Adsorption of enantiomeric and racemic cysteine on a silver electrode--SERS sensitivity to chirality of adsorbed molecules.
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, NH3+ 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 C=O 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.
Graff
Marek
M
Department of Chemistry, University of Warsaw, Pasteur Street 1, 02 093 Warsaw, Poland.
Bukowska
Jolanta
J
D000042:Absorption; D003460:Crystallization; D003545:Cysteine / Q000737:chemistry*; D017666:Deuterium Oxide / Q000737:chemistry; D004566:Electrodes; D006863:Hydrogen-Ion Concentration; D012834:Silver / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013237:Stereoisomerism
D012834:Silver; D017666:Deuterium Oxide; D003545:Cysteine