No abstract

A 24-amino acid peptide designed to solubilize integral membrane proteins has been synthesized. The design was for an amphipathic alpha helix with a "flat" hydrophobic surface that would interact with a transmembrane protein as a detergent. When mixed with peptide, 85 percent of bacteriorhodopsin and 60 percent of rhodopsin remained in solution over a period of 2 days in their native forms. The crystal structure of peptide alone showed it to form an antiparallel four-helix bundle in which monomers interact, flat surface to flat surface, as predicted.

A 108 amino acid protein was designed and constructed from a reduced alphabet of seven amino acids. The 2.9 A resolution X-ray crystal structure confirms that the protein is a four helix bundle, as it was designed to be. Hydrogen/deuterium exchange experiments reveal buried amide protons with protection factors in excess of 1 x 10(6) in the range characteristic of well protected protons in functional folded proteins (10(3)-10(8)) rather than protons in rapid exchange (0-10(2)). The protein is monomeric at 1 mM, the concentration at which the exchange experiments were undertaken, indicating that the exchange factors are due to a unique stable tertiary structure fold, and not due to any higher order quaternary structure. Thermodynamic analysis provides an estimate of the free energy of folding of -9.3 kcal mole-1 at 25 degrees C, consistent with the free energy of folding derived from the protection factors of the most protected protons, indicating that global unfolding is required for exchange of the most protected protons.

We demonstrate the use of electron spin resonance (ESR) to determine long-range distances and flexibility in water-soluble bis-peptide molecular rods. Bis-peptide oligomers with 4-8 monomer units were synthesized. ESR determined that the end-to-end length of the peptides is linearly proportional to the number of monomers. The linear shape is, therefore, easily interpreted from the data. In addition, the flexibility of the rods was quantified directly from the ESR-determined distance distribution functions. Quantitative information on chain length and flexibility is important to develop the use of these oligomers as rodlike structural elements for applications such as bivalent display of ligands and as elements of future nanoscale devices.