The membrane-intrinsic light harvesting complex from the diatom Cyclotella meneghiniana, fucoxanthin chlorophyll-a/c 2 protein (FCP), is characterized by Stark spectroscopy to obtain a quantitative measure of the excited-state dipolar properties of the constituent pigments. The electrooptical properties of the carotenoid fucoxanthin (Fx), the primary light harvester in FCP, were determined from the Stark spectrum measured in a MeTHF glass (77 K) and compared to the results from electronic-structure calculations. On photon absorption by Fx, a 17 D change in the static dipole moment (|Δμ ⃗ | exp ), and a somewhat larger |Δμ ⃗ | exp at the red edge, are measured for the S 0 → S 2 transition. The significant change in dipolar properties demonstrates that Fx undergoes photoinduced charge transfer (CT), and underscores the influence of the S 2 state on the polarity-dependent excited-state dynamics of Fx that has so far been attributed to, and discussed in terms of, the S 0 and the S 1 /ICT states. MNDO-PSDCI and SACCI-CISD calculations indicate that the -like state intrinsically possesses a dipole moment much smaller than the -like state, suggesting that solvent fields promote the mixing of these two states and accounts for the large dipole moments measured here for the S 0 → S 2 transition. These CT properties of the -like state of Fx, which are further enhanced in the protein, underpin its photosynthetic capabilities for light harvesting and energy transfer (ET). In FCP, the CT properties of the Fx's vary according to the energetic position: between 450 and 500 nm there appear to be two sets of Fx's that exhibit |Δμ ⃗ | exp values on the order of 5 and 15 D, whereas the red-most Fx's, that are very efficient in ET to chlorophyll-a (Chl-a), exhibit strikingly large |Δμ ⃗ | exp values on the order of 40 D. Such magnitudes of |ΔΔμ ⃗ | exp suggest a mechanism to enhance Coulombic coupling to promote ET from the S 2 state © XXXX American Chemical Society *To whom correspondence should be addressed. premvard@gmail.com. Tel.: +33-(0)1-69359793. Supporting Information AvailableAdditional figures and a table are included as Supporting Information. These include the molecular structures of the pigments in FCP (Scheme S1), the Stark signal and analysis of the Q y band of Chl-a (Figure S1), the Mulliken charge-difference densities between the ground-and first two excited-states from SAC-CI and TDDFT (PBE1PBE) calculations, per individual atom ( Figure S2), and solvent shifts estimated using the electrostatic properties from the MNDOCI results for the first two excited states (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access

The structures of a number of stereoisomers of carotenoids have been revealed in three-dimensional X-ray crystallographic investigations of pigment-protein complexes from photosynthetic organisms. Despite these structural elucidations, the reason for the presence of stereoisomers in these systems is not well understood. An important unresolved issue is whether the natural selection of geometric isomers of carotenoids in photosynthetic pigment-protein complexes is determined by the structure of the protein binding site or by the need for the organism to accomplish a specific physiological task. The association of cis isomers of a carotenoid with reaction centers and trans isomers of the same carotenoid with light-harvesting pigment-protein complexes has led to the hypothesis that the stereoisomers play distinctly different physiological roles. A systematic investigation of the photophysics and photochemistry of purified, stable geometric isomers of carotenoids is needed to understand if a relationship between stereochemistry and biological function exists. In this work we present a comparative study of the spectroscopy and excited state dynamics of cis and trans isomers of three different open-chain carotenoids in solution. The molecules are neurosporene (n=9), spheroidene (n=10), and spirilloxanthin (n=13), where n is the number of conjugated π-electron double bonds. The spectroscopic experiments were carried out on geometric isomers of the carotenoids purified by high performance liquid chromatography (HPLC) and then frozen to 77 K to inhibit isomerization. The spectral data taken at 77 K provide a high resolution view of the spectroscopic differences between geometric isomers. The kinetic data reveal that the lifetime of the lowest excited singlet state of a cis-isomer is consistently shorter than that of its corresponding all-trans counterpart despite the fact that the excited state energy of the cis molecule is typically higher than that of the trans molecule. Quantum theoretical calculations on an n=9 linear polyene were carried out to examine this process. The calculations indicate that the electronic coupling terms are significantly higher for the cis isomer, and when combined with the Franck-Condon factors, predict internal conversion rates roughly double those of the all-trans species. The electronic effects more than offset the decrease in coupling efficiencies associated with the higher system origin energies and explain the observed shorter cis isomer lifetimes.

Using molecular dynamics simulations in combination with scaling analysis, we have studied the effects of the solvent quality and the strength of the electrostatic interactions on the conformations of spherical polyelectrolyte brushes in salt-free solutions. The spherical polyelectrolyte brush could be in one of four conformations: (1) a star-like conformation, (2) a "star of bundles" conformation in which the polyelectrolyte chains self-assemble into pinned cylindrical micelles, (3) a micelle-like conformation with a dense core and charged corona, or (4) a conformation in which there is a thin polymeric layer uniformly covering the particle surface. These different brush conformations appear as a result of the fine interplay between electrostatic and monomer-monomer interactions. The brush thickness depends nonmonotonically on the value of the Bjerrum length. This dependence of the brush thickness is due to counterion condensation inside the brush volume. We have also established that bundle formation in poor solvent conditions for the polymer backbone can also occur in a planar polyelectrolyte brush. In this case, the grafted polyelectrolyte chains form hemispherical aggregates at low polymer grafting densities, cylindrical aggregates at an intermediate range of the grafting densities, and vertically oriented ribbon-like aggregates at high grafting densities.

Recent developments in the biophysical characterization of proteins have provided a means of directly measuring electrostatic fields by introducing a probe molecule to the system of interest and interpreting photon absorption in the context of the Stark effect. To fully account for this effect, the development of accurate atomistic models is of paramount importance. However, suitable computational protocols for evaluating Stark shifts in proteins are yet to be established. In this work, we present a comprehensive computational method to predict the change in absorption frequency of a probe functional group as a direct result of a perturbation in its surrounding electrostatic field created by a protein environment, i.e., the Stark shift. We apply the method to human aldose reductase, a key protein enzyme that catalyzes the reduction of monosaccharides. We develop a protocol based on a combination of molecular dynamics and moving-domain QM/MM methods, which achieves quantitative agreement with experiment. We outline the difficulties in predicting localized electrostatic field changes within a protein environment, and by extension the Stark shift, due to a protein site mutation. Furthermore, the combined use of Stark effect spectroscopy and computational modeling is used to predict the protonation state of ionizable residues in the vicinity of the electrostatic probe.

An exfoliated clay−polymer nanocomposite was prepared by surface‐initiated ring opening metathesis polymerization (SI‐ROMP) of norbornene on a montmorillonite (MMT) clay with a modified surface. Utilizing the hydrothermal‐silylation reaction between a norbornenyl‐bearing chlorosilane agent and silanol groups of the MMT clay, we were able to bind a metal alkylidene catalyst to the surface in order to grow poly(norbornene) chains directly from the surface using ROMP. Our approach produced nanocomposites having poly(norbornene) chains that are covalently attached to the inorganic substrate, as opposed to most conventional polymer‐clay composites that have ionically tethered chains (via the ammonium‐based modifiers of the organoclay) or physically adsorbed polymers. POLYM. ENG. SCI., 55:2349–2354, 2015. © 2015 Society of Plastics Engineers