Effects of bioconjugation on the structures and electronic spectra of CdSe: density functional theory study of CdSe-adenine complexes.

We present density functional theory (DFT) and time-dependent DFT (TD-DFT) study of the structures and electronic spectra of small CdSe nanocluster-adenine complexes Cd(n)Se(n)-adenine (n = 3, 6, 10, 13). We examine the changes in the geometries and excitation spectra of the nanoclusters induced by DNA base-binding. By comparing the results calculated for the bare (Cd(n)Se(n)), hydrogen-passivated (Cd(n)Se(n)H(2n)), as well as the corresponding adenine (Ade)-bound clusters (Cd(n)Se(n)-Ade, Cd(n)Se(n)H(2n)-Ade, Cd(n)Se(n)H(2n-2)-Ade), we find that binding with Ade slightly blue-shifts (up to 0.18 eV) the electronic excitations of bare nanoclusters but strongly red-shifts (<1.2 eV) those of hydrogen-passivated nanoclusters. Natural bond orbital analysis shows that the LUMO of Cd(n)Se(n)H(2n)-Ade is a pi* orbital located on the purine ring.