ARTICLES Intramolecular Interaction between Nitroxide Radical and Photoexcited Benzophenone Triplet Linked to Peptide Templates E. Sartori, ² A. Toffoletti, ² F. Rastrelli, ² C. Corvaja,* ,² A. Bettio, ‡ F. Formaggio, ‡ S. Oancea, ‡ and C. Toniolo ‡ Department of Physical Chemistry and Institute of Biomolecular Chemistry, CNR and Department of Organic Chemistry, UniVersity of PadoVa, I-35131 PadoVa, Italy ReceiVed: February 28, 2003; In Final Form: July 1, 2003 A series of heptapeptides and one dodecapeptide doubly labeled with a triplet precursor (4′-benzoylphenyl- alanine) 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. Introduction Quenching of excited triplets by free radicals and paramag- netic impurities was first reported many years ago. 1 This phenomenon was observed through its different effects (e.g., triplet lifetime shortening 2 and magnetic field dependence of photoconductivity). 3 In the early ‘90s, it was shown that in the presence of excited triplet species, the EPR lines of a free radical become polarized in emission or in enhanced absorption. 4 Anomalous line intensity is a consequence of the deviation of the spin level populations from the thermal equilibrium values. Such a polarization is caused by the spin selectivity of the triplet quenching process, which occurs through the formation of radical-triplet pairs (RTPs). As the RTPs produced in the doublet state by antiparallel coupling of the radical and triplet spin possess the same spin multiplicity of the pairs formed by ground- state singlet plus free radical, they decay fast, while RTPs in the quartet state, formed by parallel spin coupling, decay slowly. The radical-triplet pair mechanism (RTPM) of spin polarization is based on the selective mixing of doublet and quartet spin substates of the pair by electron spin dipolar and hyperfine interactions. These terms of the spin Hamiltonian do not commute with the Zeeman and exchange terms of the spin Hamiltonian of the pair. 4a When the doublet and the quartet are separated by a nonvanishing exchange interaction, J, the extent of mixing depends on the z component of the electron spin total angular momentum. This fact produces spin polariza- tion because the quartet components more contaminated with those of the doublet decay faster. The existing RTPM theory is based on a model in which the quartet/doublet transitions are induced by the fluctuation of J during the relative diffusion of the RTP partners. 5 Indeed, J is expected to change exponentially with the radical-triplet dis- tance. 6 The amount of polarization reaches a maximum when J is close to 1/3 and 2/3 of the Zeeman energy, because for these values, a crossing occurs between doublet and quartet spin sublevels. Even if efforts have been made to assess the relative distance between the partners in the pair, no clear-cut informa- tion is as yet available on this relationship. In addition to the knowledge of the exact dependence of J on the distance, other parameters should be considered, such as the correlation time for the diffusion process and the absolute value of the spin polarization, which is not easily obtained. 7 Moreover, it should be considered that a dependence of J exclusively on the radical- triplet distance is a reasonable assumption only for small molecules subjected to fast rotational diffusion. Under such conditions, any variation with the relative orientation of the partner is averaged out. Recently, it has been shown that RTPM operates also when the radical and the triplet are covalently bound to the same molecule. 8 This observation suggested the possibility to inves- tigate RTPs in which the partner distance range is well established. Moreover, by changing the spacer length, one could investigate the dependence of polarization intensity and kinetics on the radical-triplet distance. 9 A similar strategy was used in the study of spin-correlated radical pairs by producing two covalently linked free radicals by photoinduced Norrish I cleavage of cyclic ketones. 10 In this paper, we present a time-resolved EPR (TR-EPR) study on a series of radical-triplet pairs, whose components are covalently linked at different relative positions on a rigid oligopeptide template. The radical is the nitroxide R-amino acid TOAC (4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-4-carbox- ylic acid) (Figure 1), 11 while the triplet excited species is the benzophenone moiety of L-Bpa (4′-benzoylphenylalanine) * To whom correspondence should be addressed. E-mail: carlo.corvaja@ unipd.it. ² Department of Physical Chemistry. ‡ Department of Organic Chemistry. 6905 J. Phys. Chem. A 2003, 107, 6905-6912 10.1021/jp0345203 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/16/2003