Interaction Between TOAC Free Radical and Photoexcited Triplet Chromophores Linked to Peptide Templates Carlo Corvaja 1 Elena Sartori 1 Antonio Toffoletti 1 Fernando Formaggio 2 Marco Crisma 2 Claudio Toniolo 2 1 Department of Physical Chemistry, University of Padova, Padova 35131, Italy 2 Biopolymer Research Center, CNR, Department of Organic Chemistry, University of Padova, Padova 35131, Italy Abstract: The intramolecular quenching of photoexcited triplet 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 triplet the TOAC radical spin sublevel populations assume values that differ from the Boltzmann equilibrium values. This spin polarization effect produces EPR lines in emission whose time evolution reflects the triplet quenching rate. In particular, in a series of peptides labeled with Bpa and TOAC at successive positions in the 3 10 -helix, radical-triplet interaction was observed in all cases. However, for the peptide where Bpa and TOAC are at positions 2 and 4 the rate of triplet quenching is lower than for the other peptides in the series. In addition, the radical-excited triplet complex in the quartet spin state was observed in a peptide containing fullerene (C 60 ) as a triplet precursor and TOAC. © 2001 John Wiley & Sons, Inc. Biopolymers (Pept Sci) 55: 486 – 495, 2000 Keywords: chemical induced dynamic electron polarization; electron paramagnetic resonance; helical peptides; radical–triplet interactions; 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-car- boxylic acid INTRODUCTION Usually, organic molecules have a singlet ground state. 1 However, they can be photoexcited into a metastable triplet state by virtue of intersystem cross- ing (ISC), caused by spin orbit coupling. 2 The triplet lifetime, in the range of microseconds or even milli- seconds depending on temperature, is long compared Correspondence to: Carlo Corvaja; email: corvaja@chfi.unipd.it Contract grant sponsor: National Research Council (CNR) and the Ministry of University and Scientific and Technological Re- search (MURST) Biopolymers (Peptide Science), Vol. 55, 486 – 495 (2000) © 2001 John Wiley & Sons, Inc. 486