Structures and electronic spectra of CdSe-Cys complexes: density functional theory study of a simple peptide-coated nanocluster.

We present density functional theory (DFT) structures and time-dependent DFT electronic excitation energies of several small CdSe nanoclusters with the composition Cd(n)Se(n) (n = 3, 6, 10, 13). We examine the effects on the geometries and excitation spectra of the nanoclusters induced by two chemical changes: peptide-binding and ligand passivation. We use cysteine (Cys) and cysteine-cysteine dipeptide (Cys-Cys) as model peptides and hydrogen atoms as surface-bound solvent ligands (or stabilizing agents). By comparing the results calculated for bare, hydrogen-passivated (Cd(n)Se(n)H(2n)), as well as the corresponding Cys- and Cys-Cys- bound clusters (Cd(n)Se(n)-, Cd(n)Se(n)H(2n), -Cys, -Cys-Cys), we find that peptide-binding blue shifts the electronic excitations of bare nanoclusters, but red shifts those of hydrogen-passivated nanoclusters. The carboxyl oxygen and the sulfur atom tend to form a four-centered ring with adjacent two Cd atoms when the CdSe cluster forms covalent bonds with Cys or Cys-Cys. Further, this type of bonds may be distinguishable by significant red shifts of the excitation energies.