Probing the Nature of Charge Transfer at Nano-Bio Interfaces: Peptides on Metal Oxide Nanoparticles Pilarisetty Tarakeshwar,* ,† Julio L. Palma, ‡ Gregory P. Holland, § Petra Fromme, † Jeffery L. Yarger, † and Vladimiro Mujica † † Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States ‡ Center for Biosensors and Bioelectronics, Biodesign Institute, Arizona State University, Tempe, Arizona 85287-5001, United States § Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1030, United States * S Supporting Information ABSTRACT: Characterizing the nano-bio interface has been a long-standing endeavor in the quest for novel biosensors, biophotovoltaics, and biocompatible electronic devices. In this context, the present computational work on the interaction of two peptides, A6K (Ac-AAAAAAK-NH 2 ) and A7 (Ac-AAAAAAA-NH 2 ) with semiconducting TiO 2 nanoparticles is an effort to understand the peptide-metal oxide nanointerface. These investigations were spurred by recent experimental observations that nanostructured semiconducting metal oxides templated with A6K peptides not only stabilize large proteins like photosystem-I (PS-I) but also exhibit enhanced charge-transfer character- istics. Our results indicate that α-helical structures of A6K are not only energetically more stabilized on TiO 2 nanoparticles, but the resulting hybrids also exhibit enhanced electron transfer characteristics. This enhancement can be attributed to substantial changes in the electronic characteristics at the peptide-TiO 2 interface. Apart from understanding the mechanism of electron transfer (ET) in peptide-stabilized PS-I on metal oxide nanoparticles, the current work also has implications in the development of novel solar cells and photocatalysts. SECTION: Physical Processes in Nanomaterials and Nanostructures B iological systems provide useful cues in directing the assembly of nanoscale components into controlled and advanced structures. Most of the early efforts were predom- inantly based on using the exquisite recognition capabilities of DNA and RNA in the design and development of novel self- assembled structures. 1-6 However, recent studies have shown that polypeptides could be employed as templates in directing the assembly of π-conjugated oligomers. 5 Interestingly, the characteristics of these biologically templated oligomers were found to depend on the size, geometry, and the electronic properties of the biological template. 5 It has been shown in previous photophysical studies of photoinduced electron transfer that chromophores placed at different locations of a α-helical peptide could be used to investigate distance-dependent excitonic coupling. 5,7-9 These couplings can also be investigated by employing different sizes of α-helical peptides as bridges between the chromophores. 5,7-9 The hierarchical self-assembly of α-helical peptides also leads to the formation of 3D organogels with high dielectric constant. The unique properties of these peptide-based organogels has recently been harnessed in the development of novel organic bulk-heterojunction photovoltaic devices. 5 In this context, a recent study of dry photosystem-I (PS-I) stabilized by surfactant peptides and self-assembled on nanostructured semiconductors is interesting because a small α-helical peptide was found to aid the functioning of PS-I both as a light-harvester and charge separator in solar cells. 10 Although the exact role of the peptide in enhancing the photocurrent of PS-I adsorbed on nanostructured ZnO or TiO 2 is not known, it is intriguing that a small cationic peptide surfactant Ac-AAAAAAK-NH 2 (A6K) consisting of six alanines and a lysine at the amidated C-terminus can play such a vital role in both stabilizing dry PS-I on nanostructured semi- conductors and enhancing its photocurrent. 10,11 Since there is a parallel between our recent work in understanding the role of semiconducting oxide nanoparticles in enhancing the Raman activities of molecules adsorbed on them 12-15 and the characteristics of peptide-metal oxide nanointerfaces, 16-19 we thought it would be interesting to investigate them. In our work on surface enhanced Raman scattering (SERS) on semiconducting nanoparticles, we have shown that the enhancement of Raman activities arises from a large increase in polarizability due to charge transfer from the molecule to the semiconducting nanoparticle. 14,15 Furthermore, Received: September 2, 2014 Accepted: October 3, 2014 Published: October 3, 2014 Letter pubs.acs.org/JPCL © 2014 American Chemical Society 3555 dx.doi.org/10.1021/jz501854x | J. Phys. Chem. Lett. 2014, 5, 3555-3559