Two synthetic peptides corresponding to sequences in HIV-1LAI gp41, (aa558-595) and T20 (aa 643-678), are strong inhibitors of HIV-1 viral fusion, having EC50 values of 1 microgram/mL and 1 ng/mL, respectively. Previous work suggested that T21 forms a coiled-coil structure in PBS solution, while T20 is primarily nonhelical, and that the inhibitory action of these peptides occurs after the interaction between the viral gp120 protein and the cellular CD4 receptor [Wild, C.T., Shugars, D. C., Greenwell, T. K., McDanal, C. B., Matthews, T. J. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 9770 and references therein]. The current study uses sedimentation equilibrium (SE), circular dichroism (CD), and viral-fusion assays to quantitatively investigate peptide structure and peptide-peptide interactions. SE analyses of T21 (1-100 microM) indicate that the peptide self associates via a monomer/dimer/tetramer equilibrium; in addition, T20 is monomeric in the range of 1-10 microM and exhibits a complicated monomer/tetramer equilibrium between 20 and 100 microM. Singular value decomposition analyses of the CD spectra of T21 and T20 indicate that the helical content of these peptides in PBS solution is 90% and 20%, respectively. A structural interaction between the two peptides is detected by CD at several concentration ratios of T20:T21. These experiments emphasize that T20 interacts specifically with the tetrameric form of T21. Truncated forms of T20 also exhibit structural interactions with T21 at varying concentration ratios. The ability of T20 and the truncated peptides to interact structurally with tetrameric T21 correlates with antiviral activity. Implications of these findings are discussed in terms of proposed mechanisms of membrane fusion inhibition and the structural changes which occur in gp41 during membrane fusion.

Resonance Raman intensity analysis and picosecond time-resolved resonance Raman spectroscopy are used to elucidate the reaction dynamics of the electrocyclic ring-openings of 1,3-~yclohexadiene (CHD) and 1,3,5-cyclooctatriene (COT) as well as the hydrogen migration in 1,3,5-cycloheptatriene (CHT). The resonance Raman intensities of CHD demonstrate that evolution along the conrotatory reaction coordinate occurs immediately after photoexcitation, in agreement with the prediction of the Woodward-Hoffmann rules. The 900-cm-l optical T2 combined with the 2 X 10" fluorescence quantum yield shows that the initially prepared excited state of CHD depopulates on the 10-fs time scale due to internal conversion to a lower energy, optically dark surface. The Raman intensities of COT and C H T demonstrate that for these molecules, the initial excited-state dynamics consist principally of ring planarization with no evidence for motion along reactive coordinates. This suggests that the establishment of a planar excited-state geometry is a prerequisite for reactive pericyclic nuclear motion. Picosecond time-resolved resonance Raman Stokes and anti-Stokes spectra of the above reactions reveal that the ground-state photoproducts appear on the 10-ps time scale. Analysis of the time-resolved vibrational spectra also demonstrates that population of the ground state is followed by vibrational relaxation and single-bond isomerization of the ring-opened photoproducts on the 10-ps time scale. This work demonstrates that resonance Raman spectroscopy is a powerful methodology for elucidating condensedphase chemical reaction dynamics.

Absolute resonance Raman cross sections are measured for Nile blue 690 perchlorate dissolved in ethylene glycol with excitation at 514, 531, and 568 nm. These values and the absorption spectrum are modeled using a time-dependent wave packet formalism. The excited-state equilibrium geometry changes are quantitated for 40 resonance Raman active modes, seven of which (590, 1141, 1351, 1429, 1492, 1544, and 1640 cm−1 ) carry 70% of the total resonance Raman intensity. This demonstrates that in addition to the prominent 590 and 1640 cm−1 modes, a large number of vibrational degrees of freedom are Franck–Condon coupled to the electronic transition. After exposure of the explicit vibrational progressions, the residual absorption linewidth is separated into its homogeneous [350 cm−1 half-width at half-maximum (HWHM)] and inhomogeneous (313 cm−1 HWHM) components through an analysis of the absolute Raman cross sections. The value of the electronic dephasing time derived from this study (25 fs) compares well to previously published results. These data should be valuable in multimode modeling of femtosecond experiments on Nile blue.

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