American Society for Photobiology

ASP Conference 2016: 21-26 May 2016
Tampa Marriott Waterside Hotel & Marina

Single Session

[Schedule Grid]

5 - In honor of Jack Saltiel

Florida 1   13:00 - 17:00

Chair(s): Beth Gaillard, Igor Alabugin
5-0   13:00  Opening Remarks by Mark Wrighton, Chancellor, Washington University in Saint Louis

5-1   13:10  Synthetic Oligo- and Poly-(phenylene ethynylene)-Electrolyte/Biological Interfaces DG Whitten*, University of New Mexico ; HC Pappas, University of New Mexico; PL Donabedian, University of New Mexico; EH Hill, University of New Mexico; EY Chi, University of New Mexico; AK Neumann, University of New Mexico

Abstract: Our presentation will introduce a class of materials and compounds based on charged phenylene ethynylene polymers (CPE) and oligomers (OPE) that we have developed as fluorescence-based biosensors and antimicrobials. The focus of the presentation will be on recent findings and new applications we have developed. The first example is our discovery that certain OPE can induce or enhance bacillus spore germination in a dark process. This provides a new mode of antibacterial activity in that the resulting vegetative cell can be killed by either dark or light-activated biocidal activity of the same OPE. Another topic we will discuss is the selective binding of certain OPE with fibrils formed from aggregation of mis-folded proteins associated with Alzheimer's disease and sensing and therapeutic possibilities that might be developed from these interactions. Finally we compare the different paths used by an OPE and PPE to deactivate pathogenic Candida cells.

5-2   13:40  Tracking Photoinduced Charge Separation in DNA FD Lewis*, Northwestern University

Abstract: Recent studies of photoinduced charge separation in DNA have progressed rapidly as a consequence of collaborations with groups of laser spectroscopists and theoreticians. Recent studies of photoinduced charge separation in DNA have progressed rapidly as a consequence of collaborations with groups of laser spectroscopists and theoreticians. Old theories for the transport of positive charge (holes) and negative charge (electrons) founded on singlet step superexchange and multi-step incoherent hole hopping have been superseded by hole injection followed by formation of a conduction channel or trapped polaron. Direct evidence for formation of the trapped polaron in poly(adenine) sequences (A-tracks) of variable length has now been obtained by transient spectroscopy, as has the formation of the guanine hole trap. The efficiency of hole transport across an A-track is determined by the competition between charge recombination and charge transport in the polaron, both of which are subject to energy gap laws. Beyond five A-T base pairs the efficiency of charge separation becomes very small except when charge recombination is inhibited. Upon reaching a deep hole trap, the polaron is irreversibly trapped and decays slowly by charge recombination with the chromophore anion radical produced during the charge injection process. Electron transport is less efficient than is hole transport in DNA, because of faster charge recombination in the initially formed contact radical ion pair.

5-3   14:10  Expanding The Range Of Caged Bioinitiators: 2-PE (500 - 700 nm) Excitation Of p-Hydroxyphenacyl Caged ATP Richard S. Givens*, University of Kansas ; Christopher G. Elles, University of Kansas; Amanda L. Houk, University of Kansas

Abstract: The photoremovable protecting group, p-HydroxyPhenacyl (pHP), is demonstrably effective at delivering biological ligands for investigations such as triggering nucleotide release of ATP and GTP that activate ATPase and GTPase signal protein action associated with GAP protein which controls cell growth. Likewise, release of GABA was used to reveal the effect of GABA spillover during axon development (plasticity) during activation of the Lateral Superior Olive (LSO) to GABAergic receptors on the Medial Nucleus of the Trapezoid Body (MNTB) in the brainstem of newborn auditory systems. Developing methods to explore the intricate mechanisms, including the physical parameters of kinetic rate constants and activation barriers, requires tools that control the temporal, spatial and quantity of initiator released. The pHP chromophore has been used to release an impressive array of bioactive substrates ranging from phenols and carboxylates to phosphates and sulfonates. The release occurs within a nsec in near quantitative yield with 10 to 100% efficiency. Whereas all previous applications depended on one-photon excitation at 280 - 330 nm, we now demonstrate the same photochemistry using a Ti:Sapphire pulsed laser for broadband two-photon absorption (2-PA). The 2-PA measurements reveal a strong transition at 4.5 eV (550 nm), the lowest-energy bands of p-hydroxyacetophenone and pHP diethyl phosphate with cross sections of σ2PA = 23 ± 11 and 11 ± 6 GM, respectively. These 2-PA absorptions are superimposable on the 1-PA spectra of pHP at ~310 nm. This has been extended to pHP ATP, a classic caged initiator. The 2-PA absorption spectra are consistent with excitation to the same S3 (1ÏÏ*) excited state for both 1- and 2-PA activation. Two-photon uncaging is now possible using the visible light range 500-720 nm even at basic pH ~8.0. The fundamental spectroscopy, mechanism and representative applications will be discussed. Houk, Givens, Elles, J. Phys. Chem., 2016, 107, 0000.

5-4   14:40  Photochemistry in Confined Spaces: Medium is the Message V Ramamurthy*, Uni. Miami

Abstract: It is well known that chemical reactions, activated by heat or light, occurring in biological assemblies differ from that in conventional media such as organic solvents including water. Uniqueness of these reactions has led to continued search for new supramolecular structures (media) that would mimic biological systems. The hosts that have attracted attention includes micelles, liquid crystals, LB films, organic and inorganic hosts, clays, silica, zeolites, etc. This type of investigation under the broader title of "˜supramolecular chemistry' that emphasizes the importance of weak interactions has witnessed a renaissance since the award of Nobel Prize to Cram, Lehn and Pederson. The early investigation (pre-1900) of supramolecular photochemistry dealt with crystals while the recent one's deal with host-guest assemblies involving synthetic hosts in water. Reactions carried out in crystals as well as in host-guest assemblies in water share features and they both could be understood based on the topochemical principles established to understand reactions in crystals. This lecture will highlight a few observations from our laboratory on photoreactions in various media. Studies from our and several other laboratories have established that molecules like humans behave differently when confined. Photochemistry within a water-soluble organic capsule, V. Ramamurthy, Acc. Chem. Res., 48, 2904, 2015. Supramolecular photochemistry: From molecular crystals to water-soluble capsules, V. Ramamurthy and S. Gupta, Chem. Soc. Rev., 44, 119, 2015 Supramolecular Photochemistry as a Synthetic Tool, V. Ramamurthy and J. Sivaguru, Chem. Rev., 2016, ASAP. (Special issue on Photochemistry in Organic Synthesis)

5-6   15:10  High Energy Long-Lived Excited States in Model Duplexes and Natural DNA D. Markovitsi*, LIDYL, CEA, CNRS

Abstract: According to the picture emerged during the past decade, the energy of a UV photon absorbed directly by DNA duplexes may be redistributed among their bases. An important part of the excitations evolves toward charge transfer (CT) states which emit around 420 nm, at lower energy than the fluorescence of mono-nucleotides (330 nm). In the case duplexes with alternating guanine-cytosine or adenine-thymine sequence, a counterintuitive relaxation pathway, involving High-energy Long-lived Emitting Mixed (HELM) states, was identified. HELM states emit at 305 nm, decay on the ns time-scale and are very sensitive to structural disorder. Quantum chemistry calculations showed that they result from mixing between Frenkel excitons and purine-to-pyrimidine CT states and extend over at least four bases on both strands. The properties of HELM states were also detected in the fluorescence of calf thymus DNA. Conformational motions break the coherence of HELM states repopulating pp* states and giving rise to delayed fluorescence, which was indeed observed for this natural system. (Vaya

5-7   15:40  Chemometric Analysis of Chromoprotein Photocycles Zimanyi L*, Biological Research Centre of the Hungarian Academy of Sciences

Abstract: The archetypical microbial retinal protein, bacteriorhodopsin, undergoes a photocycle triggered by light absorption by the retinal chromophore, that is followed by a sequence of dark reactions resulting in the pumping of a proton across the cell membrane. The rigorous chemometric analysis of the spectrotemporal matrices obtained in our time resolved multichannel absorption spectroscopic experiments was initiated by Jack Saltiel in the nineties. Our work together yielded a more accurate description of the intermediate spectra and kinetics and, with the combination of singular value decomposition with exponential fit-assisted self-modeling (SVD-EFASM), we found a previously undetected intermediate. More recently, we applied similar chemometric methods to elucidate the photocycle of the soluble chromoprotein, photoactive yellow protein (PYP). PYP is a blue-light sensor from the bacterium Halorhodospira halophila with a covalently bound p-coumaric acid chromophore. SVD, exponential fit and target testing yielded intermediate spectra serving as input in the following global spectrotemporal model fit, using a sufficiently complex photocycle scheme with reversible transitions. The obtained dependence of the rate coefficients on the presence of chaotropic or kosmotropic Hofmeister co-solutes suggested that the conformational change of PYP leading to the most unfolded signaling state takes place gradually, and starts already in microseconds with the relaxation after the chromophore isomerization. At present we are studying the Hofmeister effect on the kinetics of the bacteriorhodopsin photocycle. These effects, observed almost exclusively in the millisecond range, will be discussed based on available spectroscopic and X-ray structural data, and on a recent model of the Hofmeister effect on the structural stability of proteins [Dar et al., J. Phys. Chem. B 111 (2007) 5344-5350].

5-8   16:10  Triplet States in Organometallic Conjugated Materials KS Schanze*, University of Florida

Abstract: Triplet excited states (excitons) play an important role in the application of organic materials. For example, in organic light emitting diodes, harvesting of triplet excitons affords a substantial enhancement in external quantum efficiency and luminous efficiency. In organic solar cells, recombination to produce triplet states can reduce the efficiency of photocurrent generation. Our research program has explored the properties of triplet excitons in -conjugated polymers and model oligomer systems. This work has taken advantage of the effect of spin-orbit coupling enhancement by heavy metal centers that are strongly coupled to the -conjugated electronic system. In particular, platinum(II) acetylides and orthometalled platinum(II) complexes give rise to enhanced intersystem crossing efficiency and in some cases room temperature phosphorescence. This allows facile investigation of the energetics and dynamics of triplet exciton states. Topics of interest include triplet excited state structure and delocalization in conjugated molecules, the effect of electronic structure on intersystem crossing efficiency, triplet exciton diffusion length, triplet states in organic solar cells and in near-infrared light emitting diodes.

5-9   16:40  Remarks, Beth Gaillard and Igor Alabugin, session co-chairs

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