JOSEPH R. GORD, PATRICK S. WALSH 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, Madison, WI 53706.
In order to further understand the intramolecular forces governing secondary structure formation in peptides and to provide benchmarks for the computational community, conformation-specific spectroscopy techniques have been applied to several model systems provided by Dr. Sam Gellman's research group at the University of Wisconsin-Madison. In the present work, two model / -peptides, Ac- ACPC- ACHC-NHBz and Ac- ACHC- ACPC-NHBz have been investigated using single and double resonance ultraviolet and infrared spectroscopy to elucidate their intrinsic folding propensities. The -peptide is constrained by a five-membered ring spanning the 3- 2 positions ( ACPC) and the -peptide is constrained by a six-membered ring spanning the 4- 3 positions with an additional ethyl group at 2 ( ACHC). Resonant two-photon ionization (R2PI) spectra from 37250 to 37750 cm-1 were obtained and subsequently interrogated using UV-UV hole-burning to reveal the presence of three conformations for Ac- ACPC- ACHC-NHBz, and a single conformation for Ac- ACHC- ACPC-NHBz. Resonant ion-dip infrared (RIDIR) spectra were obtained in the NH stretch region from 3300 to 3500 cm-1 and in both the amide I and II regions from 1400 to 1800 cm-1. These spectra were compared to computational predictions given by DFT calculations using the M05-2X functional with a 6-31G+(d) basis set revealing two slightly varied iterations of a bifurcated C-8/13 double ring structure for Ac- ACPC- ACHC-NHBz and one bifurcated C-9/13 double ring structure for Ac- ACHC- ACPC-NHBz. The appearance of C-13 rings was also seen in solution phase studies. This work is a complement to studies performed on pure -peptides and / -peptides.