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ABSTRACT: Photosynthetic organisms have evolved diverse light-harvesting complexes to harness light of various qualities and intensities. Photosynthetic bacteria can have (bacterio)chlorophyll Q(y) antenna absorption bands ranging from approximately 650 to approximately 1100 nm. This broad range of wavelengths has allowed many organisms to thrive in unique light environments. Roseiflexus castenholzii is a niche-adapted, filamentous anoxygenic phototroph (FAP) that lacks chlorosomes, the dominant antenna found in most green bacteria, and here we describe the purification of a full complement of photosynthetic complexes: the light-harvesting (LH) antenna, reaction center (RC), and core complex (RC-LH). By high-performance liquid chromatography separation of bacteriochlorophyll and bacteriopheophytin pigments extracted from the core complex and the RC, the number of subunits that comprise the antenna was determined to be 15 +/- 1. Resonance Raman spectroscopy of the carbonyl stretching region displayed modes indicating that 3C-acetyl groups of BChl a are all involved in molecular interactions probably similar to those found in LH1 complexes from purple photosynthetic bacteria. Finally, two-dimensional projections of negatively stained core complexes and the LH antenna revealed a closed, slightly elliptical LH ring with an average diameter of 130 +/- 10 A surrounding a single RC that lacks an H-subunit but is associated with a tetraheme c-type cytochrome.
Biochemistry 09/2010; 49(35):7524-31. · 3.42 Impact Factor
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Angewandte Chemie International Edition 02/2008; 47(4):693-6. · 13.45 Impact Factor
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Journal of the American Chemical Society 10/2007; 129(36):11018-9. · 9.91 Impact Factor
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Hooi Ling Kee,
Christine Kirmaier, Qun Tang,
James R Diers,
Chinnasamy Muthiah,
Masahiko Taniguchi,
Joydev K Laha,
Marcin Ptaszek,
Jonathan S Lindsey,
David F Bocian,
Dewey Holten
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ABSTRACT: The optical absorption spectra and redox properties are presented for 24 synthetic zinc chlorins and 18 free base analogs bearing a variety of 3,13 (beta) and 5,10,15 (meso) substituents. Results are also given for a zinc and free base oxophorbine, which contain the keto-bearing isocyclic ring present in the natural photosynthetic pigments such as chlorophyll a. Density functional theory calculations were carried out to probe the effects of the types and positions of substituents on the characteristics (energies, electron distributions) of the frontier molecular orbitals. A general finding is that the 3,13 positions are more sensitive to the effects of auxochromes than the 5,10,15 positions. The auxochromes investigated (acetyl>ethynyl>vinyl>aryl) cause a significant redshift and intensification of the Qy band upon placement at the 3,13 positions, whereas groups at the 5,10,15 positions result in much smaller redshifts that are accompanied by a decrease in relative Qy intensity. In addition, the substituent-induced shifts in first oxidation and reduction potentials faithfully track the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The calculations show that the LUMO is shifted more by substituents than the HOMO, which derives from the differences in the electron densities of the two orbitals at the substituent sites. The trends in the substituent-induced effects on the wavelengths and relative intensities of the major features (By, Bx, Qx, Qy) in the near-UV to near-IR absorption bands are well accounted for using Gouterman's four-orbital model, which incorporates the effects of the substituents on the HOMO-1 and LUMO+1 in addition to the HOMO and LUMO. Collectively, the results and analysis presented herein and in the companion paper provide insights into the effects of substituents on the optical absorption, redox and other photophysical properties of the chlorins. These insights form a framework that underpins the rational design of chlorins for applications encompassing photomedicine and solar-energy conversion.
Photochemistry and Photobiology 06/2007; 83(5):1125-43. · 2.41 Impact Factor
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ABSTRACT: Successful realization of highly crystalline and densely packed Ag2S nanocrystal (NC) films has been achieved by directly converting precursor molecules, Ag(SCOPh), on preheated substrates. When an aliquot of Ag(SCOPh) solution dissolved in trioctylphosphine (TOP) is applied on preheated solid substrates at 160 degrees C, such as SiO2/Si, H-terminated Si, and quartz. Ag2S NC thin films have been formed with instant phase and color changes of the precursor solutions from pale yellow homogeneous solution to black solid films. The average diameter of individual Ag2S NCs forming thin films is ca. 25 nm, as confirmed by examining both isolated Ag2S NCs from thin films and as-made thin film samples by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Powder X-ray diffraction (XRD) pattern shows that the synthesized Ag2S NCs have well-defined monoclinic acanthite phase. Direct precursor conversion process has resulted in densely packed Ag2S NCs with reduced interparticle distances owing to efficient removal of TOP during the reaction. Compared to the devices fabricated by the drop-coating process, Ag2S thin film devices fabricated by direct precursor conversion process have shown a ca. 300-fold increased conductance. Such Ag2S NC devices have also displayed reliable photoresponses upon white light illumination with high photosensitivity (S approximately equal to 1).
Langmuir 03/2007; 23(5):2800-4. · 4.19 Impact Factor
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ABSTRACT: Methyl-coenzyme M reductase (MCR) catalyzes the final step in methane biosynthesis by methanogenic archaea and contains a redox-active nickel tetrahydrocorphin, coenzyme F430, at its active site. Spectroscopic and computational methods have been used to study a novel form of the coenzyme, called F330, which is obtained by reducing F430 with sodium borohydride (NaBH4). F330 exhibits a prominent absorption peak at 330 nm, which is blue shifted by 100 nm relative to F430. Mass spectrometric studies demonstrate that the tetrapyrrole ring in F330 has undergone reduction, on the basis of the incorporation of protium (or deuterium), upon treatment of F430 with NaBH4 (or NaBD4). One- and two-dimensional NMR studies show that the site of reduction is the exocyclic ketone group of the tetrahydrocorphin. Resonance Raman studies indicate that elimination of this pi-bond increases the overall pi-bond order in the conjugative framework. X-ray absorption, magnetic circular dichroism, and computational results show that F330 contains low-spin Ni(II). Thus, conversion of F430 to F330 reduces the hydrocorphin ring but not the metal. Conversely, reduction of F430 with Ti(III) citrate to generate F380 (corresponding to the active MCR(red1) state) reduces the Ni(II) to Ni(I) but does not reduce the tetrapyrrole ring system, which is consistent with other studies [Piskorski, R., and Jaun, B. (2003) J. Am. Chem. Soc. 125, 13120-13125; Craft, J. L., et al. (2004) J. Biol. Inorg. Chem. 9, 77-89]. The distinct origins of the absorption band shifts associated with the formation of F330 and F380 are discussed within the framework of our computational results. These studies on the nature of the product(s) of reduction of F430 are of interest in the context of the mechanism of methane formation by MCR and in relation to the chemistry of hydroporphinoid systems in general. The spectroscopic and time-dependent DFT calculations add important insight into the electronic structure of the nickel hydrocorphinate in its Ni(II) and Ni(I) valence states.
Biochemistry 11/2006; 45(39):11915-33. · 3.42 Impact Factor
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ABSTRACT: Selective formation of metallic Ag and semiconducting Ag(2)S nanocrystals has been achieved via a modified hot-injection process from a single-source precursor molecule, Ag(SCOPh), which can potentially generate both [Ag] and [AgS] fragments simultaneously. When the precursor molecules are injected into a preheated reaction system at 160 degrees C, spherical Ag(2)S nanocrystals are directly obtained even without a molecular activator, such as alkylamines. Mixtures of Ag and Ag(2)S or pure metallic Ag nanocrystals are obtained if the precursor molecules are injected at lower than 160 degrees C or room temperature. These results are attributed to the direct transfer of thermal energies to precursor molecules, which are enough to dissociate S-C as well as Ag-S bonds simultaneously. Detailed characterizations about the produced nanocrystals have been performed using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), as well as energy-dispersive X-ray (EDX) spectrum.
Langmuir 04/2006; 22(6):2802-5. · 4.19 Impact Factor
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ABSTRACT: Methyl-coenzyme M reductase (MCR) catalyzes methane formation from methyl-coenzyme M (methyl-SCoM) and N-7-mercaptoheptanoylthreonine phosphate (CoBSH). MCR contains a nickel hydrocorphin cofactor at its active site, called cofactor F(430). Here we present evidence that the macrocyclic ligand participates in the redox chemistry involved in catalysis. The active form of MCR, the red1 state, is generated by reducing another spectroscopically distinct form called ox1 with titanium(III) citrate. Previous electron paramagnetic resonance (EPR) and (14)N electron nuclear double resonance (ENDOR) studies indicate that both the ox1 and red1 states are best described as formally Ni(I) species on the basis of the character of the orbital containing the spin in the two EPR-active species. Herein, X-ray absorption spectroscopic (XAS) and resonance Raman (RR) studies are reported for the inactive (EPR-silent) forms and the red1 and ox1 states of MCR. RR spectra are also reported for isolated cofactor F(430) in the reduced, resting, and oxidized states; selected RR data are reported for the (15)N and (64)Ni isotopomers of the cofactor, both in the intact enzyme and in solution. Small Ni K-edge energy shifts indicate that minimal electron density changes occur at the Ni center during redox cycling of the enzyme. Titrations with Ti(III) indicate a 3-electron reduction of free cofactor F(430) to generate a stable Ni(I) state and a 2-electron reduction of Ni(I)-ox1 to Ni(I)-red1. Analyses of the XANES and EXAFS data reveal that both the ox1 and red1 forms are best described as hexacoordinate and that the main difference between ox1 and red1 is the absence of an axial thiolate ligand in the red1 state. The RR data indicate that cofactor F(430) undergoes a significant conformational change when it binds to MCR. Furthermore, the vibrational characteristics of the ox1 state and red1 states are significantly different, especially in hydrocorphin ring modes with appreciable C=N stretching character. It is proposed that these differences arise from a 2-electron reduction of the hydrocorphin ring upon conversion to the red1 form. Presumably, the ring-reduction and ligand-exchange reactions reported herein underlie the enhanced activity of MCR(red1), the only form of MCR that can react productively with the methyl group of methyl-SCoM.
Journal of the American Chemical Society 12/2002; 124(44):13242-56. · 9.91 Impact Factor
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Hooi Ling Kee,
Christine Kirmaier, Qun Tang,
James R Diers,
Chinnasamy Muthiah,
Masahiko Taniguchi,
Joydev K Laha,
Marcin Ptaszek,
Jonathan S Lindsey,
David F Bocian,
Dewey Holten
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ABSTRACT: Understanding the effects of substituents on the spectra of chlorins is essential for a wide variety of applications. Recent developments in synthetic methodology have made possible systematic studies of the properties of the chlorin macrocycle as a function of diverse types and patterns of substituents. In this paper, the spectral, vibrational and excited-state decay characteristics are examined for a set of synthetic chlorins. The chlorins bear substituents at the 5,10,15 (meso) positions or the 3,13 (beta) positions (plus 10-mesityl in a series of compounds) and include 24 zinc chlorins, 18 free base (Fb) analogs and one Fb or zinc oxophorbine. The oxophorbine contains the keto-bearing isocyclic ring present in the natural photosynthetic pigments (e.g. chlorophyll a). The substituents cause no significant perturbation to the structure of the chlorin macrocycle, as evidenced by the vibrational properties investigated using resonance Raman spectroscopy. In contrast, the fluorescence properties are significantly altered due to the electronic effects of substituents. For example, the fluorescence wavelength maximum, quantum yield and lifetime for a zinc chlorin bearing 3,13-diacetyl and 10-mesityl groups (662 nm, 0.28, 6.0 ns) differ substantially from those of the parent unsubstituted chlorin (602 nm, 0.062, 1.7 ns). Each of these properties of the lowest singlet excited state can be progressively stepped between these two extremes by incorporating different substituents. These perturbations are associated with significant changes in the rate constants of the decay pathways of the lowest excited singlet state. In this regard, the zinc chlorins with the red-most fluorescence also have the greatest radiative decay rate constant and are expected to have the fastest nonradiative internal conversion to the ground state. Nonetheless, these complexes have the longest singlet excited-state lifetime. The Fb chlorins bearing the same substituents exhibit similar fluorescence properties. Such combinations of factors render the chlorins suitable for a range of applications that require tunable coverage of the solar spectrum, long-lived excited states and red-region fluorescence.
Photochemistry and Photobiology 83(5):1110-24. · 2.41 Impact Factor
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ABSTRACT: Understanding the changes in molecular structure of tetrapyrrole macrocycles upon derivatization of the organic framework is essential for diverse studies ranging from metal complexation to formation of supramolecular assemblies. New, sparsely substituted free base chlorin, 17-oxochlorin, phorbine and 131-oxophorbine macrocycles provide benchmarks for naturally occurring hydroporphyrins and have been examined here by X-ray crystallography, resonance Raman spectroscopy, and density functional theoretical (DFT) calculations. The macrocycles contain no substituents other than a geminal-dimethyl group attached to the reduced, pyrroline ring. The X-ray studies indicate that the benchmark compounds exhibit only slight distortion from planarity, which increases along the series porphine < chlorin < oxochlorin < phorbine < oxophorbine. The elongated CβCβ bond distance due to sp3 versus sp2 hybridization in the pyrroline ring (ring D) of the (oxo)chlorins and (oxo)phorbines (1.52–1.54 Å) versus that of porphine (1.35 Å) is accompanied by altered bond angles in ring D. Introduction of ring E (exocyclic ring) in a chlorin to give the phorbine or oxophorbine causes alteration of the bond angles at many sites in the framework of the macrocycle; for example, the bond angles of N3C14C15 in the (oxo)phorbine are widened by ∼11° compared to those of porphine or the analogous sparsely substituted chlorin. As a result, the shape of the macrocycle core changes along the series of porphine (nearly square), (oxo)chlorin (kite-shaped), and (oxo)phorbine (right-angled trapezoid), and the core size increases in the order porphine < phorbine ∼ oxophorbine < oxochlorin ∼ chlorin. Comparison of the bond distances and angles in ring E of the phorbine versus oxophorbine indicates that the shortening of the C13C131 bond owing to the presence of the oxo-group is quite small, only 0.024 Å; thus, the unsymmetrical structure of ring E does not appear to be due to conjugation with the C131O group but may be a characteristic feature of the (oxo)phorbine framework. The X-ray data further indicate that the lengths of the oxochlorin C17O and oxophorbine C131O groups are essentially identical, a result also predicted by DFT calculations. Regardless, the observed frequencies for the stretching vibrations of the C17O (1721 cm−1) and C131O (1701 cm−1) groups are different and suggest that conjugation of the latter group with the π-system of the macrocycle is greater than that of the former group. Collectively, the studies provide new insights into the individual factors that give rise to the overall structural characteristics of various macrocycles.
Journal of Molecular Structure. 979:27-45.
