Amino Acid Adhesion on TiO2 Surface DFT Model Francesco Buonocore Caterina Arcangeli, Massimo Celino, Ivo Borriello ENEA - C.R. Casaccia and NAST Centre Computational Material Science and technology (CMAST) Laboratory ”Biosystems, Energy, and Cultural Heritage: Materials Enhancement for technological application” July 3rd Università di Roma Tor Vergata

Density functional theory (DFT) let us model the following interactions that molecular dynamics cannot describe: Covalent bond formation Electron charge distribution Therefore we give a close look to the region of the amino acid/TiO 2 surface interaction by means of DFT methods Why density functional theory?

We focus on the following amino acids -NH-C-(NH 2 ) 2 -CH 2 -COO Terminal groups of side-chains interacting with TiO 2 surface R D

Bulk O are bonded to 3 Ti atoms -> O(3c) Bulk Ti are bonded to 6 O atoms -> Ti(6c) Low coordinated Ti(5c) and O(2c) on surface layer: they represent the most reactive points of the surface TiO 2 (101) anatase reconstructed surface Anatase is the most probable phase of TiO2 oxide surface formation The 101 orientation represents the most stable recontruction in TiO2 anatase phase TiO2 (101) anatase recontruction Ti(6c)O(2c)Ti(5c)O(3c)

ARGININE on TiO 2 (101) anatase: bound configurations 5) O(2c) & O(2c) O(2c)-H bonds lenght lying in 1.7 – 2.2 Å 2) O(2c) 3) O(2c) & O(3c) 4) 2 x O(2c) Free ARG Free ARG = 10 Å far away from TiO 2 surface

ASPARTIC ACID on TiO 2 (101) anatase : bound configurations 6) 2 x Ti(5c) COOH towards O(3c) Ti-O bonds lenght lying in 1.9 – 2.2 Å 2) Ti(5c) 4) 2 x Ti(6c) 5) 2 x Ti(5c) COOH towards O(2c) Free ASP Free ASP = 10 Å far away from TiO 2 surface

ARGININE on TiO 2 (101) anatase: charge density analysis Negative charge density difference charge depletion charge density Positive charge density difference charge accumulation Electron charge is removed from H and TiO 2 Electron charge moves on N and H-O bond

ASPARTIC ACID on TiO 2 (101) anatase: charge density analysis Negative charge density difference charge depletion charge density Positive charge density difference charge accumulation Electron charge moves on O-Ti bond and TiO 2 Electron charge is not removed from TiO 2

The mediation of a water layer can be included in the DFT models The role of van der Waals interactions can also be investigated How to include environmental water effects? Challenges

The adhesion of the amino acid side-chains to TiO2 substrate has been modeled for arginine and aspartic acid. The most stable configurations and their binding energies have been calculated. Arginine (positive ion) adhesion does involve a charge transfer from TiO2 to amino acid. In aspartic acid (negative ion) this effect is reversed. DFT models provide information complementary to that provided by MD Conclusions