Exciton Annihilation and Energy Transfer in Self-Assembled Peptide-Porphyrin Complexes Depends on Peptide Secondary Structure

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 (TPPS4) and a synthetic 22-residue polypeptide The polypeptide was designed and previously shown to bind three TPPS4 monomers via electrostatic interactions between the sulfonate groups and canonic lysine residues Additionally the peptide induces formation of TPPS4 J-aggregates in acidic solutions when the peptide secondary structure is disordered In neutral solutions, the peptide adopts an alpha-helical secondary structure that can bind TPPS4 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 TPPS4 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 TPPS4 monomers in a peptide TPPS4 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 alpha-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