Efficient singlet-singlet (SSET) and triplet-triplet (TTET) energy transfer between fluorenyl and either phenylbenzoyl or naphthyl chromophores in enantiomerically pure dipeptides occur via through-space intramolecular dipole-induced dipole (FÃƒÂ¶rster-SSET) and electron exchange (Dexter-TTET) mechanisms as demonstrated by, UV-visible absorption, fluorescence, phosphorescence and laser flash photolysis measurements. Unprotected, optically pure 3-(2-fluorenyl)-L-alanine (8, Fla) and its C- and N-protected derivatives were synthesized using a modified Sorensen procedure, and were subsequently used for solution-phase synthesis of the dipeptides (9, Bpa-Fla and 10,Npa-Fla) and solid-phase synthesis of a 15-residue peptide (7). UV absorption spectra of the dipeptides indicate that the chromophores do not interact electronically in the ground state nor do their photophysical characteristics change when incorporated into the peptides. Dipeptide bichromophore fluorescence and phosphorescence results show efficient SSET (kSSET > 109 s-1) and TTET (kTTET > 104 s-1). Fluorescence and phosphorescence spectra produced by excitation at donor absorbance wavelengths show complete energy transfer in 9 and efficient energy transfer in 10. Donor-acceptor separation (transfer separation distance) was calculated from the spectral overlap of the donor fluorescence spectrum and the acceptor UV absorption spectrum using the FÃƒÂ¶rster equation for energy transfer. With the spectral overlap value, assumed orientation of the chromophores in the dipeptide and the fluorescence quantum yield of the donor, a "critical" FÃƒÂ¶rster transfer distance was calculated for the two dipeptides (9, 10) to be 14.6 Ãƒâ€¦ and 15.1 Ãƒâ€¦, respectively. The actual distance calculated from dipeptide fluorescence spectra, using an assumed FÃƒÂ¶rster distance was 9.1 Ãƒâ€¦ and 8.7 Ãƒâ€¦, respectively. HyperChem MM3 modeling of the two dipeptides yielded an average inter-chromophore separation of 11 ±3 Ãƒâ€¦, comparable to the experimental values. Laser flash photolysis of the two dipeptides revealed near complete TTET. The spectral analysis of each dipeptide showed essentially 100% acceptor triplet present, although the donor in each case absorbed all or most of the incident light.
Worcester Polytechnic Institute
Chemistry & Biochemistry
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Ferguson, David C., "Intramolecular Energy Transfer in Polychromophores that Utilize Peptide Bridging Groups" (2000). Masters Theses (All Theses, All Years). 750.
triplet energy transfer, chromophores, peptides, Peptides, Synthesis, Energy transfer