Lavoisier Ramos

Washington University in St. Louis, Saint Louis, MO, United States

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Publications (9)27.38 Total impact

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    SOJ Biochemistry. 02/2014; 1(1):12.
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    ABSTRACT: Seven perylene-porphyrin dyads were examined with the goal of identifying those most suitable for components of light-harvesting systems. The ideal dyad should exhibit strong absorption by the perylene in the green, undergo rapid and efficient excited-state energy transfer from perylene to porphyrin, and avoid electron-transfer quenching of the porphyrin excited state by the perylene in the medium of interest. Four dyads have different perylenes at the p-position of the meso-aryl group on the zinc porphyrin. The most suitable perylene identified in that set was then incorporated at the m- or o-position of the zinc porphyrin, affording two other dyads. An analogue of the o-substituted architecture was prepared in which the zinc porphyrin was replaced with the free base porphyrin. The perylene in each dyad is a monoimide derivative; the perylenes differ in attachment of the linker (either via a diphenylethyne linker at the N-imide or an ethynylphenyl linker at the C9 position) and the number (0-3) of 4-tert-butylphenoxy groups (which increase solubility and slightly alter the electrochemical potentials). In the p-linked dyad, the monophenoxy perylene with an N-imide diphenylethyne linker is superior in providing rapid and essentially quantitative energy transfer from excited perylene to zinc porphyrin with minimal electron-transfer quenching in both toluene and benzonitrile. The dyads with the same perylene at the m- or o-position exhibited similar results except for one case, the o-linked dyad bearing the zinc porphyrin in benzonitrile, where significant excited-state quenching is observed; this phenomenon is facilitated by close spatial approach of the perylene and porphyrin and the associated thermodynamic/kinetic enhancement of the electron-transfer process. Such quenching does not occur with the free base porphyrin because electron transfer is thermodynamically unfavorable even in the polar medium. The p-linked dyad containing a zinc porphyrin attached to a bis(4-tert-butylphenoxy)perylene via an ethynylphenyl linker at the C9 position exhibits ultrafast and quantitative energy transfer in toluene; the same dyad in benzonitrile exhibits ultrafast (<0.5 ps) perylene-to-porphyrin energy transfer, rapid (∼5 ps) porphyrin-to-perylene electron transfer, and fast (∼25 ps) charge recombination to the ground state. Collectively, this study has identified suitable perylene-porphyrin constructs for use in light-harvesting applications.
    The Journal of Physical Chemistry B 11/2010; 114(45):14249-64. · 3.61 Impact Factor
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    ABSTRACT: The absorption maximum of blue proteorhodopsin (BPR) is the most blue-shifted of all retinal proteins found in archaea or bacteria, with the exception of sensory rhodopsin II (SRII). The absorption spectrum also exhibits a pH dependence larger than any other retinal protein. We examine the structural origins of these two properties of BPR by using optical spectroscopy, homology modeling, and molecular orbital theory. Bacteriorhodopsin (BR) and SRII are used as homology parents for comparative purposes. We find that the tertiary structure of BPR based on SRII is more realistic with respect to free energy, dynamic stability, and spectroscopic properties. Molecular orbital calculations including full single- and double-configuration interaction within the chromophore pi-electron system provide perspectives on the wavelength regulation in this protein and indicate that Arg-95, Gln-106, Glu-143, and Asp-229 play important, and in some cases pH-dependent roles. A possible model for the 0.22 eV red shift of BPR at low pH is examined, in which Glu-143 becomes protonated and releases Arg-95 to rotate up into the binding site, altering the electrostatic environment of the chromophore. At high pH, BPR has spectroscopic properties similar to SRII, but at low pH, BPR has spectroscopic properties more similar to BR. Nevertheless, SRII is a significantly better homology model for BPR and opens up the question of whether this protein serves as a proton pump, as commonly believed, or is a light sensor with structure-function properties more comparable to those of SRII. The function of BPR in the native organism is discussed with reference to the results of the homology model.
    Biochemistry 03/2006; 45(6):1579-90. · 3.38 Impact Factor
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    ABSTRACT: Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-tert-butylphenyl) group show low fluorescence yields (approximately 0.06) and short excited-singlet-state lifetimes (approximately 500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (approximately 0.9) and long excited-state lifetimes (approximately 6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of approximately 60 degrees with respect to the dipyrrin framework whereas the angle is approximately 80 degrees for mesityl or o-tolyl groups. The calculated potential energy surface for the phenyl-substituted complex indicates that the excited state has a second, lower energy minimum in which the nonhindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a complex bearing the sterically hindered o-tolyl or mesityl group at the 5-position, leading to a high radiative probability involving conformations at or near the initial Franck-Condon form of the excited state. These combined results demonstrate the critical role of aryl-ring rotation in governing the excited-state dynamics of this class of widely used dyes.
    The Journal of Physical Chemistry B 11/2005; 109(43):20433-43. · 3.61 Impact Factor
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    ABSTRACT: The photochemistry of the 13-desmethyl (DM) analogue of bacteriorhodopsin (BR) is examined by using spectroscopy, molecular orbital theory, and chromophore extraction followed by conformational analysis. The removal of the 13-methyl group permits the direct photochemical formation of a thermally stable, photochemically reversible state, P1(DM) (lambda(max) = 525 nm), which can be generated efficiently by exciting the resting state, bR(DM) with yellow or red light (lambda > 590 nm). Chromophore extraction analysis reveals that the retinal configuration in P1(DM) is 9-cis, identical to that of the retinal configuration in the native BR P1 state. Fourier transform infrared and Raman experiments on P1(DM) indicate an anti configuration around the C15=N bond, as would be expected of an O-state photoproduct. However, low-temperature spectroscopy and ambient, time-resolved studies indicate that the P1(DM) state forms primarily via thermal relaxation from the L(D)(DM) state. Theoretical studies on the BR binding site show that 13-dm retinal is capable of isomerizing into a 9-cis configuration with minimal steric hindrance from surrounding residues, in contrast to the native chromophore in which surrounding residues significantly obstruct the corresponding motion. Analysis of the photokinetic experiments indicates that the Arrhenius activation energy of the bR(DM) --> P1(DM) transition in 13-dm-BR is less than 0.6 kcal/mol (vs 22 +/-5 kcal/mol measured for the bR --> P (P1 and P2) reaction in 85:15 glycerol:water suspensions of wild type). Consequently, the P1(DM) state in 13-dm-BR can form directly from all-trans, 15-anti intermediates (bR(DM) and O(DM)) or all-trans, 15-syn (K(D)(DM)/L(D)(DM)) intermediates. This study demonstrates that the 13-methyl group, and its interactions with nearby binding site residues, is primarily responsible for channeling one-photon photochemical and thermal reactions and is limited to the all-trans and 13-cis species interconversions in the native protein.
    The Journal of Physical Chemistry B 08/2005; 109(33):16142-52. · 3.61 Impact Factor
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    ABSTRACT: For visual pigments, a covalent bond between the ligand (11-cis-retinal) and receptor (opsin) is crucial to spectral tuning and photoactivation. All photoreceptors have retinal bound via a Schiff base (SB) linkage, but only UV-sensitive cone pigments have this moiety unprotonated in the dark. We investigated the dynamics of mouse UV (MUV) photoactivation, focusing on SB protonation and the functional role of a highly conserved acidic residue (E108) in the third transmembrane helix. On illumination, wild-type MUV undergoes a series of conformational changes, batho --> lumi --> meta I, finally forming the active intermediate meta II. During the dark reactions, the SB becomes protonated transiently. In contrast, the MUV-E108Q mutant formed significantly less batho that did not decay through a protonated lumi. Rather, a transition to meta I occurred above approximately 240 K, with a remarkable red shift (lambda(max) approximately 520 nm) accompanying SB protonation. The MUV-E108Q meta I --> meta II transition appeared normal but the MUV-E108Q meta II decay to opsin and free retinal was dramatically delayed, resulting in increased transducin activation. These results suggest that there are two proton donors during the activation of UV pigments, the primary counterion E108 necessary for protonation of the SB during lumi formation and a second one necessary for protonation of meta I. Inactivation of meta II in SWS1 cone pigments is regulated by the primary counterion. Computational studies suggest that UV pigments adopt a switch to a more distant counterion, E176, during the lumi to meta I transition. The findings with MUV are in close analogy to rhodopsin and provides further support for the importance of the counterion switch in the photoactivation of both rod and cone visual pigments.
    Proceedings of the National Academy of Sciences 01/2004; 101(4):941-6. · 9.81 Impact Factor
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    ABSTRACT: The bacteriorhodopsin branched-photocycle intermediates P and Q are studied with respect to chromophore isomeric content, photochemical origin, kinetic heterogeneity, and photoreversibility. These blue-shifted species are compared to products with similar spectroscopic properties generated via thermal denaturation. We observe that the thermal and photochemical species differ in both isomeric content and binding site environment. Sequential two-photon activation of glycerol suspensions of bacteriorhodopsin containing low concentrations of water (<15% v/v water/glycerol) form high yields of P state but almost no Q state. This observation is attributed to the role that water plays in the hydrolysis reaction that converts the P state to Q. Relatively large photostationary state populations of both intermediates can be generated in both 85% v/v glycerol suspensions and polyacrylamide gels. At ambient temperature, both intermediates can be fully converted back to bR with blue light. Chromophore extraction and HPLC analysis reveal that P, Q, and the spectrally similar thermal products have a 9-cis retinal chromophore. The thermal products are also found to contain small amounts of the 7-cis isomer. Time-resolved spectroscopy reveals that the P state is actually comprised of two components with maximum absorptivities at 445 and 525 nm. The molar absorptivities of the chromophore band maxima of the P and Q states in an 85% v/v glycerol/water suspension at pH 7 are ε(P445) = 47 000, ε(P525) = 39 000, and ε(Q) = 33 000 M-1 cm-1. Kinetic analyses support previous studies indicating that the P state is formed predominantly from the O state, which is consistent with the branched-photocycle model. However, other paths to the P state are possible, including direct excitation of the small population of blue membrane present in solution. We examine the hypothesis that the branched-photocycle evolved as a photochromic sunscreen for UVA and UVB protection.
    J. Phys. Chem. B. 11/2002; 106:13352-13361.
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    ABSTRACT: The photochemical and subsequent thermal reactions of the mouse short-wavelength visual pigment (MUV) were studied by using cryogenic UV-visible and FTIR difference spectroscopy. Upon illumination at 75 K, MUV forms a batho intermediate (lambda(max) approximately 380 nm). The batho intermediate thermally decays to the lumi intermediate (lambda(max) approximately 440 nm) via a slightly blue-shifted intermediate not observed in other photobleaching pathways, BL (lambda(max) approximately 375 nm), at temperatures greater than 180 K. The lumi intermediate has a significantly red-shifted absorption maximum at 440 nm, suggesting that the retinylidene Schiff base in this intermediate is protonated. The lumi intermediate decays to an even more red-shifted meta I intermediate (lambda(max) approximately 480 nm) which in turn decays to meta II (lambda(max) approximately 380 nm) at 248 K and above. Differential FTIR analysis of the 1100-1500 cm(-1) region reveals an integral absorptivity that is more than 3 times smaller than observed in rhodopsin and VCOP. These results are consistent with an unprotonated Schiff base chromophore. We conclude that the MUV-visual pigment possesses an unprotonated retinylidene Schiff base in the dark state, and undergoes a protonation event during the photobleaching cascade.
    Biochemistry 09/2002; 41(31):9842-51. · 3.38 Impact Factor
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    ABSTRACT: Seven perylene-porphyrin dyads were examined with the goal of identifying those most suitable for components of light-harvesting systems. The ideal dyad should exhibit strong absorption by the perylene in the green, undergo rapid and efficient excited-state energy transfer from perylene to porphyrin, and avoid electron-transfer quenching of the porphyrin excited state by the perylene in the medium of interest. Four dyads have different perylenes at the p-position of the meso-aryl group on the zinc porphyrin. The most suitable perylene identified in that set was then incorporated at the m-or o-position of the zinc porphyrin, affording two other dyads. An analogue of the o-substituted architecture was prepared in which the zinc porphyrin was replaced with the free base porphyrin. The perylene in each dyad is a monoimide derivative; the perylenes differ in attachment of the linker (either via a diphenylethyne linker at the N-imide or an ethynylphenyl linker at the C9 position) and the number (0-3) of 4-tert-butylphenoxy groups (which increase solubility and slightly alter the electrochemical potentials). In the p-linked dyad, the monophenoxy perylene with an N-imide diphenylethyne linker is superior in providing rapid and essentially quantitative energy transfer from excited perylene to zinc porphyrin with minimal electron-transfer quenching in both toluene and benzonitrile. The dyads with the same perylene at the m-or o-position exhibited similar results except for one case, the o-linked dyad bearing the zinc porphyrin in benzonitrile, where significant excited-state quenching is observed; this phenomenon is facilitated by close spatial approach of the perylene and porphyrin and the associated thermodynamic/kinetic enhancement of the electron-transfer process. Such quenching does not occur with the free base porphyrin because electron transfer is thermodynamically unfavorable even in the polar medium. The p-linked dyad containing a zinc porphyrin attached to a bis(4-tert-butylphenoxy)perylene via an ethynylphenyl linker at the C9 position exhibits ultrafast and quantitative energy transfer in toluene; the same dyad in benzonitrile exhibits ultrafast (<0.5 ps) perylene-to-porphyrin energy transfer, rapid (∼5 ps) porphyrin-to-perylene electron transfer, and fast (∼25 ps) charge recombination to the ground state. Collectively, this study has identified suitable perylene-porphyrin constructs for use in light-harvesting applications.

Publication Stats

163 Citations
27.38 Total Impact Points

Institutions

  • 2005–2010
    • Washington University in St. Louis
      • Department of Chemistry
      Saint Louis, MO, United States
  • 2004–2006
    • University of Connecticut
      • Department of Chemistry
      Storrs, CT, United States
  • 2002
    • Syracuse University
      Syracuse, New York, United States