Figure 3.

Room-temperature Raman spectra obtained from the aqueous samples of free AAs together with their lowest energy zwitterionic species. Raman spectra of (A) Ser, (B) Phe, (C) Glu^1– (Glu with a deprotonated side chain), and (D) Glu⁰ (Glu with a protonated side chain). The characteristic Raman bands sensitive to the Glu side-chain protonaton/deprotonation are marked by pink arrows in (C) and (D). Lowest energy conformers of (E) Ser, (F) Phe, (G) Glu^1–, and (H) Glu⁰. Side-chain torsion angles values and global orientations are reported at the right side of the graphical representations (E–H). See text for details. Theoretical level: DFT/B3LYP/6-311++G(d,p).