Cholesterol enhances influenza binding avidity by controlling nanoscale receptor clustering.

Influenza virus infects cells by binding to sialylated glycans on the cell surface. While the chemical structure of these glycans determines hemagglutinin-glycan binding affinity, bimolecular affinities are weak, so binding is avidity-dominated and driven by multivalent interactions. Here, we show that membrane spatial organization can control viral binding. Using single-virus fluorescence microscopy, we demonstrate that the sterol composition of the target membrane enhances viral binding avidity in a dose-dependent manner. Binding shows a cooperative dependence on concentration of receptors for influenza virus, as would be expected for a multivalent interaction. Surprisingly, the ability of sterols to promote viral binding is independent of their ability to support liquid-liquid phase separation in model systems. We develop a molecular explanation for this observation via molecular dynamics simulations, where we find that cholesterol promotes small-scale clusters of glycosphingolipid receptors. We propose a model whereby cholesterol orders the monomeric state of glycosphingolipid receptors, reducing the entropic penalty of receptor association and thus favoring multimeric complexes without phase separation. This model explains how cholesterol and other sterols control the spatial organization of membrane receptors for influenza and increase viral binding avidity. A natural consequence of this finding is that local cholesterol concentration in the plasma membrane of cells may alter the binding avidity of influenza virions. Furthermore, our results demonstrate a form of cholesterol-dependent membrane organization that does not involve lipid rafts, suggesting that cholesterol's effect on cell membrane heterogeneity is likely the interplay of several different factors.

FTIR and resonance Raman studies of nitric oxide binding to H93G cavity mutants of myoglobin.

Nitric oxide (NO) binds to the myoglobin (Mb) cavity mutant, H93G, forming either a five- or six-coordinate Fe-NO complex. The H93G mutation eliminates the covalent attachment between the protein and the proximal ligand, allowing NO to bind H93G possibly from the proximal side of the heme rather than the typical diatomic binding pocket on the distal side. The question of whether NO binds on the distal or proximal side was addressed by FTIR spectroscopy of the N-O vibrational frequency nuN(-O) for a set of Mb mutants that perturb the electrostatic environment of the heme pocket. Vibrational spectra of five- and six-coordinate MbNO complexes indicate that nu(N-O) shifts (by as much as 26 cm(-1)) to higher energies for the distal mutants H64V and H64V/H93G relative to the energies of wild-type and H93G MbNO, while nu(N-O) is not affected by the proximal side mutation S92A/H93G. This result suggests that NO binds on the distal side of heme in the five- and six-coordinate MbNO complexes of H93G. Additionally, values of the Fe-NO vibrational frequency nu(Fe-NO) as measured by resonance Raman spectroscopy are reported for the distal and proximal double mutants of H93G. These results suggest that nu(Fe-NO) is not very sensitive to mutations that perturb the electrostatic environment of the heme pocket, leading to the observation that nu(N-O) and nu(Fe-NO) are not quantitatively correlated for the MbNO complexes presented here. Furthermore, nu(N-O) and nu(Fe-NO) do not correlate well with equilibrium constants for imidazole binding to the five-coordinate MbNO complexes of the H93G double mutants. The data presented here do not appear to support the presence of pi-back-bonding or an inverse trans effect of NO binding in Mb mutants that alter the electrostatic environment of the heme pocket.

Polymer-supported lipid bilayers on benzophenone-modified substrates.

Solid-supported lipid membranes are important for their roles in fundamental biophysical research as well as in applications such as biosensors. In our study, lipopolymers containing alkyl side chains were synthesized and a mixture of the lipopolymer and free lipids was preorganized at the air-water interface and then transferred to a solid substrate using the Langmuir-Blodgett technique. A photochemical reaction between a substrate-functionalized benzophenone and C-H bonds on the lipopolymer was used to attach the lipopolymers to the substrate. The final assembly of the membrane was completed by vesicle fusion. Langmuir film experiments at the air-water interface indicate tighter molecular packing for the lipopolymers with 28 mol % alkyl side chains than for the ones with 22 mol %. Atomic force microscopy images point to phase separation of lipopolymers on the substrates due to their dewetting from hydrophobic surfaces. However, a mixture of lipopolymers and free lipids formed a smooth film on the same substrate. After the addition of the second lipid layer on the lipopolymer/free lipid layer, the fluorescence images of the polymer-supported bilayer suggested that the distal lipid layer is homogeneous on the micrometer scale. The relaxation of the fluorescent probe lipids was analyzed after application of an electric field to determine their diffusion coefficient; the distal lipid layer was mobile with an average diffusion coefficient of approximately 0.1 microm(2)/s. Moreover, the immobile fraction of the lipids in the distal layer was estimated to be around 15%.

High refractive index substrates for fluorescence microscopy of biological interfaces with high z contrast.

Total internal reflection fluorescence microscopy is widely used to confine the excitation of a complex fluorescent sample very close to the material on which it is supported. By working with high refractive index solid supports, it is possible to confine even further the evanescent field, and by varying the angle of incidence, to obtain quantitative information on the distance of the fluorescent object from the surface. We report the fabrication of hybrid surfaces consisting of nm layers of SiO(2) on lithium niobate (LiNbO(3), n = 2.3). Supported lipid bilayer membranes can be assembled and patterned on these hybrid surfaces as on conventional glass. By varying the angle of incidence of the excitation light, we are able to obtain fluorescent contrast between 40-nm fluorescent beads tethered to a supported bilayer and fluorescently labeled protein printed on the surface, which differ in vertical position by only tens of nm. Preliminary experiments that test theoretical models for the fluorescence-collection factor near a high refractive index surface are presented, and this factor is incorporated into a semiquantitative model used to predict the contrast of the 40-nm bead/protein system. These results demonstrate that it should be possible to profile the vertical location of fluorophores on the nm distance scale in real time, opening the possibility of many experiments at the interface between supported membranes and living cells. Improvements in materials and optical techniques are outlined.

Electrostatic and conformational effects on the electronic structures of distortional isomers of a mixed-valence binuclear Cu complex.

The electronic structure of the binuclear copper complex [Cu(2)(L)](3+) [L = N(CH(2)CH(2)N(H)CH(2)CH(2)N(H)CH(2)CH(2))(3)N] has been investigated by resonance Raman and electroabsorption spectroscopy. Crystallographic Cu(2) distances of 2.364(1) and 2.415(1) A determined for the nitrate and acetate salts, respectively, are consistent with a substantial metal-metal interaction. The Cu-Cu bonding interaction in the binuclear complex is modulated both in the solid state and in solution by the ligand environment through coupling to ligand torsional modes that are, in turn, stabilized by hydrogen bonding. Electroabsorption data on the three major visible and near-infrared electronic transitions of Cu(2)L, lambda(max) (epsilon(max)) = 1000 nm ( approximately 1200 M(-1) cm(-1)), 748 nm (5600 M(-1) cm(-1)), and 622 nm (3350 M(-1) cm(-1)), reveal a difference dipole moment between the ground and excited states (Deltamu(A)) because of symmetry breaking. The difference polarizability for all three of the transitions is negative, indicating that the ground state is more polarizable than the excited state. A general model to explain this behavior in terms of the proximity of accessible transitions involving copper d electrons is proposed to explain the larger polarizability of the ground state. Raman excitation profiles (REPs) provide evidence for multiple conformational states of [Cu(2)(L)](3+). Separate REPs were obtained for each of the components of the two major Raman bands for nu(1) (a Cu-Cu stretching mode) and nu(2) (a Cu-Cu-N(eq) bending mode). The Raman data along with quantum chemical ZINDO/S CI calculations provide evidence for isomeric forms of Cu(2)L with strong coupling between the conformation of L and the Cu-Cu bond length.

Spatially localized generation of nucleotide sequence-specific DNA damage.

Psoralens linked to triplex-forming oligonucleotides (psoTFOs) have been used in conjunction with laser-induced two-photon excitation (TPE) to damage a specific DNA target sequence. To demonstrate that TPE can initiate photochemistry resulting in psoralen-DNA photoadducts, target DNA sequences were incubated with psoTFOs to form triple-helical complexes and then irradiated in liquid solution with pulsed 765-nm laser light, which is half the quantum energy required for conventional one-photon excitation, as used in psoralen + UV A radiation (320-400 nm) therapy. Target DNA acquired strand-specific psoralen monoadducts in a light dose-dependent fashion. To localize DNA damage in a model tissue-like medium, a DNA-psoTFO mixture was prepared in a polyacrylamide gel and then irradiated with a converging laser beam targeting the rear of the gel. The highest number of photoadducts formed at the rear while relatively sparing DNA at the front of the gel, demonstrating spatial localization of sequence-specific DNA damage by TPE. To assess whether TPE treatment could be extended to cells without significant toxicity, cultured monolayers of normal human dermal fibroblasts were incubated with tritium-labeled psoralen without TFO to maximize detectable damage and irradiated by TPE. DNA from irradiated cells treated with psoralen exhibited a 4- to 7-fold increase in tritium activity relative to untreated controls. Functional survival assays indicated that the psoralen-TPE treatment was not toxic to cells. These results demonstrate that DNA damage can be simultaneously manipulated at the nucleotide level and in three dimensions. This approach for targeting photochemical DNA damage may have photochemotherapeutic applications in skin and other optically accessible tissues.

19F NMR of trifluoroacetyl-labeled cysteine mutants of myoglobin: structural probes of nitric oxide bound to the H93G cavity mutant.

Nitric oxide (NO) binds to the myoglobin (Mb) cavity mutant, H93G, forming either a 5- or 6-coordinate Fe--NO heme complex. The H93G mutation replaces the proximal histidine of Mb with glycine, allowing exogenous ligands to occupy the proximal binding site. In the absence of the covalently attached proximal ligand, NO could bind to H93G from the proximal side of the heme rather than the typical diatomic binding pocket on the distal side when the 5-coordinate complex forms. The question of whether NO binds on the distal or proximal side was addressed by (19)F NMR. Site-directed mutagenesis was used to introduce unique cysteine residues at the protein surface on either the distal (S58C) or proximal (L149C) side, approximately equidistant from and perpendicular to the heme plane of both wild-type and H93G Mb. The cysteine thiols were alkylated with 3-bromo-1,1,1-trifluoroacetone to attach a trifluoroacetyl group at the mutation site. (19)F NMR spectra of 5-coordinate, NO bound S58C/H93G and L149C/H93G double mutants depict peaks with line widths of 100 and 23 Hz, respectively. As fluorine peaks broaden with increasing proximity to paramagnetic centers, such as 5-coordinate Fe--NO, the (19)F NMR data are consistent with NO binding in the distal heme pocket of H93G, even in the absence of a sixth axial ligand. Additionally, (19)F NMR spectra are reported for deoxy, oxy, CO, met CN, and met H(2)O forms of the labeled cysteine mutants. These results demonstrate that the fluorine probes are sensitive to subtle conformational changes in the protein structure due to ligation and oxidation state changes of the heme iron in Mb.

A photolysis-triggered heme ligand switch in H93G myoglobin.

Resonance Raman spectroscopy and step-scan Fourier transform infrared (FTIR) spectroscopy have been used to identify the ligation state of ferrous heme iron for the H93G proximal cavity mutant of myoglobin in the absence of exogenous ligand on the proximal side. Preparation of the H93G mutant of myoglobin has been previously reported for a variety of axial ligands to the heme iron (e.g., substituted pyridines and imidazoles) [DePillis, G., Decatur, S. M., Barrick, D., and Boxer, S. G. (1994) J. Am. Chem. Soc. 116, 6981-6982]. The present study examines the ligation states of heme in preparations of the H93G myoglobin with no exogenous ligand. In the deoxy form of H93G, resonance Raman spectroscopic evidence shows water to be the axial (fifth) ligand to the deoxy heme iron. Analysis of the infrared C-O and Raman Fe-C stretching frequencies for the CO adduct indicates that it is six-coordinate with a histidine trans ligand. Following photolysis of CO, a time-dependent change in ligation is evident in both step-scan FTIR and saturation resonance Raman spectra, leading to the conclusion that a conformationally driven ligand switch exists in the H93G protein. In the absence of exogenous nitrogenous ligands, the CO trans effect stabilizes endogenous histidine ligation, while conformational strain favors the dissociation of histidine following photolysis of CO. The replacement of histidine by water in the five-coordinate complex is estimated to occur in < 5 micros. The results demonstrate that the H93G myoglobin cavity mutant has potential utility as a model system for studying the conformational energetics of ligand switching in heme proteins such as those observed in nitrite reductase, guanylyl cyclase, and possibly cytochrome c oxidase.

Cell adhesion to protein-micropatterned-supported lipid bilayer membranes.

A new method for constructing controlled interfaces between cells and synthetic supported lipid bilayer membranes is reported. Microcontact printing is used to define squares and grid lines of fibronectin onto glass, which subsequently direct the self-assembly of fluid lipid bilayers onto the complementary, uncoated regions of the surface. Features of fibronectin as small as 5 microm effectively control the lateral organization of the lipid bilayers. These fibronectin barriers also facilitate the adhesion of endothelial cells, which exhibit minimal adhesion to fluid supported lipid bilayers alone. Cells selectively adhere to the features of fibronectin, spanning over and exposing the cells to the intervening regions of supported lipid bilayer. Cell spreading is correlated with both the geometry and dimensions of the fibronectin barriers. Importantly, lipids underlying adherent cells are laterally mobile, suggesting that, in contrast to the regions of fibronectin, cells were not in direct contact with the supported membrane. Protein micropatterning thus provides a valuable tool for controlling supported membranes and for juxtaposing anchorage-dependent cells with lipid bilayers. These systems should be generally useful for studying specific interactions between cells and biomolecules incorporated into supported membranes, and as an approach for integrating living cells with synthetic, laterally complex surfaces.

Molecular transport and organization in supported lipid membranes.

The mechanism by which vesicles spontaneously form supported lipid bilayer membranes on glass surfaces is becoming better understood and this knowledge is the basis of a technology of patterning membrane arrays and controlling composition. Controlled interactions between supported membranes and cells, particularly from the immune system, provide direct insight into cell-cell surface interactions.

The H93G myoglobin cavity mutant as a versatile template for modeling heme proteins: ferrous, ferric, and ferryl mixed-ligand complexes with imidazole in the cavity.

One of the difficulties in preparing accurate ambient-temperature model complexes for heme proteins, particularly in the ferric state, has been the generation of mixed-ligand adducts: complexes with different ligands on either side of the heme. The difference in the accessibility of the two sides of the heme in the H93G cavity mutant of myoglobin (Mb) provides a potential general solution to this problem. To demonstrate the versatility of H93G Mb for the preparation of heme protein models, numerous mixed-ligand adducts of ferrous, ferric, and ferryl imidazole-ligated H93G (H93G(Im) Mb) have been prepared. The complexes have been characterized by electronic absorption and magnetic circular dichroism (MCD) spectroscopy in comparison to analogous derivatives of wild type Mb. The starting ferric H93G(Im) Mb state spectroscopically resembles wild-type ferric Mb as expected for a complex containing a single imidazole in the proximal cavity and water bound on the distal side. Addition of a sixth ligand to ferric H93G(Im) Mb, whether charge neutral (imidazole) or anionic (cyanide and azide), results in formation of six-coordinate low-spin complexes with MCD characteristics similar to those of parallel derivatives of wild-type ferric Mb. Reduction of ferric H93G(Im) Mb and subsequent exposure to either CO, NO, or O2 produces ferrous complexes (deoxy, CO, NO, and O2) that consistently exhibit MCD spectra similar to the analogous ferrous species of wild-type ferrous Mb. Most interestingly, reaction of ferric H93G(Im) Mb with H2O2 results in the formation of a stable high-valent oxoferryl complex with MCD characteristics that are essentially identical to those of oxoferryl wild-type Mb. The generation of such a wide array of mixed-ligand heme complexes demonstrates the efficacy of the H93G Mb cavity mutant as a template for the preparation of heme protein model complexes.

Excited-state electronic asymmetry of the special pair in photosynthetic reaction center mutants: absorption and Stark spectroscopy.

The electronic absorption line shape and Stark spectrum of the lowest energy Q(y)() transition of the special pair in bacterial reaction centers contain a wealth of information on mixing with charge transfer states and electronic asymmetry. Both vary greatly in mutants that perturb the chemical composition of the special pair, such as the heterodimer mutants, and in mutants that alter interactions between the special pair and the surrounding reaction center protein, such as those that add or remove hydrogen bonds. The conventional and higher-order Stark spectra of a series of mutants are presented with the aim of developing a systematic description of the electronic structure of the excited state of the special pair that initiates photosynthetic charge separation. The mutants L168HF, M197FH, L131LH and L131LH/M160LH/M197FH are known to have different hydrogen-bonding patterns to the special pair; however, they exhibit Stark effects that are very similar to wild type. By contrast, the addition of a hydrogen bond to the M-side keto carbonyl group of the special pair in M160LH greatly affects both the absorption and Stark spectra. The heterodimer special pairs, L173HL and M202HL, exhibit much larger Stark effects than wild type, with the greatest effect in the M-side mutant. Double mutants that combine the M-side heterodimer and a hydrogen-bond addition to the L-side of the special pair decrease the magnitude of the Stark effect. These results suggest that the electronic asymmetry of the dimer can be perturbed either by the formation of a heterodimer or by adding or deleting a hydrogen bond to a keto carbonyl group. From the pattern observed, it is concluded that the charge transfer state P(L)(+)P(M)(-) has a larger influence on the excited state of the dimer in wild type than the P(L)(-)P(M)(+)charge transfer state. Furthermore, asymmetry can be varied continuously, from extreme cases in which the heterodimer and hydrogen-bond effects work together, to cases in which hydrogen bonding offsets the effects of the heterodimer, to cases in which the homodimer is perturbed by hydrogen bonds. This leads to a unified model for understanding the effects of perturbations on the electronic symmetry of the special pair, and this can be connected with perturbations on the properties of many other systems such as donor-acceptor-substituted polyenes.

Hydrogen bonding modulates binding of exogenous ligands in a myoglobin proximal cavity mutant.

In the sperm whale myoglobin mutant H93G, the proximal histidine is replaced by glycine, leaving a cavity in which exogenous imidazole can bind and ligate the heme iron (Barrick, D. (1994) Biochemistry 33, 6545-6554). Structural studies of this mutant suggest that serine 92 may play an important role in imidazole binding by serving as a hydrogen bond acceptor. Serine 92 is highly conserved in myoglobins, forming a well-characterized weak hydrogen bond with the proximal histidine in the native protein. We have probed the importance of this hydrogen bond through studies of the double mutants S92A/H93G and S92T/H93G incorporating exogenous imidazole and methylimidazoles. (1)H NMR spectra reveal that loss of the hydrogen bond in S92A/H93G does not affect the conformation of the bound imidazole. However, the binding constants for imidazoles to the ferrous nitrosyl complex of S92A/H93G are much weaker than in H93G. These results are discussed in terms of hydrogen bonding and steric packing within the proximal cavity. The results also highlight the importance of the trans diatomic ligand in altering the binding and sensitivity to perturbation of the ligand in the proximal cavity.

Brownian ratchets: molecular separations in lipid bilayers supported on patterned arrays.

Brownian ratchets use a time-varying asymmetric potential that can be applied to separate diffusing particles or molecules. A new type of Brownian ratchet, a geometrical Brownian ratchet, has been realized. Charged, fluorescently labeled phospholipids in a two-dimensional fluid bilayer were driven in one direction by an electric field through a two-dimensional periodic array of asymmetric barriers to lateral diffusion fabricated from titanium oxide on silica. Diffusion spreads the phospholipid molecules in the orthogonal direction, and the asymmetric barriers rectify the Brownian motion, causing a directional transport of molecules. The geometrical ratchet can be used as a continuous molecular sieve to separate mixtures of membrane-associated molecules that differ in electrophoretic mobility and diffusion coefficient.

Assignment of the heme axial ligand(s) for the ferric myoglobin (H93G) and heme oxygenase (H25A) cavity mutants as oxygen donors using magnetic circular dichroism.

UV-visible absorption and magnetic circular dichroism (MCD) data are reported for the cavity mutants of sperm whale H93G myoglobin and human H25A heme oxygenase in their ferric states at 4 degreesC. Detailed spectral analyses of H93G myoglobin reveal that its heme coordination structure has a single water ligand at pH 5.0, a single hydroxide ligand at pH 10.0, and a mixture of species at pH 7.0 including five-coordinate hydroxide-bound, and six-coordinate structures. The five-coordinate aquo structure at pH 5 is supported by spectral similarity to acidic horseradish peroxidase (pH 3.1), whose MCD data are reported herein for the first time, and acidic myoglobin (pH 3.4), whose structures have been previously assigned by resonance Raman spectroscopy. The five-coordinate hydroxide structure at pH 10.0 is supported by MCD and resonance Raman data obtained here and by comparison with those of other known five-coordinate oxygen donor complexes. In particular, the MCD spectrum of alkaline ferric H93G myoglobin is strikingly similar to that of ferric tyrosinate-ligated human H93Y myoglobin, whose MCD data are reported herein for the first time, and that of the methoxide adduct of ferric protoporphyrin IX dimethyl ester (FeIIIPPIXDME). Analysis of the spectral data for ferric H25A heme oxygenase at neutral pH in the context of the spectra of other five-coordinate ferric heme complexes with proximal oxygen donor ligands, in particular the p-nitrophenolate and acetate adducts of FeIIIPPIXDME, is most consistent with ligation by a carboxylate group of a nearby glutamyl (or aspartic) acid residue.

Electric field-induced critical demixing in lipid bilayer membranes.

Electric fields can induce lateral reorganization of lipids in fluid bilayer membranes. The resulting concentration profiles readily are observed in planar-supported bilayers by epifluorescence microscopy. When a fluorescently labeled lipid was used to probe the field-induced separation of cardiolipin from egg-phosphatidylcholine, an enhanced sensitivity to the electric field was observed that is attributed to a critical demixing effect. A thermodynamic model of the system was used to analyze the results. The observed concentration profiles can be understood if the lipid mixture has a critical temperature equal to 75 degrees K. The steady-state distribution of lipids under the influence of an electric field is very sensitive to demixing effects, even at temperatures well above the critical temperature for spontaneous phase separation, and this may have significant consequences for organization and structural changes in natural cell membranes.

Electric field-induced reorganization of two-component supported bilayer membranes.

Application of electric fields tangent to the plane of a confined patch of fluid bilayer membrane can create lateral concentration gradients of the lipids. A thermodynamic model of this steady-state behavior is developed for binary systems and tested with experiments in supported lipid bilayers. The model uses Flory's approximation for the entropy of mixing and allows for effects arising when the components have different molecular areas. In the special case of equal area molecules the concentration gradient reduces to a Fermi-Dirac distribution. The theory is extended to include effects from charged molecules in the membrane. Calculations show that surface charge on the supporting substrate substantially screens electrostatic interactions within the membrane. It also is shown that concentration profiles can be affected by other intermolecular interactions such as clustering. Qualitative agreement with this prediction is provided by comparing phosphatidylserine- and cardiolipin-containing membranes.

Crystal structure and photodynamic behavior of the blue emission variant Y66H/Y145F of green fluorescent protein.

The crystal structure of a blue emission variant (Y66H/Y145F) of the Aequorea victoria green fluorescent protein has been determined by molecular replacement and the model refined. The crystallographic R-factor is 18.1% for all data from 20 to 2.1 A, and the model geometry is excellent. The chromophore is non-native and is autocatalytically generated from the internal tripeptide Ser65-His66-Gly67. The final electron density maps indicate that the formation of the chromophore is complete, including 1,2 dehydration of His66 as indicated by the planarity of the chromophore. The chromophore is in the cis conformation, with no evidence for any substantial fraction of the trans configuration or uncyclized apoprotein, and is well-shielded from bulk solvent by the folded protein. These characteristics indicate that the machinery for production of the chromophore from a buried tripeptide unit is not only intact but also highly efficient in spite of a major change in chromophore chemical structure. Nevertheless, there are significant rearrangements in the hydrogen bond configuration around the chromophore as compared to wild-type, indicating flexibility of the active site. pH titration of the intact protein and the chromopeptide (pKa1 = 4.9 +/- 0.1, pKa2 = 12.0 +/- 0.1) suggests that the predominant form of the chromophore in the intact protein is electrically neutral. In contrast to the wild-type protein [Chattoraj, M., King, B. A., Bublitz, G. U., & Boxer, S. G. (1996) Proc. Natl. Acad. Sci. U.S.A., 8362-8367], femtosecond fluorescence up-conversion spectroscopy of the intact protein and a partially deuterated form strongly suggests that excited-state proton transfer is not coupled to fluorescence emission.

Temperature dependence of the Qy resonance Raman spectra of bacteriochlorophylls, the primary electron donor, and bacteriopheophytins in the bacterial photosynthetic reaction center.

Qy-excited resonance Raman spectra of the accessory bacteriochlorophylls (B), the bacteriopheophytins (H), and the primary electron donor (P) in the bacterial photosynthetic reaction center (RC) of Rhodobacter sphaeroides have been obtained at 95 and 278 K. Frequency and intensity differences are observed in the low-frequency region of the P vibrational spectrum when the sample is cooled from 278 to 95 K. The B and H spectra exhibit minimal changes of frequencies and relative intensities as a function of temperature. The mode patterns in the Raman spectra of B and H differ very little from Raman spectra of the chromophores in vitro. The Raman scattering cross sections of B and H are 6-7 times larger than those for analogous modes of P at 278 K. The cross sections of B and of H are 3-4 times larger at 95 K than at 278 K, while the cross sections of P are approximately constant with temperature. The temperature dependence of the Raman cross sections for B and H suggests that pure dephasing arising from coupling to low-frequency solvent/protein modes is important in the damping of their excited states. The weak Raman cross sections of the special pair suggest that the excited state of P is damped by very rapid (<<30 fs) electronic relaxation processes. These resonance Raman spectra provide information for developing multimode vibronic models of the excited-state structure and dynamics of the chromophores in the RC.

Large molecular third-order optical nonlinearities in polarized carotenoids.

Garito and co-workers have suggested a mechanism to dramatically increase the second hyperpolarizability, gamma, in linear pi-electron-conjugated molecules. Polarization is introduced that leads to a difference between the dipole moments of the molecule's ground state and excited state. Here a series of carotenoids was examined that had increasing intramolecular charge transfer (ICT) from the polyenic chain to the acceptor moiety in the ground state, and gamma was measured for these compounds as a function of wavelength by third-harmonic generation. The compound with the greatest ICT exhibited a 35-fold enhancement of gammamax (the gamma measured at the peak of the three-photon resonance) relative to the symmetric molecule beta-carotene, which itself has one of the largest third-order nonlinearities known. Stark spectroscopic measurements revealed the existence of a large difference dipole moment, Delta mu, between the ground and excited state. Quantum-chemical calculations underline the importance of interactions involving states with large Delta mu.