A study of aliphatic amino acids using simulated vibrational circular dichroism and Raman optical activity spectra

TL;DR

This study uses density functional theory to simulate vibrational optical activity spectra of aliphatic amino acids, validating models with experimental data and revealing spectral signatures of side chains and functional groups in different phases.

Contribution

It provides validated DFT models for simulating VOA spectra of aliphatic amino acids in gas and solution phases, highlighting spectral features related to side chains and functional groups.

Findings

IR region signatures of alkyl side chains

Raman region reveals functional group information

Chiral carbons dominate spectra in gas phase, methyl groups in solution

Abstract

Vibrational optical activity (VOA) spectra, such as vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectra, of aliphatic amino acids are simulated using density functional theory (DFT) methods in both gas phase (neutral form) and solution (zwitterionic form), together with their respective infrared (IR) and Raman spectra of the amino acids. The DFT models, which are validated by excellent agreements with the available experimental Raman and ROA spectra of alanine in solution, are employed to study other aliphatic amino acids. The inferred (IR) intensive region (below 2000 cm-1) reveals the signature of alkyl side chains, whereas the Raman intensive region (above 3000 cm-1) contains the information of the functional groups in the amino acids. Furthermore, the chiral carbons of the amino acids (except for glycine) dominate the VCD and ROA spectra in the gas phase, but the methyl group vibrations produce stronger VCD and ROA signals in solution. The C-H related asymmetric vibrations dominate the VOA spectra (i.e., VCD and ROA) > 3000 cm-1 reflecting the side chain structures of the amino acids. Finally the carboxyl and the C(2)H modes of aliphatic amino acids, together with the side chain vibrations, are very active in the VCD/IR and ROA/Raman spectra, which makes such the vibrational spectroscopic methods a very attractive means to study biomolecules.