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 Carlo Corvaja,* [a] Elena Sartori, [a] Antonio Toffoletti, [a] Fernando Formaggio, [b] Marco Crisma, [b] Claudio Toniolo,* [b] Jean-Paul Mazaleyrat, [c] and Michel Wakselman [c] Abstract: Two hexapeptides, each bear- ing one photoactive a-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorp- tion measurements indicate that a 3 10 - helical 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  after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser pop- ulates the excited states localized on the chromophores. An intramolecular inter- action between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through- bond or through-space, we have pre- pared linear and cyclic TOAC-Bin di- peptide units. A CIDEP study revealed that a through-space intramolecular in- teraction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational stud- ies of peptides in solution. Keywords: chromophores ´ CIDEP ´ EPR spectroscopy ´ helical pep- tides ´ intramolecular interaction Introduction The unique stereochemistry of peptides containing C a -tetra- substituted a-amino acids, [1] that severely restrict their con- formational freedom, allows for their exploitation as precise molecular rulers, [2] scaffolding blocks in the de novo design of protein and enzyme mimetics, [3] and suitable models for spectroscopic studies. [4] Aib (a-aminoisobutyric acid, Fig- ure 1), the prototype of C a -tetrasubstituted a-amino acids, is a strong helix promoter. In particular, Aib homo-peptides and Aib-rich, short oligopeptides adopt predominantly the 3 10 - helical conformation both in solution and in the crystal state. [1] A similar behavior was found for the two other C a -tetrasub- Figure 1. Chemical formulas of amino acids and sequences of peptides discussed in this work. stituted a-amino acids discussed in this work, the nitroxide- based TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino- 4-carboxylic acid) [4a, 4c, 5] and the axially chiral, photoexcitable Bin (2',1':1,2;1'',2'':3,4-dinaphthcyclohepta-1,3-diene-6-amino- 6-carboxylic acid). [6] The 3 10 -helix, the third principal long-range structural element occurring in globular proteins beside the a-helix [a] Prof. C. Corvaja, Dr. E. Sartori, Dr. A. Toffoletti Department of Physical Chemistry, University of Padova 35131 Padova (Italy) Fax: ( 39) 049 827-5135 E-mail: corvaja@chfi.unipd.it [b] Prof. C. Toniolo, Dr. F. Formaggio, Dr. M. Crisma Biopolymer Research Centre, CNR Department of Organic Chemistry, University of Padova 35131 Padova (Italy) Fax: ( 39) 049 827-5239 E-mail: biop02@chor.unipd.it [c] Dr. J.-P. Mazaleyrat, Dr. M. Wakselman SIRCOB, Ba Ãt. Lavoisier, University of Versailles 78000 Versailles (France) FULL PAPER Chem.Eur.J. 2000, 6, No. 15  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0947-6539/00/0615-2775 $ 17.50+.50/0 2775