Computational study of cysteine interaction with the rutile TiO2 (110) surface

The nature of interaction of the amino acid cysteine (HS–CH2CH(NH2)COOH), as a model multifunctional group amino acid, was studied theoretically on a rutile TiO2 (110) surface using the DFT method. A large number of adsorption modes were investigated through the three functional groups: the carboxyl (COOH), thiol (SH) and amino (NH2) groups. Two main stable forms of adsorption via the carboxyl group were found: bridging (O, O) (with adsorption energies, Ea between 1.1 and 1.5 eV) and (O, N) (Ea = 1.4 eV). The bridging (O, O) adsorption mode had zwitterionic forms (with the H atom of the SH transferred to the NH2 group making S− and NH3+) which were nearly isoenergetic with the dissociated form (Ea = 1.35 eV). The mono-dentate carboxyl adsorption mode had no stable zwitterionic forms. The thiol and amino groups of cysteine affect bonding by creating hydrogen bonds to the surface. These cause small changes of the adsorption energy (up to 0.4 eV); they also modify the ordering of the last six (lone pair) orbitals of cysteine which in turn affect its nature as a hole acceptor.