Intramolecular interaction between nitroxide radical and photoexcited benzophenone triplet linked to peptide templates

A series of heptapeptides and one dodecapeptide doubly labeled with a triplet precursor (4'-benzoylphenylalanine) and a nitroxide (4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-4-carboxylic acid) have been synthesized by solution methods and studied through their FT-IR absorption and time-resolved EPR spectra with UV laser pulse excitation. All of the oligopeptides show EPR nitroxide lines strongly polarized in emission because of the intramolecular interaction between the free radical and the photoexcited triplet. The kinetics of the time evolution of the EPR lines is analyzed to study the radical-triplet interaction in the series of heptapeptides characterized by diverging radical and triplet relative positions in the amino acid sequence.