Interaction between TOAC free radical and Photoexcited triplet chromophores linked to peptide templates

The intramolecular quenching of photoexcited tripler states by free radicals linked to peptide templates was studied by time-resolved electron paramagnetic resonance (EPR) with pulsed laser excitation. The systems investigated are 3(10)-helix forming peptides, having in the amino acid sequence the free radical 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) and a triplet precursor. such as Bin. Bpa, or Trp, incorporated at different relative positions. Upon interaction with the excited tripler the TOAC medical spin sublevel populations assume values that differ from the Boltzmann equilibrium values. This spin polarization effect products EPR lines in emission whose time evolution reflects the tripler quenching rare. In particular, in a series of peptides labeled with Bpa and TOAC at successive positions in the 3(10)-helix, radical-tripler interaction was observed in all cases. However. for the peptide where Bpa and TOAC are at positions 2 and 4 the rare of tripler quenching is lower than for the other peptides in the series. In addition, the radical-excited triplet complex in the quarter spin state was observed in a peptide containing fullerene (C-60) as a tripler precursor and TOAC. (C) 2001 John Wiley & Sons, Inc.