Quenching mechanism in bichromophoric 310-helical AIB-based peptides, modulated by chain length-dependent topologies

The photophysical and structural features of a series of linear, Aib-based peptides were investigated in methanol solution. These compounds have the general formula P(Aib)nN, where Aib is α-aminoisobutyric acid, N naphthalene, and P the monomethylated protoporphyrin IX, the two latter molecules being covalently attached to the peptide N- and C-termini, respectively, while n = 3, 6, 9, 12, and 15. According to IR and 1H NMR, in all cases the backbone chain populates a 310-helical structure. Both steady-state and time-resolved fluorescence measurements show a strong quenching of the N emission, whose efficiency depends on the chain length. A corresponding increase of the P fluorescence intensity was also observed, suggesting the occurrence of long-range energy transfer from singlet N* to P, though the N emission quenching parallels the enhancement of P fluorescence intensity in the short compounds only, i.e., for n = 3, 6, and 9. In the longer peptides (n = 12 and 15) a competitive quenching mechanism, possibly an electron-transfer process from P ground-state to 1N*, is likely to occur. Transient absorption spectra of P(Aib)6N and P(Aib)15N in aereated methanol solution show marked differences between the two peptides, suggesting different deexcitation pathways. Molecular mechanics calculations show differences in the topology as the chain length of the peptides increases, which are thought to be primarily responsible for the singlet energy transfer vs electron-transfer competition.