FAST CONFORMATIONAL RELAXATION IN MODEL PEPTIDES

University Of Pennsylvania, Philadelphia PA

Investigators

Linked publications & trials

Abstract

DESCRIPTION: (adapted from applicant?s abstract) The investigators plan to obtain a detailed description of how changes in charge, dipole, temperature and conformational restrictions lead to changes in secondary structural units, such as beta-turns, alpha-helices and beta-hairpins. The proposed work will be accomplished through the design of small, chemically defined model peptides and proteins that lack much of the complexity of natural systems, but still contain the essential elements believed to be required for forming a particular secondary structure. In each proposed area, molecules have been designed that contain phototriggers to allow measurement of conformational changes on time scales ranging from femtoseconds to milliseconds. Specifically the proposed research can be divided into three main areas: 1) disulfide photocleavable crosslinks to induce folding of peptides, 2) the effects of transient dipoles on peptide conformation and folding, and 3) temperature jump experiments to monitor the effect of specific peptide modifications on peptide stability and folding. In the first area the investigators plan to a) develop improved disulfide crosslinking functionality to slow down the recombination of the generated thiyl radicals, b) analyze changes in their prototypical peptide 2, and c) to use the disulfide pairs to stabilize a beta-hairpin conformation in a peptide. The investigators plan to accomplish goal (a) by the incorporation of triplet sensitizers (such as benzophenone) to excite the disulfides directly into the less reactive triplet state, and by introducing an electron donor as one of the disulfide partners (such as nitrothiotyrosine) such that electron transfer could take place between the caged thiyl radical pair thus lowering reactivity. The use of mixed disulfides between a single amino acid and a small thiol-containing molecule is proposed within this context for use in protein folding studies. Proposed work under goal (b) include modifying the helical potential of peptide 2 to study potential folding events after photolysis of the constrained peptides, by incorporating helix breaking residues such as Gly, Bal, and Pro, or by the addition of helix stabilizing lactam bridges. Additionally the role of 4 Ala or Gly residues appended to each termini of PCP on the recombination of the thiyl radicals will be evaluated, as well as modifications to the position of the disulfide within the peptide sequence. In each case the rate of radical recombination and peptide relaxation will be monitored by transient IR spectroscopy. The proposed work in area 2 on the effects of transient dipoles on peptide structure includes a) synthesis of helical peptides containing ruthenium probes which rapidly change their charge or dipole moment upon light absorption, b) use of Stark spectroscopy to determine the strength of the photo-induced dipole with the partial positive charge of the dipole on a fully helical peptide, c) measure the change in the helicity of a partially folded helical peptide in response to the photoinduced dipole by time resolved FTIR, and d) measure the kinetics of field-induced helix formation at specific sites along the peptide chain by 13C-labeling individual carbonyl groups also by time resolved FTIR. The above experiments will also be applied to a potential beta-turn containing peptide. Finally, in area 3, temperature jump experiments are proposed to monitor helical peptide stability as a function of a) different N-terminal helical caps, b) incorporation of specific ion pairs, and c) aromatic-aromatic interactions.