A Raman, Sers and UV-Circular dichroism spectroscopic study of N-acetyl-L-Cisteine in aqueous solutions

Abstract

The aim of this work is to evaluate the vibrational and structural properties of N-acetyl-L-cysteine (NAC),and its molecular structure and electronic properties in relation to the action of thiol and amine groupsat different pH. Raman and Surface Enhanced Raman Spectroscopy (SERS) spectra were measured inaqueous solution. The influence of an aqueous environment on the NAC spectra was simulated bymeans of an implicit (polarizable continuum model) method. SERS spectra indicate that the S atom isinteracting with the surface through the sulfur atom. One of the consequences of the interaction withthe surface is the deprotonation of the SH group, as revealed by the disappearance of the n(S?H) band.The calculations performed for the Ag?NAC complex confirm the experimental data obtained by SERS,where the S?Ag interaction is the most important. These results are very interesting when one canformulate the drug feasibility of NAC using silver nanoparticles as a carrier. Raman spectra were measuredto compare the behavior of different functional groups in the molecule, both in the solid phase and inaqueous solution at different pH. Apparent ionization constants (pK0 values) for the S?H group at highionic strengths were calculated from the intensity of the 2580 cm1 frequency as a function of pH. UVand circular dichroism spectra were also measured in aqueous solution at different pH. Finally, the studywas completed with natural bond orbital (NBO) analysis to determine the presence of hyper-conjugativeinteractions. It is important to observe the behavior of the C2?N bond with the delocalization effect; asthe pH increases the hyperconjugative interaction of this bond decreases in the same way as in the caseof nCN. The way in which the LP pO1 - s*C2?N interaction and nCN decrease is an inverse reflectionof the fractional ionization aSH.