Peptide-sandwiched protoporphyrin compounds mimicking hemoprotein structures in solution

A series of covalently bound peptide-protoporphyrin-peptide compounds, also carrying naphthalene (N) to allow a photophysical investigation, were synthesized. Their general formula is P(nN)(2), where P refers to protoporphyrin IX, and n to the number of amino acids in the sequence Boc-Leu-Leu-Lys-(Ala)(x) -Leu-Leu-Lys-OtBu of each backbone chain (x = 0-3; n = x + 6). Their structural features in methanol solution were investigated by ir and CD spectra, and by steady-state and time resolved fluorescence experiments as well. The ir spectra indicate that intramolecularly H-bonded conformations form, and CD data in both methanol and water-methanol mixture suggest the presence of alpha-helix structure. Quenching of excited naphthalene takes place by electronic energy transfer from singlet N* to P ground state. Fluorescence decays coupled with molecular mechanics calculations indicate that 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, while the two N groups are externally situated. 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. Experimental photophysical parameters satisfactorily compare with those obtained by theoretical calculations, within the Förster mechanism for long-range energy transfer, only when the mutual orientation of the chromophores was also taken into account. This implies that interconversion among conformational substates of probes linkages is slow on the time scale of the energy transfer process.