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Berlin Brandenburg

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  • 1
    Language: English
    In: Journal of the American Chemical Society, 24 April 2013, Vol.135(16), pp.6300-6
    Description: Monolayers of the redox protein Cytochrome C (CytC) can be electrostatically formed on an H-terminated Si substrate, if the protein- and Si-surface are prepared so as to carry opposite charges. With such monolayers we study electron transport (ETp) via CytC, using a solid-state approach with macroscopic electrodes. We have revealed that currents via holo-CytC are almost 3 orders of magnitude higher than via the heme-depleted protein (→ apo-CytC). This large difference in currents is attributed to loss of the proteins' secondary structure upon heme removal. While removal of only the Fe ion (→ porphyrin-CytC) does not significantly change the currents via this protein at room temperature, the 30-335 K temperature dependence suggests opening of a new ETp pathway, which dominates at high temperatures (〉285 K). These results suggest that the cofactor plays a major role in determining the ETp pathway(s) within CytC.
    Keywords: Electron Transport ; Cytochromes C -- Chemistry ; Heme -- Chemistry ; Iron -- Chemistry ; Silicones -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 2
    Language: English
    In: Journal of the American Chemical Society, 07 November 2012, Vol.134(44), pp.18221-4
    Description: Electrons can migrate via proteins over distances that are considered long for nonconjugated systems. The nanoscale dimensions of proteins and their enormous structural and chemical flexibility makes them fascinating subjects for exploring their electron transport (ETp) capacity. One particularly attractive direction is that of tuning their ETp efficiency by "doping" them with small molecules. Here we report that binding of retinoate (RA) to human serum albumin (HSA) increases the solid-state electronic conductance of a monolayer of the protein by 〉2 orders of magnitude for RA/HSA ≥ 3. Temperature-dependent ETp measurements show the following with increasing RA/HSA: (a) The temperature-independent current magnitude of the low-temperature (300-fold), suggesting a decrease in the distance-decay constant of the process. (b) The activation energy of the thermally activated regime (〉190 K) decreases from 220 meV (RA/HSA = 0) to 70 meV (RA/HSA ≥ 3).
    Keywords: Serum Albumin -- Chemistry ; Tretinoin -- Analogs & Derivatives
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 3
    Language: English
    In: Journal of the American Chemical Society, 09 September 2015, Vol.137(35), pp.11226-9
    Description: We observe temperature-independent electron transport, characteristic of tunneling across a ∼6 nm thick Halorhodopsin (phR) monolayer. phR contains both retinal and a carotenoid, bacterioruberin, as cofactors, in a trimeric protein-chromophore complex. This finding is unusual because for conjugated oligo-imine molecular wires a transition from temperature-independent to -dependent electron transport, ETp, was reported at ∼4 nm wire length. In the ∼6 nm long phR, the ∼4 nm 50-carbon conjugated bacterioruberin is bound parallel to the α-helices of the peptide backbone. This places bacterioruberin's ends proximal to the two electrodes that contact the protein; thus, coupling to these electrodes may facilitate the activation-less current across the contacts. Oxidation of bacterioruberin eliminates its conjugation, causing the ETp to become temperature dependent (〉180 K). Remarkably, even elimination of the retinal-protein covalent bond, with the fully conjugated bacterioruberin still present, leads to temperature-dependent ETp (〉180 K). These results suggest that ETp via phR is cooperatively affected by both retinal and bacterioruberin cofactors.
    Keywords: Temperature ; Carotenoids -- Chemistry ; Halorhodopsins -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 4
    Language: English
    In: Journal of the American Chemical Society, 02 March 2011, Vol.133(8), pp.2421-3
    Description: The temperature dependence of current-voltage values of electron transport through proteins integrated into a solid-state junction has been investigated. These measurements were performed from 80 up to 400 K [above the denaturation temperature of azurin (Az)] using Si/Az/Au junctions that we have described previously. The current across the ∼3.5 nm thick Az junction was temperature-independent over the complete range. In marked contrast, for both Zn-substituted and apo-Az (i.e., Cu-depleted Az), thermally activated behavior was observed. These striking temperature-dependence differences are ascribed to the pivotal function of the Cu ion as a redox center in the solid-state electron transport process. Thus, while Cu enabled temperature-independent electron transport, upon its removal the polypeptide was capable only of supporting thermally activated transport.
    Keywords: Temperature ; Azurin -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 5
    Language: English
    In: Journal of the American Chemical Society, 07 March 2012, Vol.134(9), pp.4169-76
    Description: Electron transport (ETp) across bacteriorhodopsin (bR), a natural proton pump protein, in the solid state (dry) monolayer configuration, was studied as a function of temperature. Transport changes from thermally activated at T 〉 200 K to temperature independent at 〈130 K, similar to what we have observed earlier for BSA and apo-azurin. The relatively large activation energy and high temperature stability leads to conditions where bR transports remarkably high current densities above room temperature. Severing the chemical bond between the protein and the retinal polyene only slightly affected the main electron transport via bR. Another thermally activated transport path opens upon retinal oxime production, instead of or in addition to the natural retinal. Transport through either or both of these paths occurs on a background of a general temperature-independent transport. These results lead us to propose a generalized mechanism for ETp across proteins, in which tunneling and hopping coexist and dominate in different temperature regimes.
    Keywords: Temperature ; Bacteriorhodopsins -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 6
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 08 January 2013, Vol.110(2), pp.507-12
    Description: Measuring solid-state electron transport (ETp) across proteins allows studying electron transfer (ET) mechanism(s), while minimizing solvation effects on the process. ETp is, however, sensitive to any static (conformational) or dynamic (vibrational) changes in the protein. Our macroscopic measurements allow extending ETp studies to low temperatures, with the concomitant resolution of lower current densities, because of the larger electrode contact areas. Thus, earlier we reported temperature-independent ETp via the copper protein azurin (Az), from 80 K until denaturation, whereas for apo-Az ETp was temperature dependent above 180 K. Deuteration (H/D substitution) may provide mechanistic information on the question of whether the ETp involves H-bonds in the solid state. Here we report results of kinetic deuterium isotope effect (KIE) measurements on ETp through holo-Az as a function of temperature (30-340 K). Strikingly, deuteration changed ETp from temperature independent to temperature dependent above 180 K. This H/D effect is expressed in KIE values between 1.8 (340 K) and 9.1 (≤ 180 K). These values are remarkable in light of the previously reported inverse KIE on ET in Az in solution. We ascribe the difference between our KIE results and those observed in solution to the dominance of solvent effects in the latter (larger thermal expansion in H(2)O than in D(2)O), whereas in our case the KIE is primarily due to intramolecular changes, mainly in the low-frequency structural modes of the protein caused by H/D exchange. The observed high KIE values are consistent with a transport mechanism that involves through-H-bonds of the β-sheet structure of Az, likely also those in the Cu coordination sphere.
    Keywords: Protein Conformation ; Alcaligenes Faecalis -- Chemistry ; Azurin -- Metabolism ; Electron Transport -- Physiology
    ISSN: 00278424
    E-ISSN: 1091-6490
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  • 7
    Language: English
    In: Biochemistry, 19 February 2013, Vol.52(7), pp.1290-301
    Description: Xanthorhodopsin (xR) is a retinal protein that contains, in addition to the retinal chromophore, a carotenoid (salinixanthin) that functions as a light-harvesting antenna [Balashov, S. P., et al. (2005) Science 309, 2061-2064]. The center-center distance between the two polyene chains is 12-13 Å, but the distance between the two rings of retinal and salinixanthin is surprisingly small (~5 Å) with an angle of ~45° [Luecke, H., et al. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 16561-16565]. We aimed to clarify the role of the β-ionone ring in the binding of retinal to apo-xR, as well as a possible role that the β-ionone ring plays in fixation of the salinixanthin 4-keto ring. The binding of native retinal and series of synthetic retinal analogues modified in the β-ionone ring to apo-xR was monitored by absorption and circular dichroism (CD) spectroscopies. The results indicate that the β-ionone ring modification significantly affected formation of the retinal-protein covalent bond as well as the pigment absorption and CD spectra. It was observed that several retinal analogues, modified in the retinal β-ionone ring, did not bind to apo-xR and did not form the pigment. Also, none of these analogues induced the fixation of the salinixanthin 4-keto ring. In addition, we show that the native retinal within its binding site adopts exclusively the 6-s-trans ring-chain conformation.
    Keywords: Bacterial Proteins -- Chemistry ; Carotenoids -- Chemistry ; Glycosides -- Chemistry ; Retinaldehyde -- Chemistry ; Rhodopsins, Microbial -- Chemistry
    ISSN: 00062960
    E-ISSN: 1520-4995
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  • 8
    Language: English
    In: Biochemistry, 26 May 2015, Vol.54(20), pp.3164-72
    Description: A member of the retinal protein family, halorhodopsin, acts as an inward light-driven Cl(-) pump. It was recently demonstrated that the Natronomonas pharaonis halorhodopsin-overproducing mutant strain KM-1 contains, in addition to the retinal chromophore, a lipid soluble chromophore, bacterioruberin, which binds to crevices between adjacent protein subunits. It is established that halorhodopsin has several chloride binding sites, with binding site I, located in the retinal protonated Schiff base vicinity, affecting retinal absorption. However, it remained unclear whether cations also bind to this protein. Our electron paramagnetic resonance spectroscopy examination of cation binding to the halorhodopsin mutant KM-1 reveals that divalent cations like Mn(2+) and Ca(2+) bind to the protein. Halorhodopsin has a high affinity for Mn(2+) ions, which bind initially to several strong binding sites and then to binding sites that exhibit positive cooperativity. The binding behavior is pH-dependent, and its strength is influenced by the nature of counterions. Furthermore, the binding strength of Mn(2+) ions decreases upon removal of the retinal chromophore from the protein or following bacterioruberin oxidation. Our results also indicate that Mn(2+) ions, as well as Cl(-) ions, first occupy binding sites other than site I. The observed synergetic effect between cation and anion binding suggests that while Cl(-) anions bind to halorhodopsin at low concentrations, the occupancy of site I requires a high concentration.
    Keywords: Halorhodopsins -- Chemistry ; Manganese -- Chemistry
    ISSN: 00062960
    E-ISSN: 1520-4995
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  • 9
    Language: English
    In: Journal of the American Chemical Society, Dec 28, 2011, Vol.133(51), p.20922-20932
    Description: Femtosecond spectroscopy in conjunction with extraction of coherent vibrational signatures are used to demonstrate the utility of cis-to-trans photoisomerization, as in visual pigments. The results revealed that cis-to-trans photoisomerization is ballistic and over in a fraction of a picosecond, while the reverse is nearly 10 times slower and kinetically reminiscent of other microbial rhodopsins and provides a new test case for appreciating medium effects on primary events in retinal proteins (RPs).
    Keywords: Active Sites (Biochemistry) -- Research ; Isomerization -- Analysis ; Photochemistry -- Research ; Rhodopsin -- Optical Properties ; Rhodopsin -- Chemical Properties
    ISSN: 0002-7863
    Source: Cengage Learning, Inc.
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  • 10
    Language: English
    In: Journal of the American Chemical Society, 16 February 2011, Vol.133(6), pp.1626-9
    Description: A VIS pump/hyperspectral NIR probe study of all-trans-retinal protonated Schiff base (RPSB) in ethanol is presented. Upon irradiation, a short-lived absorption band covers the recorded range of λ = 1-2 μm. It decays to reveal the tail of S(1) emission at λ 〈 1.3 μm, along with a residual absorption at longer wavelengths, both of which decay with the known kinetics of internal conversion to S(0). The existence of this hitherto unrecorded excited-state absorption deep in the NIR will require a revision of current models for RPSB electronic structure. The phenomenological similarity of these observations with ultrafast NIR studies of carotenoids raises the question of whether three, and not two, electronic states participate in RPSB photochemistry as well. The relevance of these observations to retinal protein photochemistry is discussed.
    Keywords: Photochemical Processes ; Spectrum Analysis ; Eye Proteins -- Chemistry ; Retina -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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