Crystal and molecular structure and absolute configuration of ethyl chlorophyllide a-dihydrate. Model for the different spectral forms of chlorophyll a

Chow, H. C.; SERLIN, ROBERT; Strouse, Charles E.

doi:10.1021/ja00858a006

Cited by 300 publications

(197 citation statements)

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“…The small-angle neutron scattering data are too limited to provide detailed arrangements of the chlorophyll macrocycles, but x-ray crystal structures for chlorophyll derivatives provide valuable guidelines for specific models. From the crystal structures of ethylchlorophyllide a 2H2O and other derivatives of Chla and Chlb (8,12), it is possible to suggest some basic features of the coordination and hydrogen-bonding interactions in the micelles. In the crystal structures, a molecule of water is coordinated to the central magnesium atom and hydrogen-bonded to the ring V keto C=O function of another chlorophyll molecule.…”

Section: Resultsmentioning

confidence: 99%

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll

Worcester

Michalski

Katz

1986

Proc. Natl. Acad. Sci. U.S.A.

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Micelles of hydrated chlorophyll a (P740), bacteriochlorophyll a (P865), bacteriochlorophyll c (P750), and pheophytin a prepared in organic media have been studied by small-angle neutron scattering to determine their shape, size, and mass per unit length. All of the micelles are hollow cylinders of well-defined size. The P740 and P750 cylinders are essentially monolayers of macrocycles crosslinked by water, probably in an arrangement similar to that of crystals of chlorophyll derivatives. The P865 micelle is more nearly a bilayer of macrocycles. We show that the curvature necessary to form cylinders probably results from intrinsic curvature of the five-coordinated chlorophyll macrocycle. Studies of P740 micelle formation and the disaggregating effects of another nucleophile (pyridine) are described. As the P750 micelles are nearly identical in size and optical spectra to the rod-shaped structures observed in chlorosomes, and the P865 micelles have optical properties very similar to the in vivo properties of the long-wavelength antenna of purple photosynthetic bacteria, we propose that features of the hydrated cylindrical micelles of these chlorophylls provide good models for antenna chlorophyll in photosynthetic bacteria.Many artificial chlorophyll systems (colloidal dispersions, films, monolayers, adsorbates) are of interest because their visible absorption maxima are red-shifted relative to the corresponding monomeric chlorophyll, a feature that is also characteristic of chlorophylls in vivo (1). Of particular importance are the chlorophyll/water micelles formed in nonpolar organic solvents. Chlorophyll a/water micelles so formed are red-shifted from 660 nm (monomer) to 740 nm and are photoactive in red light (2). Bacteriochlorophyll a/water micelles (P865) have red-shifted optical spectra that strongly resemble those of intact purple photosynthetic bacteria (3). As no direct structural information on these systems exists, we have undertaken a small-angle neutron-scattering study of chlorophyll/water micelles in organic media, using the In- MATERIALS AND METHODS Materials. Chlorophyll a (Chla), bacteriochlorophyll a (Bchla), Chlorobium chlorophyll (Bchlc), and pheophytin a (Pheoa) were prepared by literature methods (4). Purity was established by HPLC and by californium-252 plasma desorption mass spectroscopy (5). Deuterated solvents (Aldrich) contained <1% 1H.Chlorophyll/water micelles were prepared by addition of microliter amounts of 'H20 or 2H20 to previously dried chlorophyll dissolved (=10 mg/ml) in a nonpolar solvent.Mild sonication (30 sec) was used to disperse the water. Rigorous drying of the chlorophylls and the solvents (6) before hydration was necessary as the structures formed from undried or partially dried starting materials can vary. Samples for small-angle neutron scattering were sealed in 2-to 5-mm-thick quartz cells. Neutron Scattering. Chlorophylls of natural isotopic composition were studied in deuterated solvents to enhance the scattering intensity in much the same way neutrons h...

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Section: Resultsmentioning

confidence: 99%

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll

Worcester

Michalski

Katz

1986

Proc. Natl. Acad. Sci. U.S.A.

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“…This is in contrast to the crystallographic results for the chlorophyll derivatives in which the oxygen atom of a water molecule is coordinated to the Mg atom in an almost vertical direction. 58,59 The difference in coordination may be interpreted in terms of the differences between a water molecule and the macrocycle-attached hydroxyethyl group for which large conformational restrictions could be expected. The Mg‚‚‚O distances were measured to be about 2.9 Å, similar to that estimated for the BChlide d dimer.…”

Section: Discussionmentioning

confidence: 99%

Complete Assignment of¹H NMR Spectra and Structural Analysis of Intact BacteriochlorophyllcDimer in Solution

et al. 1999

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Intact farnesyl (3 R)-bacteriochlorophyll (BChl) c in carbon tetrachloride forms a stable dimer at room temperature characterized by two resonances resolved for each individual proton in the NMR spectrum and by a long wavelength shift of the Q y absorption band to 710 nm. All the proton resonances are precisely assigned on the basis of two-dimensional H−C and H−H correlation experiments. Authentic farnesyl acetate is used for assistance in the assignment. Extensive nuclear Overhauser effects (NOE) are observed, from which distances between intermolecular proton pairs are evaluated. Geometry of the macrocycles determined from the distance information and refined by a molecular mechanics program is found to clearly explain the observed complexation shifts. Strong intermolecular NOE signals observed for 10-H/201-H and 10-H/21-H exclude a face-to-face arrangement but support an antiparallel “piggy-back” conformation for the BChl c dimer. Farnesyl protons do not show significant complexation shifts, and it is suggested that the farnesyl side chain may adopt a folding-back conformation with most of the group fluctuating around the periphery of the macrocycle in a restricted motion. The two-dimensional exchange experiment demonstrates that molecules in the dimer experience slow exchange between the two nonequivalent configurations with an exchange rate constant of about 1.8 s-1. Finally, the stereochemical effect of chirality at the 31 position on the aggregation behavior and possible relationships among 680, 710, and 740 nm species are discussed.

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“…Electronic spin densities were calculated by using Gaussian 98 (Gaussian, Carnegie, PA) on an IBM SP2 scalable computer. The nuclear coordinates for the ethyl-chlorophyllide a were taken from the x-ray structure and used without further optimization (29). In the single-point calculations the LanL2DZ basis set was used (30)(31)(32)(33).…”

Section: Mas Nmr Measurementsmentioning

confidence: 99%

Photochemically induced nuclear spin polarization in reaction centers of photosystem II observed by¹³C-solid-state NMR reveals a strongly asymmetric electronic structure of the P₆₈₀.+ primary donor chlorophyll

Matysik¹,

Alia²,

Gast³

et al. 2000

Proc. Natl. Acad. Sci. U.S.A.

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We report 13 C magic angle spinning NMR observation of photochemically induced dynamic nuclear spin polarization (photo-CIDNP) in the reaction center (RC) of photosystem II (PS2). The light-enhanced NMR signals of the natural abundance 13 C provide information on the electronic structure of the primary electron donor P680 (chlorophyll a molecules absorbing around 680 nm) and on the pz spin density pattern in its oxidized form, P680 .؉ . Most centerband signals can be attributed to a single chlorophyll a (Chl a) cofactor that has little interaction with other pigments. The chemical shift anisotropy of the most intense signals is characteristic for aromatic carbon atoms. The data reveal a pronounced asymmetry of the electronic spin density distribution within the P 680 . ؉ . PS2 shows only a single broad and intense emissive signal, which is assigned to both the C-10 and C-15 methine carbon atoms. The spin density appears shifted toward ring III. This shift is remarkable, because, for monomeric Chl a radical cations in solution, the region of highest spin density is around ring II. It leads to a first hypothesis as to how the planet can provide itself with the chemical potential to split water and generate an oxygen atmosphere using the Chl a macroaromatic cycle. A local electrostatic field close to ring III can polarize the electronic charge and associated spin density and increase the redox potential of P680 by stabilizing the highest occupied molecular orbital, without a major change of color. This field could be produced, e.g., by protonation of the keto group of ring V. Finally, the radical cation electronic structure in PS2 is different from that in the bacterial RC, which shows at least four emissive centerbands, indicating a symmetric spin density distribution over the entire bacteriochlorophyll macrocycle.

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Crystal and molecular structure and absolute configuration of ethyl chlorophyllide a-dihydrate. Model for the different spectral forms of chlorophyll a

Cited by 300 publications

References 10 publications

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll

Complete Assignment of¹H NMR Spectra and Structural Analysis of Intact BacteriochlorophyllcDimer in Solution

Photochemically induced nuclear spin polarization in reaction centers of photosystem II observed by¹³C-solid-state NMR reveals a strongly asymmetric electronic structure of the P₆₈₀.+ primary donor chlorophyll

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Crystal and molecular structure and absolute configuration of ethyl chlorophyllide a-dihydrate. Model for the different spectral forms of chlorophyll a

Cited by 300 publications

References 10 publications

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll

Small-angle neutron scattering studies of chlorophyll micelles: Models for bacterial antenna chlorophyll

Complete Assignment of1H NMR Spectra and Structural Analysis of Intact BacteriochlorophyllcDimer in Solution

Photochemically induced nuclear spin polarization in reaction centers of photosystem II observed by13C-solid-state NMR reveals a strongly asymmetric electronic structure of the P680.+ primary donor chlorophyll

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Complete Assignment of¹H NMR Spectra and Structural Analysis of Intact BacteriochlorophyllcDimer in Solution

Photochemically induced nuclear spin polarization in reaction centers of photosystem II observed by¹³C-solid-state NMR reveals a strongly asymmetric electronic structure of the P₆₈₀.+ primary donor chlorophyll