The binding energies and excitation energies of dopamine interacting with TiO2 clusters of different sizes representing an anatase surface are calculated. Results indicate that bidentate adsorption of dopamine on undercoordinated sites containing a Ti=O double bond is more stable than monodentate or molecular adsorption on the (101) anatase surface. A large red shift in the absorption spectrum of TiO2-dopamine is calculated, in agreement with experimental studies. We also investigate how the optical properties of TiO2-dopamine are affected by adding carboxyl thymine and subsequent base pairing with adenine. (c) 2005 Published by Elsevier B.V.