C5H11NO2, (2S)-2-amino-3-methylbutanoic acid] under three representative pH conditions of 6.00 (zwitterionic species), 1.00 (protonated species, cations), and 13.00 (deprotonated species, anions). Both non-chiroptical IR–Raman and chiroptical VCD vibrational techniques were used for the analysis of the l-valine under the pH conditions. Likewise, theoretical DFT (B3LYP and M06-2X) and ab initio MP2 studies were undertaken to consider the three possible forms (zwitterionic, cationic, or anionic) of this amino acid in aqueous solution, with a comparison of the solvent treatment using a continuum solvation model (PCM) and an hybrid solvation model (PCM plus explicit water molecules), that is, the zwitterionic l-Val surrounded by five explicit water molecules, the anionic l-Val by five and the cation l-Val by eight. While the recorded IR and Raman spectra were reasonably interpreted using only the implicit solvation model, the hybrid solvation model, which includes explicit water molecules, improved the reproduction of the complex IR–Raman–VCD features, especially in the carbonyl and amino vibration regions where the intermolecular amino acid–water hydrogen bond interactions occur. The present work reveals that IR, Raman, and VCD vibrational spectroscopies combined with quantum chemical calculations are helpful complementary techniques to characterize flexible systems, as amino acids, which present several conformers.

Structural behavior of l-valine at different pH.Figure optionsDownload as PowerPoint slide