Somatostatin, bombesin, and other flexible peptide hormones adopt multiple conformations in solution, yet bind with high specificity to membrane receptors. Empirical structure-activity studies have discovered more rigid analogs with enhanced activity, suggesting that only a subset of conformers of the natural peptide are recognized by the receptor. Molecules tailored to fit the receptor binding site should have higher affinity and specificity for the receptor. Although factors other than binding affect biological activity, a unifying structural approach to peptide design is of key importance. Presently, there are few techniques for determining peptide structure in such complex systems. The long term goal of the proposed research is to develop fluorescence methods to probe the structure and dynamics of flexible peptides. Fluorescence is a sensitive technique and the aromatic amino acids are intrinsic reporters of peptide structure. However, in most cases the structural and chemical basis of the multiexponenrial fluorescence decays of peptides with even a single aromatic residue are not understood. Our strategy is to design tryptophan and tyrosine derivatives, whose fluorescence emission may be directly interpreted in structural terms. The proposed modifications will constrain side chain bond rotations and extend the aromatic ring system. Excited-state properties will be determined by steady-state and time-resolved fluorescence. Ground- state properties will be determined by X-ray diffraction, molecular mechanics, and NMR. The success of this approach is apparent from preliminary studies of a constrained tryptophan derivative. The effects of peptide environment on the fluorescence of this derivative will be characterized in simple model peptides and rigid somatostatin analog. Finally, the constrained derivative will be incorporated into semi-rigid somatostatin analogs and flexible bombesin analogs and used to probe solution structural and dynamical features relevant to biological activity. Future work will focus on peptide structure and dynamics in complex environments, including soluble protein and membrane receptor complexes. In addition, some of the proposed derivatives may have applications as fluorescence probes for protein conformational transitions as well as tools for peptide design.
