CIDEP effects of intramolecular quenching of singlet and triplet excited states by nitroxide radicals in oligopeptides: A potentially useful new method for investigating peptide secondary structures in solution

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa ) and one nitroxide-containing TOAC residue, have been synthesized and full y characterized. FT-IR absorption measurements indicate that a 3(10)-helica l conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 Angstrom after on e complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. A n intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spi n-selective decay pathway possible, that produces transient spin polarizati on. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear a nd cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-s pace intramolecular interaction is operative. The observation of spin polar ization makes the two helical hexapeptides suitable models to test the poss ibility of application of this novel technique to conformational studies of peptides in solution.