Hydroxy and methoxy perylene quinones are synthesized in an attempt to isolate the essential spectroscopic and biological features of light-induced antiviral agents such as hypericin and hypocrellin. Unlike their naturally occurring counterparts, these synthetic quinones bear the carbonyl, hydroxyl, and methoxy groups in the "bay region." The hydroxy and methoxy compounds have rich absorption spectra with broad features in the visible (approximately 450-800 nm) and relatively more intense and narrow features at wavelengths < or = 350 nm. High-level ab initio quantum mechanical calculations assign the features in the absorption spectra to electronic transitions from S0 to S2 and to higher-lying electronic states. The calculations indicate that in the ground state the trans dihydroxy isomer is 12.5 kcal/mol lower in energy than the cis dihydroxy isomer and is thus the only species present. The lowest-energy trans methoxy ground state isomer and the lowest-energy cis methoxy ground state isomer are found to be degenerate. An additional cis methoxy isomer 6.3 kcal/mol higher in energy than the global minimum is assumed to contribute to the spectrum and is also considered. Finally, the synthetic compounds exhibit similar light-induced antiviral activity to each other, but significantly less than that of hypericin.

The nuclear-electronic orbital (NEO) method for the calculation of mixed nuclear-electronic wave functions is presented. Both electronic and nuclear molecular orbitals are expressed as linear combinations of Gaussian basis functions. In the NEO-HF (Hartree-Fock) method, the energy corresponding to the single-configurational mixed nuclear-electronic wave function is minimized with respect to the molecular orbitals. Multiconfigurational approaches are implemented to include significant correlation effects. In the NEO-CI (configuration interaction) method, the energy corresponding to the multiconfigurational mixed nuclear-electronic wave function is minimized with respect to the CI coefficients. In the NEO-MCSCF (multiconfigurational self-consistent-field) method, the energy is minimized with respect to the molecular orbitals as well as the CI coefficients. Analytic gradient expressions are presented for NEO-HF and NEO-MCSCF. These analytic gradients allow the variational optimization of the centers of the nuclear basis functions. They also enable the location and characterization of geometry stationary points and the generation of minimum energy paths and dynamic reaction paths. The advantages of the NEO approach are that nuclear quantum effects are incorporated during the electronic structure calculation, the Born-Oppenheimer separation of electrons and nuclei is avoided, excited vibrational-electronic states may be calculated, and its accuracy may be improved systematically. Initial applications are presented to illustrate the computational feasibility and accuracy of this approach.

A parallel implementation of the gradient of the second order Moeller-Plesset (MP2) energy is used to investigate the structure of Silicocene. Molecular symmetry and frozen core orbitals are exploited to minimise the computation time. The distributed data algorithm harnesses the aggregate memory of the given platform to store four subclasses of transformed electron repulsion integrals with up to four occupied molecular orbital (MO) indices and at most two virtual MO indices. Terms of the MP2 Lagrangian involving three virtual MO indices are computed separately and the integrals not stored. The scheme permits the entire gradient to be completed with just six evaluations of the regular electron repulsion integrals and four evaluations of the derivative electron repulsion integrals. Results obtained on the Cray T3D demonstrate good scaling properties with the number of nodes.

An effective fragment model is developed to treat solvent effects on chemical properties and reactions. The solvent, which might consist of discrete water molecules, protein, or other material, is treated explicitly using a model potential that incorporates electrostatics, polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water-formamide are in good agreement with fully ab initio calculations.

Using time-resolved single photon counting, fluorescence decay in photosystem I (PS I) was analyzed in mutant strains of Chlamydomonas reinhardtii that lack photosystem II. Two strains are compared: one with a wild-type PS I core antenna (120 chlorophyll a/P700) and a second showing an apparent reduction in core antenna size (60 chlorophyll a/P700). These data were calculated from the lifetimes of core antenna excited states (75 and 45 ps, respectively) and from pigment stoichiometries. Fluorescence decay in wild type PS I is composed of two components: a fast 75-ps decay that represents the photochemically limited lifetime of excited states in the core antenna, and a minor (less than 10%) 300-800 ps component that has spectral characteristics of both peripheral and core antenna pigments. Temporal and spectral properties of the fast PS I decay indicate that (a) excitations are nearly equilibrated among the range of spectral forms present in the PS I core antenna, (b) an average excitation visits a representative distribution of core antenna spectral forms on all pigment-binding subunits regardless of the origin of the excitation, (c) reduction in core antenna size does not alter the range of antenna spectral forms present, and (d) transfer from peripheral antennae to the PS I core complex is rapid (less than 5 ps).

ChemInformVolume 19, Issue 26 Natural Products ChemInform Abstract: Time-Resolved Spectroscopy of Tryptophan Conformers in a Supersonic Jet L. A. PHILIPS, L. A. PHILIPS Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorS. P. WEBB, S. P. WEBB Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorS. J. III MARTINEZ, S. J. III MARTINEZ Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorG. R. FLEMING, G. R. FLEMING Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorD. H. LEVY, D. H. LEVY Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this author L. A. PHILIPS, L. A. PHILIPS Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorS. P. WEBB, S. P. WEBB Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorS. J. III MARTINEZ, S. J. III MARTINEZ Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorG. R. FLEMING, G. R. FLEMING Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this authorD. H. LEVY, D. H. LEVY Dep. Chem., Univ. Chicago, Chicago, IL 60637, USASearch for more papers by this author First published: June 28, 1988 https://doi.org/10.1002/chin.198826318Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume19, Issue26June 28, 1988 RelatedInformation

The temporal and spectral properties of fluorescence decay in isolated photosystem I (PS I) preparations from algae and higher plants were measured using time-correlated single photon counting. Excitations in the PS I core antenna decay with lifetimes of 15-40 ps and 5-6 ns. The fast decay results from efficient photochemical quenching by P700, whereas the slow decay is attributed to core antenna complexes lacking a trap. Samples containing core and peripheral antenna complexes exhibited an additional intermediate lifetime (150-350 ps) decay. The PS I core antenna is composed of several spectral forms of chlorophyll a that are not temporally resolved in the decays. Analysis of the temporal and spectral properties of the decays provides a description of the composition, structure, and dynamics of energy transfer and trapping reactions in PS I. The core antenna size dependence of the spectral properties and the contributions of the spectral forms to the time-resolved decays show that energy is not concentrated in the longest wavelength absorbing pigments but is nearly homogenized among the spectral forms. These data suggest that the "funnel" description of antenna structure and energy transfer (Seely, G. R. 1973. J. Theor. Biol. 40:189-199) may not be applicable to the PS I core antenna.

Fluorescence upconversion experiments with subpicosecond resolution are described. The time dependent fluorescence spectra of several probe molecules in polar solvents are measured and used to construct the Stokes shift correlation function C(t). The correlation functions decay nonexponentially in contrast to the predictions of simple Debye/Onsager continuum theory. At high values of the static dielectric constants the solvation times are much slower than predicted from standard continuum theory. Generalizations of continuum theory to include non-spherical shapes, non-Debye dielectric response and saturation effects are discussed. The experimental data are qualitatively in accord with predictions based on a frequency and position dependent dielectric response.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTime-resolved spectroscopy of tryptophan conformers in a supersonic jetLaura A. Philips, S. P. Webb, Selso J. Martinez, G. R. Fleming, and Donald H. LevyCite this: J. Am. Chem. Soc. 1988, 110, 5, 1352–1355Publication Date (Print):March 1, 1988Publication History Published online1 May 2002Published inissue 1 March 1988https://pubs.acs.org/doi/10.1021/ja00213a005https://doi.org/10.1021/ja00213a005research-articleACS PublicationsRequest reuse permissionsArticle Views366Altmetric-Citations81LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

The isomerization of t-stilbene (stilbene h12 ) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution. In the jet we find that almost all effect of full deuteration (stilbene d12 ) is produced by deuteration of the two ethylinic hydrogens only (stilbene d2 ). Complete deuteration of the phenyl rings (stilbene d10 ) has rather little influence on the decay of the jet-cooled molecule. Nonexponential decays are found at intermediate excess energies in the jet-cooled system, with the degree of nonexponentiality decreasing with increasing excess energy. The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h12 and d2. Using similar parameters the calculations consistently place the stilbene d2 and stilbene d10 curves in the wrong order. Our results suggest extensive but not complete vibrational relaxation in the isolated molecule. Vibrational redistribution rapidly becomes complete in the presence of buffer gas. In thermal samples the isomerization rates of stilbene h12 and stilbene d10 are identical over a wide range of solvents and temperatures. By contrast the isomerization rates in stilbene d2 and stilbene d12 are 1.4 and 1.5 times slower than in stilbene h12. Again, these ratios appear constant over a wide range of experimental conditions.

ChemInformVolume 18, Issue 10 Preparative Organic Chemistry ChemInform Abstract: Hydration Dynamics of Protons from Photon Initiated Acids J. LEE, J. LEE Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorG. W. ROBINSON, G. W. ROBINSON Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorS. P. WEBB, S. P. WEBB Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorL. A. PHILIPS, L. A. PHILIPS Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorJ. H. CLARK, J. H. CLARK Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this author J. LEE, J. LEE Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorG. W. ROBINSON, G. W. ROBINSON Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorS. P. WEBB, S. P. WEBB Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorL. A. PHILIPS, L. A. PHILIPS Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this authorJ. H. CLARK, J. H. CLARK Picosecond Quantum Radiat. Lab., Tex. Tech Univ., Lubbock, TX 79409, USASearch for more papers by this author First published: March 10, 1987 https://doi.org/10.1002/chin.198710151Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume18, Issue10March 10, 1987 RelatedInformation

We have examined the photophysics of energy migration and trapping in photosystem I by investigating the spectral and temporal properties of the fluorescence from the core antenna chlorophylls as a function of the antenna size. Time-correlated single photon counting was used to determine the fluorescence lifetimes in the isolated P700 chlorophyll a-protein complex and in a mutant of Chlamydomonas reinhardtii that lacks the photosystem II reaction center complex. The fluorescence decay in both types of sample is dominated by a fast (15-45 psec) component that is attributed to the lifetime of excitations in the photosystem I core antenna. These excitations decay primarily by an efficient photochemical quenching on P700. The measured lifetimes show a linear relationship to the core antenna size. A linear dependence of the excitation lifetime on antenna size was predicted previously in a lattice model for excitation migration and trapping in arrays of photosynthetic pigments [Pearlstein, R.M. (1982) Photochem. Photobiol. 35, 835-844]. Based on this model, our data predict a time constant for photochemical charge separation in the photosystem I reaction center of 2.8 +/- 0.7 or 3.4 +/- 0.7 psec, assuming monomeric or dimeric P700, respectively. The predicted average single-step transfer time for excitation transfer between core antenna pigments is 0.21 +/- 0.04 psec. Under these conditions, excitation migration in photosystem I is near the diffusion limit, with each excitation making an average of 2.4 visits to the reaction center before photoconversion.