Exciton Annihilation and Energy Transfer in Self-Assembled Peptide-Porphyrin Complexes Depends on Peptide Secondary Structure Darius Kuciauskas, §, * ,† Juris Kiskis, ‡ Gregory A. Caputo,* ,† and Vidmantas Gulbinas* ,‡ Department of Chemistry and Biochemistry, Rowan UniVersity, 201 Mullica Hill Road, Glassboro, New Jersey 08108, United States and Institute of Physics, Centre for Physical and Technological Sciences, SaVanoriu 238, Vilnius, Lithuania ReceiVed: September 12, 2010; ReVised Manuscript ReceiVed: October 19, 2010 We used picosecond transient absorption and fluorescence lifetime spectroscopy to study singlet exciton annihilation and depolarization in self-assembled aggregates of meso-tetra(4-sulfonatophenyl)porphine (TPPS 4 ) and a synthetic 22-residue polypeptide. The polypeptide was designed and previously shown to bind three TPPS 4 monomers via electrostatic interactions between the sulfonate groups and cationic lysine residues. Additionally, the peptide induces formation of TPPS 4 J-aggregates in acidic solutions when the peptide secondary structure is disordered. In neutral solutions, the peptide adopts an R-helical secondary structure that can bind TPPS 4 with high affinity but J-aggregate formation is inhibited. Detailed analysis of excitation- power dependent transient absorption kinetics was used to obtain rate constants describing the energy transfer between TPPS 4 molecules in an aggregate under acidic and neutral conditions. Independently, such analysis was confirmed by picosecond fluorescence emission depolarization measurements. We find that energy transfer between TPPS 4 monomers in a peptide-TPPS 4 complex is more than 30 times faster in acidic aqueous solution than in neutral solutions (9 vs 279 ps). This result was attributed to a conformational change of the peptide backbone from disordered at low pH to R-helical at neutral pH and suggests a new approach to control intermolecular energy transfer with possible applications in fluorescent sensors or biomimetic light harvesting antennas. Introduction The absorption of solar energy and conversion into usable chemical energy is a common theme found in plants and photosynthetic bacteria. This absorption and conversion employs a variety of specialized, membrane-bound components that serve as absorbing antennas and reaction centers. 1 The light-harvesting antennas are evolutionarily optimized for efficient absorption of solar radiation as well as excited state energy transfer to photochemical reaction centers. Such antennas consist of several to several hundred light absorbing chromophores arranged in well-defined structures. Excitonic interactions between the light- absorbing chromophores lead to red shifts in the absorption spectra and efficient energy transfer between the antenna components. 1 Mimicry of photosynthetic antenna structures is potentially useful for third-generation photovoltaics and can potentially aid in understanding these complex biological systems. 2 Numerous peptides and proteins designed to bind porphyrins have been successfully synthesized and characterized. The peptide-porphyrin binding in these proteins is mediated either through amino acid side chains chelating a metal ion at the center of the porphyrin or through amino acid interactions with the porphyrin side chains. The metal-chelating constructs typically are designed such that specific amino acid side chains favorably interact with the metal ion, similar to the histidine- based chelation of iron in hemoglobin. 3-10 Many of these constructs are based on the biologically relevant four-helix bundle motif, which allows positioning of the porphyrin at the helical interface. The alternative design is one in which the porphyrin is bound through side-chain interactions. These designs use smaller, synthetic peptides that lack significant tertiary and quartenary structure, unlike the 4-helix bundles. 11-13 Recently we designed and characterized self-assembled light- harvesting antennas consisting of a 22-residue peptide and three anionic meso-tetra(4-sulfonatophenyl)porphine (TPPS 4 ) mono- mers (Scheme 1). 15 The cationic lysine side chains were used to bind sulfonate groups of TPPS 4 via electrostatic interactions. Using absorption, fluorescence, resonant light scattering (RLS), and circular dichroism (CD) spectroscopy, we have shown that one peptide with nine lysine residues binds three porphyrin monomers (i.e., 3 lysine residues per TPPS 4 molecule). In acidic solutions (pH 1.8 and 3.6) where the peptide has disordered secondary structure, peptide-bound porphyrins were shown to form J-aggregates. In neutral solutions (pH 7.6), porphyrin binding to the peptide induces R-helical secondary structure for a portion of the peptide. However, TPPS 4 J-aggregates are not formed at neutral pH. The interplay between peptide secondary structure and the ability of TPPS 4 (or other porphyrins) to form excitonically coupled J-aggregates could be useful for the rational design of new peptide-porphyrin based antennas. Only dynamic studies with sufficient (fs/ps) time resolution could directly examine energy transfer in such self-assembled aggregates. This system poses an additional complication in that all porphyrins are assumed to be equivalent. Therefore, monitor- ing time-dependent spectral changes to reveal energy transfer pathways is not possible. In light of this, we used ultrafast nonlinear spectroscopy based on singlet exciton annihilation. * To whom correspondence should be addressed. E-mail: (D.K.) Darius.Kuciauskas@nrel.gov; (G.A.C.) caputo@rowan.edu; (V.G.) vidgulb@ktl.mii.lt. † Rowan University. ‡ Centre for Physical and Technological Sciences. § Current address: National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO 80401. J. Phys. Chem. B 2010, 114, 16029–16035 16029 10.1021/jp108685n  2010 American Chemical Society Published on Web 11/11/2010