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 synt hesized 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-(Al a)(x)-Leu-Leu-Lys of each backbone chain (x = 0-3; n = x + 6). Their struct ural 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 o f alpha-helix structure. Quenching of excited naphthalene takes place by el ectronic energy transfer from singlet N* to P ground state. Fluorescence de cays coupled with molecular mechanics calculations indicate that two confor mers for each dimeric peptide are the major contributors to the observed ph enomena. These conformers are characterized by a globular, protein-like str ucture, where the protoporphyrin resides in a central packet, while the two N groups are externally situated. Of the four N linkages in the two confor mers, three of them attain a very similar steric arrangement around the cen tral P molecule, in terms of both center-to-center distance and mutual orie ntation, while the fourth experiences a different steric disposition as com pared to the others. Experimental photophysical parameters satisfactorily c ompare with those obtained by theoretical calculations, within the Forster mechanism for long-range energy transfer, only when the mutual orientation of the chromosomes was also taken into account. This implies that interconv ersion among conformational substates of probes linkages is slow on the tim e scale of the energy transfer process. (C) 2000 John Wiley & Sons, Inc.