15min:

RYOJI KUSAKA, Department of Chemistry, Purdue University, West Lafayette, IN 47907, and Department of Chemistry, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan; DI ZHANG, PATRICK WALSH, JOSEPH GORD AND TIMOTHY S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN 47907; BRIAN F. FISHER AND SAMUEL H. GELLMAN, Department of Chemistry, University of Wisconsin, Madison, WI 53706.

Synthetic foldamers composed of heterogeneous backbones offer constructs for building unique secondary structures. / -peptides juxtapose the -amino acid sub-units typical of proteins with -amino sub-units. Gellman and co-workers have developed efficient syntheses of / -peptides that incorporate a cyclohexyl ring constraint at the ³- ⁴ bond to limit backbone torsional mobility, and found that they form helices held together by C=O( i ) H-N( i +3) H-bonds composing 12-membered rings both in solution and in crystalline form. This talk will present a detailed study of the single-conformation double-resonance UV and IR spectroscopy of Ac-Ala- _ACHC-NH-benzyl ( ) and Ac- _ACHC-Ala-NH-benzyl ( ) capped peptides, in which _ACHC residues are constrained by a cis cyclohexyl ring at ³- ⁴ bond with an ethyl group at ² position. The two and peptides have three amide groups that are the minimum length necessary to form a 12-membered H-bond. Conformational assignments were made using the NH stretch, C=O stretch (amide I), and NH bend (amide II) regions of the IR spectrum with the aid of DFT calculations. The double-resonance UV and IR spectroscopy uncovered the presence of 6 conformers for and 4 conformers for . In the two peptides, three of ten structures incorporate bifurcated double rings made of 12-membered C=O(1) H-N(3) ring, which is the first stage of the 12-helix, and 7- or 9-membered C=O(1) H-N(2) ring via nearest-neighbor interaction. The other seven structures are constructed based on 5-, 7-, and 9-membered nearest-neighbor H-bonds. The similarities and differences between structures observed for the two two and peptides will be discussed.