Conformational Statistics and Energetics Analysis of Sequential Peptides Undergoing Intramolecular Transfer of Excitation Energy

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The photophysics of short linear peptides of general formula Boc−Leu−Leu−Lys(P)−(AA)n−Leu−Leu−Lys(N)−OtBu, where AA = Ala or Aib (α-aminoisobutyric acid), and P and N are protoporphyrin IX and naphthalene, respectively, covalently bound to ε-amino groups of lysine side-chains, were investigated in water/methanol 75/25 (v/v) solution by steady-state and time-resolved fluorescence experiments. Quenching of the excited naphthyl chromophore takes place by electronic energy transfer to the porphyrin ground state, and proceeds on a time scale of 3−8 ns. A minor and slower fluorescence lifetime measures the decay of the exciplexes. Quenching efficiencies exhibit a different trend, depending on whether AA = Ala or Aib, indicating differences in the structural features of the two series of peptides. Consistently, CD spectra suggest that the former compounds populate α-helical structures, while the latter ones possibly attain a 310-helix conformation, in agreement with the proven ability of Aib to form 310-helices in solution. The increased percentage of intramolecular H-bonds in the P(Aib)nN as compared to the corresponding P(Ala)nN peptides, as determined by IR spectra in dilute CD3OD or CDCl3 solution, confirms this conclusion. The fluorescence results were satisfactorily described by a dipole−dipole interaction mechanism, provided the mutual orientations of N and P groups are taken into account, which implies that interconversion among conformational substates of chromophore linkage is slow on the time scale of the transfer process. Conformational statistics analysis shows a rather wide interprobe separation distance distribution for each peptide, owing to the aliphatic portion of the side-chains carrying the chromophores, but theoretical conformational analysis indicates that only a few energetically favored conformers are the major contributors to the energy transfer process.