Adsorption of Enantiomeric and Racemic Cysteine on a Silver Electrode − SERS Sensitivity to Chirality of Adsorbed Molecules

Abstract

The adsorption of both (L-and D-) enantiomeric forms of cysteine on the silver electrode surface was studied by surface-enhanced Raman scattering spectroscopy (SERS) as a function of electrode potential and pH value of the solution. It was demonstrated that at potentials more positive than -0.7 V (for pH 3) or -0.8 V (for pH 2 or lower), in acidic environment L-cysteine molecules are adsorbed mainly as P H (gauche) conformer, in zwitterionic form with the COO -groups close to the surface. At more negative potentials, NH 3 + groups deprotonate at the surface with simultaneous weakening of the interaction of the carboxylic groups with the surface. Spectroscopic evidence for at least partial protonation of the COO -groups at strongly acidic solutions was given by observing the CdO stretching band at frequency lowered by about 30 cm -1 in comparison with that observed for crystalline cysteine hydrochloride. It points to the considerable enhancement of the strength of hydrogen bonds and may be ascribed to the formation of cyclic L-cysteine dimers at the electrode surface. In neutral and alkaline solutions, adsorbed L-cysteine molecules have deprotonated amino groups at wide potential range. Similar spectroelectrochemical experiments were performed for D-cysteine and for a racemic mixture of D,L-cysteine. As expected, results for D-cysteine were similar to those for L-cysteine. However, for racemic mixture at acidic pH, the spectral effects corresponding to potential-induced transition from adsorbed zwitterions to neutral molecule were considerably smaller. This effect was discussed in terms of stereoselective dimerization of cysteine molecule at the electrode surface.

Adsorption of seleno-l-cystine on silver electrode surfaces investigated by surface-enhanced Raman scattering

“…Not only the presence of the m s (COO À ) band but also the relative intensity changes of all the modes in the SERS spectra from the normal spectrum indicated that selenocysteine should interact with the Ag surfaces. This result may be in line with the recent reports that the carboxylate moiety may coordinate with the metal surfaces [12][13][14][15].…”

“…On an Au electrode, cysteine was reported to be adsorbed via its carboxylate moiety [13]. Moreover, an irreversible disulfide bond formation was observed above 0.2 V. Reorientations of cysteine with the applied potential have been reported on silver electrode surfaces by means of SERS [14,15]. At more positive potentials, cysteine molecules are adsorbed with the positively charged protonated amino group that is stabilized by the co-adsorbed chloride ion.…”

Protonation and deprotonation of cysteine and cystine monolayers probed by impedance spectroscopy

“…Moreover, a potential-induced re-orientation of adsorbed cys has been observed by in situ spectroelectrochemical measurements using a silver electrode [36,37]. The spectroelectrochemical data indicate that, in the range between À0.3 and À0.6 V (vs. SCE), cys in 0.1 M KCl adsorbs on silver by the sulphur group with the protonated amine moiety pointing towards the surface.…”

Effect of concentration and pH on the surface-enhanced Raman scattering of captopril on nano-colloidal silver surface

“…1. The assignment of Raman bands of the captopril was done by the assistance of the published studies on selected organic structures [20][21][22][23][24][25][26] and the PED estimated from the DFT calculations. The observed Raman and SERS bands, calculated values, assignments and PED (%) are given in Table 1.…”