Arginine interactions with anatase TiO2 (100) surface and the perturbation of 49Ti NMR chemical shifts – a DFT investigation: relevance to Renu-Seeram bio solar cell
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Density functional theoretical calculations have been utilized to investigate the interaction of the amino acid arginine with the (100) surface of anatase and the reproduction of experimentally measured 49Ti NMR chemical shifts of anatase. Significant binding of arginine through electrostatic interaction and hydrogen bonds of the arginine guanidinium protons to the TiO2 surface oxygen atoms is observed, allowing attachment of proteins to titania surfaces in the construction of bio-sensitized solar cells. GIAO-B3LYP/6-31G(d) NMR calculation of a three-layer model based on the experimental structure of this TiO2 modification gives an excellent reproduction of the experimental value (-927 ppm) within +/- 7 ppm, however, the change in relative chemical shifts, EFGs and CSA suggest that the effect of the electrostatic arginine binding might be too small for experimental detection.
KeywordsAmino acid binding DFT NMR simulation TiO2
Interaction of TiO2 with the mutants of bacteriorhodopsin (bR) is important in the design of efficient biosensitized solar cells (BSSC). Application of biologically-derived, inexpensive light harvesting protein sensitizers for sensitizing titanium dioxide (TiO2) nano particles and fibers, instead of the traditional, toxic and expensive chemical based dyes in dye-sensitized solar cells (DSSCs), is an attractive proposition.
Protein-sensitized solar cells emulate the design of DSSCs except that the dye molecules are replaced by light harvesting biomolecules as sensitizer. The impetus for this replacement comes from potentially high efficiency of conversion that light harvesting protein complexes promise, their ability to self-assemble on a surface , their low cost of production by biotechnology and room temperature processing and their long lifetimes even at higher temperature (100°C) . An interesting candidate for a protein sensitizer is bacteriorhodopsin (bR) which is a trans-membrane protein found in the archae Halobacterium salinarum . Absorption of photon in bR occurs principally through the chromophoric group and the charge separation that occurs eventually leads to shedding of electrons. Upon absorption of light, there is a shift of electron density from the β-ionone ring of the chromophore toward the Schiff base, which is comparable to a 2.6 Å displacement of a single electron down the polyene chain  and is in the opposite direction of the initial photovoltage spike that is observed in bR . This short duration charge shift is a critical component of the primary event because it sets in motion events that lead to the photoisomerization of the chromophore .
TiO2 has been in the focus of numerous investigations since the demonstration of the original DSSC photovoltaic cell . Researchers have proposed the possibility of using hybrid architecture of nanoparticles and nanofibres [8, 9, 10, 11, 12] to facilitate better electron transport. Given the advancement in electrospinning for generating large surface-to-volume ratio nanofibres in larger-scale , the electrospun TiO2 nanofibres have been shown to increase solar energy efficiency up to 6.2 % in DSSC .
The basic hypothesis for the design of such a BSSC, for which we propose the name Renu-Seeram cell is that bR is attached by chemisorption to TiO2, and upon photoexcitation ejects electrons into the conduction band of semiconducting metal oxide completing the circuit and electrons flow from counter electrode to the conductive ITO glass plate. An iodide electrolyte recharges bR.
In order to shed some light on this interaction, we set out to investigate computationally the binding of arginine, which has been identified as a potential terminal amino acid in bR, on the (100) (or identical by symmetry the (010)) surface of TiO2 anatase  in order to derive a model for the interaction of TiO2 to bR mutants.
All calculations were carried out using the Gaussian 03  suite of programs. Density functional theory (DFT) with the hybrid B3LYP functional [17, 18, 19] in combination with Pople’s 6-31G(d) [20, 21] was used for the optimizations of the ground state geometries of a TiO2 anatase model, arginine and a complex of arginine on anatase and the simulation of the chemical shifts. The default convergence criteria and integration grid of the program were used. The nature of all found local minimum structures was confirmed by the lack of an imaginary harmonic vibrational frequency . Absolute magnetic shieldings were determined with the GIAO (gauge-including atomic orbital) method [23, 24, 25] as implemented in Gaussian 03. The obtained shielding tensors were referenced against titanium tetrachloride to yield relative chemical shifts. This approach has been used successfully for the determination of chemical 49Ti shifts previously . The simulations of NMR tensor parameters were performed using the SIMPSON program 
Results and discussion
When a protein such as bR or its mutants is brought onto a surface such as anatase or ZnO, it will be either covalently attached or chemisorbed. It is therefore reasonable to expect subtle electronic perturbations of 49Ti chemical shift anisotropy (CSA or Δσ) and the quadrupole coupling, Cq, which provide useful tools to probe subtle interactions at the atomic level. It should be possible to compare the DFT derived changes in electric field gradient and CSA with experimental values .
The well-established B3LYP/6-31G(d) level of theory has been chosen because of its previously demonstrated superior performance in the 49Ti NMR determination . Herein, relative chemical shifts of the Ti atoms of this model are computed compared against the available experimental NMR data. In our model there are four titanium atoms which have a “complete” oxygen-saturated environment – in the middle layer of the side-on view of Fig. 1 – which can serve as representatives of the bulk titanium atoms. These Ti atoms are calculated to have isotropic chemical shifts (relative to TiCl4) between -921 and -922 ppm, in excellent agreement with the experimental value for anatase of -927 ppm . Shifts of those titanium atoms on the surface which have five oxygen contacts are in the range of -450 to -530 ppm.
Relative 13C chemical shifts (in ppm) of arginine before and after attachment to a TiO2 anatase surface
Arginine on anatase
The relative 49Ti chemical shifts of the four central Ti atoms with oxygen saturation (see above) are influenced by the arginine attachment by only a few ppm (between 1 and 9 ppm, values range from -913 to -928 ppm). This appears too small to be easily detectable in a standard NMR experiment. However, the surface titanium atoms are much more strongly influenced: the induced changes in relative chemical shifts range from -83 to +93 ppm.
It is clear from the data in Fig. 4 that the level of theory seems to overestimate both the electric field gradient (EFG) and the CSA. The 47Ti (nuclear spin 5/2) data is dominated by the quadrupolar coupling which is directly proportional to the EFG tensor and therefore shows the effects of this parameter, while in the 49Ti (nuclear spin 7/2) data it is evident that the CSA interaction dominates this lineshape at the simulated magnetic field. The key point is that the effects are similar for the models with and without the arginine present. The implication is that the electrostatics of the Ti atoms on the surface are not strongly impacted by the binding of the amino acid. Although it is possible with some effort to experimentally distinguish surface atoms from the bulk, measuring such a surface effect with NMR seems difficult, despite the above mentioned calculated changes in relative chemical shifts.
The nature of the attachment of arginine on the anatase (100) surface is clearly based on electrostatic interaction together with hydrogen bonds. It appears that an increased number of these weak interactions lead to stabilization of the complex between our (100) anatase model and the amino acid, indicating that the attachment of bR onto a TiO2 surface is possible in the Renu-Seeram Cell . Additionally, the small energetic changes between some of the found orientations suggest that one can expect dynamic behavior of arginine on the surface. The correct DFT reproduction of 49Ti NMR chemical shifts for anatase gives confidence in the reliability of the employed approach for the evaluation of this property. Detailed computational investigation of both 13C (arginine) and 49Ti (TiO2) NMR data show very little change in predicted relative chemical shifts upon binding. This is further confirmed by the simulation of the titanium spectra based on experimental and calculated data. It suggests that NMR investigation of the protein binding on titania nanoparticles seems unfeasible. Other techniques such as SERS on TiO2  might give additional insight into the attachment mode.
RK gratefully acknowledges generous allocation of computer time at the Center for Scientific Computing, Uni Oldenburg and financial support by the Deutsche Forschungsgemeinschaft DFG.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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