We hypothesize that the structural states of the peptides are a monomeric alpha helix and an aggregated antiparallel beta sheet. Conditions encouraging aggregation tend to favor sheet; conditions discouraging aggregation tend to favor helix. Consideration of such solution-dependent conformational changes may affect de novo protein design and have a bearing on certain biological processes.

A novel photoreactive substance P (SP) analogue has been synthesized by solid-phase peptide synthesis methodology to incorporate the amino acid p-benzoyl-L-phenylalanine [L-Phe(pBz)] in place of the Phe8 residue of SP. [Phe8(pBz)]SP was equipotent with SP in competing for SP binding sites on rat submaxillary gland membranes and had potent sialagogic activity in vivo. In the absence of light, the 125I-labeled Bolton-Hunter conjugate of [Phe8(pBz)]SP bound in a saturable and reversible manner to an apparently homogeneous class of binding sites (Bmax = 0.2 pmol/mg of membrane protein) with an affinity KD = 0.4 nM. The binding of 125I-[Phe8(pBz)]SP was inhibited competitively by various tachykinin peptides and analogues with the appropriate specificity for SP/NK-1 receptors. Upon photolysis, up to 70% of the specifically bound 125I-[Phe8(pBz)]SP underwent covalent linkage to two polypeptides of Mr = 53,000 and 46,000, identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Quantitative analysis of the inhibitory effects of SP and related peptides on 125I-[Phe8(pBz)]SP photoincorporation indicated that the binding sites of the two photolabeled polypeptides have the same peptide specificity, namely, that typical of NK-1-type SP receptors. In addition, the labeling of the two polypeptides was equally sensitive to inhibition by guanyl-5'-yl imidodiphosphate, a nonhydrolyzable analogue of GTP. Further information on the relationship between the two labeled SP binding sites was provided by enzymatic digestion studies: the Mr = 46,000 polypeptide contains N-linked carbohydrates and is derived most likely from the higher molecular weight species by proteolytic nicking.(ABSTRACT TRUNCATED AT 250 WORDS)

The first measurements of time-resolved circular dichroism in the far-UV spectral region (far-UV-TRCD) with time resolution on the order of lO-'s are presented. The capability of making such measurements is demonstrated with ground-and excited-state CD spectra of (A)-Ru(bpy)3*+ between 190 and 290 nm as well as with CD spectra of a short peptide measured at different pHs. The properties of the far-UV-TRCD technique are discussed in detail, and comparisons between the static CD spectra obtained by this technique and those obtained by a conventional CD spectrometer are presented. Possible improvements to the current technique and application of the technique to resolve biophysical problems such as the kinetics of peptide helix+oil transition are also discussed.

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