A vibrational and conformational characterization of arginine at different pH values investigated using Raman spectroscopy combined with DFT calculations

In the present study, the Raman spectra of an aqueous solution of arginine (Arg) at three different pH values (2.0, 9.0, and 10.5) were recorded in the spectral range 500–1800 cm −1 by using an excitation wavelength of 514 nm. The wavenumber position and intensity of the Raman bands lying in the spectral range 1000–1500 cm −1 changed with pH. The change in the peak parameters with pH was mainly attributed to the change of the molecular structure to zwitterionic (ZW), cationic zwitterionic (CATZW), or protonated (PRO) form. The presence of these conformers at a particular pH value is investigated by analyzing the experimental and theoretical Raman spectra. The structural, spectroscopic, and thermodynamical features of ZW, CATZW, and PRO conformers of Arg in the gas phase and the aqueous solution phase were investigated by using density functional theory (DFT) calculations at B3LYP/6‐31 + G(d,p) and M062X/6‐31 + G(d,p) levels of theory. The spectroscopic changes observed for the Arg–(water) 10 complex in the gas phase (explicit model) and in the bulk solvent water environment (implicit model) are explained in terms of the role that water molecules play in the stabilization or destabilization of the active sites of Arg. The structure of the most stable conformer of the ZW, CATZW, and PRO forms of Arg at the pH values of 2.0, 9.0, and 10.5 was determined by comparing the experimental and theoretical spectral features and performing a Boltzmann population analysis. The most stable conformer of Arg at a particular pH, as obtained by spectral analysis, is also well supported by the results obtained from a thermodynamical analysis. Copyright © 2016 John Wiley & Sons, Ltd.