Singlet-Singlet and Triplet-Triplet Energy Transfer in Bichromophoric Cyclic Peptides

Guler, Mustafa Ozgur

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Singlet-Singlet and Triplet-Triplet Energy Transfer in Bichromophoric Cyclic Peptides

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Intramolecular singlet-singlet (SSET) and triplet-triplet (TTET) energy transfer have been studied in two cyclic octapeptides, 1A and 2A, and their open chain analogs, 1B and 2B. The peptides are constructed by a solid phase synthetic technique from enantiomerically pure amino acids with alternating chirality. Cyclic peptides with this arrangement of amino acids preferentially adopt flat, disk-like conformations where the peptide side chains lie on the outside of the ensemble. In 1A, benzophenone and naphthalene chromophores are incorporated as 4-benzoyl-L-phenylalanine and 2-naphtyl-L-alanine at positions 1 and 5 in the peptide sequence while in 2A, these chromophores occupy positions 1 and 3. Molecular modeling studies indicate that the interchromophore separation is larger in 1A than in 2A. This difference in separation is apparent from the observation of TTET energy transfer in 2A, which is consistent with the short range nature of TTET. Low temperature phosphorescence results indicate that intramolecular TTET is efficient in 2A and 2B and occurs with a rate of kTTET > 9.4x103 s-1. Intramolecular SSET occurs efficiently within these cyclic and open chain peptides. 1A undergoes intramolecular SSET from the naphthalene chromophore to the benzophenone chromophore with kSSET > 3.7x107 s-1, while in 2A with kSSET >3.0x107 s-1. Results obtained by modeling, UV-Visible spectroscopy, fluorescence and phosphorescence spectroscopies and transient absorption experiments are described.

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