Amino Acid Adhesion on TiO2 Surface DFT Model

Francesco BuonocoreCaterina Arcangeli, Massimo Celino, Ivo Borriello

ENEA - C.R. Casaccia and NAST CentreComputational Material Science and technology (CMAST) Laboratorywww.afs.enea.it/project/cmast

”Biosystems, Energy, and Cultural Heritage:Materials Enhancement for technological application”

July 3rd 2013 - 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/TiO2 surface interaction by means of DFT methods

Why density functional theory?

We focus on the following amino acids

-NH-C-(NH2)2-CH2-COO

Terminal groups of side-chains interacting with TiO2 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

TiO2 (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 TiO2 (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)

1 2 3 4 5

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

configuration

Bin

din

g E

ne

rgy (

eV

)Free ARG

Free ARG = 10 Å far away from TiO2 surface

ASPARTIC ACID on TiO2 (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)

1 2 3 4 5 6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6B

ind

ing

En

erg

y (

eV

)

configuration

Free ASP

Free ASP = 10 Å far away from TiO2 surface

ARGININE on TiO2 (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 TiO2

Electron charge moves on N and H-O bond

ASPARTIC ACID on TiO2 (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 TiO2

Electron charge is not removed from TiO2

• 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