Photophysical and structural features of covalently bound peptide-protoporphyrin-peptide compounds carrying naphthalene chromophores

The photophysics of a series of covalently bound peptide-protoporphyrin-peptide compounds carrying naphthalene (N) were investigated in methanol solution by steady-state and time-resolved fluorescence experiments, and by transient absorption spectra as well. The general formula of the series is P(xN)(2), where P refers to protoporphyrin IX, and x to the number of amino acids in the sequence Boc-Leu-Leu-Lys-(Ala)(n)-Leu-Leu-Lys-OtBu of each backbone chain (n = 0-3). Quenching of excited naphthalene takes place by electronic energy transfer from excited naphthalene to ground-state porphyrin and proceeds on a time scale of about 2 ns and 20-30 ns. IR spectra in methanol indicate that intramolecularly H-bonded conformations form, and CD data in both methanol and water-methanol mixtures suggest the presence of ct-helix structure. According to fluorescence decay data coupled with molecular mechanics calculations, two conformers for each dimeric peptide are the major contributors to the observed phenomena. These conformers are characterized by a globular, protein-like structure, where the protoporphyrin resides in a central pocket, making it amenable to metalation for mimicking hemoprotein, while the two N groups are externally located. Of the four N linkages in the two conformers, three of them attain a very similar steric arrangement around the central P molecule, in terms of both center-to-center distance and mutual orientation, while the fourth experiences a different steric disposition as compared to the others. Where the theoretical structures and photophysical properties were correlated within the Forster-type mechanism, the kinetics of the energy transfer were well reproduced for all compounds investigated only when the mutual orientation of the chromophores was also taken into account. This implies that interconversion among conformational substates of probe linkages is slow on the time scale of the transfer process.