Quenching mechanisms in bichromophoric, 3(10)-helical Aib-based peptides, modulated by chain-length-dependent topologies

The photophysical and structural features of a series of lineal, Aib-based peptides were investigated in methanol solution. These compounds have the g eneral formula P(Aib)(n)N, where Aib is alpha-aminoisobutyric acid, N napht halene, and P the monomethylated protoporphyrin IX, the two latter molecule s being covalently attached to the peptide N- and C-termini, respectively, while n = 3, 6, 9, 12, and 15. According to IR and H-1 NMR, in all cases th e backbone chain populates a 3(10)-helical structure. Both steady-state and time-resolved fluorescence measurements show a strong quenching of the N e mission, whose efficiency depends on the chain length. A corresponding incr ease of the P fluorescence intensity was also observed, suggesting the occu rrence of long-range energy transfer from singlet N* to P, though the N emi ssion quenching parallels the enhancement of P fluorescence intensity in th e 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-tran sfer process from P ground-state to N-1*, is likely to occur. Transient abs orption spectra of P(Aib)(6)N and P(Aib)(15)N in aereated methanol solution show marked differences between the two peptides, suggesting different dee xcitation pathways. Molecular mechanics calculations show differences in th e topology as the chain length of the peptides increases, which are thought to be primarily responsible for the singlet energy transfer vs: electron-t ransfer competition.