David W. Randall

Electronic structures of active sites in electron transfer metalloproteins: contributions to reactivity

Abstract Many electron transfer centers in biology involve metal complexes, that exhibit unique spectral features. These reflect highly covalent electronic structures, which contribute to the electron transfer function of the protein. The blue copper center has a highly covalent copperthiolate bond, which promotes long range electron transfer. The Cu A center is a mixed valence binuclear complex that is completely delocalized even in low symmetry protein environments. The [2Fe2S] center is valence localized in the mixed valence Fe(III)Fe(II) oxidation state, while the mixed valence [2Fe2S] sub-sites in [4Fe4S] clusters are completely valence delocalized. Factors which contribute to electron localization/delocalization in these mixed valence sites are experimentally evaluated using a variety of spectroscopic and electronic structural methods. These include the very powerful technique of ligand K-edge X-ray absorption spectroscopy for determining the covalency of ligandmetal bonds.

Electronic structure contributions to electron transfer in blue Cu and Cu(A).

A centers are summarized and their relation to intra- and inter-protein electron transfer (ET) kinetics are described. Specific contributions of the electronic structures of these two broad classes of Cu ET proteins to HAB, λ, and ΔE° are discussed. Also, the role of the protein structure in determining key geometric features which define the electronic structures of the metal sites in these proteins is considered.

Carbon dioxide exchange characteristics of C4 Hawaiian Euphorbia species native to diverse habitats

SummaryThe characteristics of the photosynthetic apparatus of 11 Hawaiian Euphorbia species, all of which possess C4 photosynthesis but range from arid habitat, drought-deciduous shrubs to mesic or wet forest evergreen trees and shrubs, were investigated under uniform greenhouse conditions. Nine species exhibited CO2 response curves typical of C4 plants, but differed markedly in photosynthetic capacity. Light-saturated CO2 uptake rates ranged from 48 to 52 μmol m-2 s-1 in arid habitat species to 18 to 20 μmol m-2 s-1 in mesic and wet forest species. Two possessed unusual CO2 response curves in which photosynthesis was not saturated above intercellular CO2 pressures [p(CO2)] of 10 to 15 Pa, as typically occurs in C4 plants.Both leaf (g′1) and mesophyll (g′m) conductances to CO2 varied widely between species. At an atmospheric p(CO2) of 32 Pa, g′1 regulated intercellular p(CO2) at 12–15 Pa in most species, which supported nearly maximum CO2 uptake rates, but did not result in excessive transpiration. Intercellular p(CO2) was higher in the two species with unusual CO2 response curves. This was especially apparent in E. remyi, which is native to a bog habitat. The regulation of g′1 and intercellular p(CO2) yielded high photosynthetic water use efficiencies (P/E) in the species with typical CO2 response curves, whereas P/E was much lower in E. remyi.Photosynthetic capacity was closely related to leaf nitrogen content, whereas correlations with leaf morphological characteristics and leaf cell surface area were not significant. Thus, differences in photosynthetic capacity may be determined primarily by investment in the biochemical components of the photosynthetic apparatus rather than by differences in diffusion limitations. The lower photosynthetic capacities in the wet habitat species may reflect the lower light availability. However, other factors, such as reduced nutrient availability, may also be important.

Pulsed 1H and 55Mn ENDOR studies of dinuclear Mn(III)Mn(IV) model complexes

The high resolution pulsed EPR technique of ESE-ENDOR is applied to bis-μ-oxo dinuclear Mn(III)Mn(IV) model complexes ligated by either 2,2′-bipyridine (bipy) or 1,10-phenanthro-line (phen). Such complexes, which model a building block of which the Mn4 cluster of the oxygen evolving complex of photosystem II is thought to be contracted, are strongly anti-ferromagnetically coupled, class II mixed valence compounds. The 1H hyperfine couplings between ligand protons and the dinuclear spin centre reflect significant couplings to each Mn ion: a situation which requires a more complex analysis than that used in the case of a simple single point-dipole. 55Mn ENDOR spectra of phen recorded at field positions across the EPR envelope reveal its Mn hyperfine and quadrupole coupling constants. These ENDOR spectra are discussed in relation to a general analysis of 55Mn ENDOR spectra, including the importance of angle and m I selection, in addition to obtaining ENDOR spectra across the EPR envelope in cases where 55Mn ...

Spectroscopic comparison of the five-coordinate [Cu(SMeIm)(HB(3,5-iPr2pz)3)] with the four-coordinate [Cu(SCPh3)(HB(3,5-iPr2pz)3)]: effect of coordination number increase on a blue copper type site

Abstract A variety of spectroscopic techniques have been applied to characterize and compare two model complexes with relevance to red and blue copper centers in metalloproteins, [Cu(SMeIm)(HB(3,5-iPr 2 pz) 3 )] ( 1 ) and [Cu(SCPh 3 )(HB(3,5-iPr 2 pz) 3 )] ( 2 ), which are five- and four-coordinate, respectively. The key spectral differences in low temperature absorption and magnetic circular dichroism (MCD) include an increase in the Sσ→Cu charge transfer (CT) band intensity at 370 nm and a decrease in the absorption intensity at ∼570 nm for the Sπ→Cu CT. The energies of the d→d transitions in 1 are increased relative to 2 reflecting a more tetragonal geometry and a stronger ligand field. S K-edge X-ray absorption spectroscopy (XAS) measurements demonstrate a less covalent thiolate Cu interaction in the HOMO of 1 (15% Sp) compared to 2 (52% Sp). XAS at the Cu L-edge indicates that the Cu d-character in the HOMO of 1 has increased relative to that of 2 . The electronic perturbation resulting from the increased coordination number has been evaluated. The thiolate rotates in the NNS plane resulting in increased σ overlap with the Cu. Additionally, the Cu–S bond length increases. The associated reduced covalency of the thiolate can contribute to the function of perturbed blue copper sites in proteins.