Joseph L. Hughes

Intramolecular light induced activation of a Salen–MnIII complex by a ruthenium photosensitizer

We have designed a molecular system consisting of a heteroleptic [Ru(bpy)(2)L](2+) chromophore covalently linked to a Mn(III)-Salen unit. We demonstrate the light induced oxidation of the Mn(III) center in this putative photo-catalyst assembly to a Mn(IV) high spin intermediate. Both oxidation states have been characterized by transient absorption and EPR techniques.

Identification of the QY Excitation of the Primary Electron Acceptor of Photosystem II: CD Determination of Its Coupling Environment

Low-temperature absorption and CD spectra, measured simultaneously, are reported from Photosystem II (PS II) reduced with sodium dithionite. Spectra were obtained using PS II core complexes before and after photoaccumulation of Pheo(D1)(-), the anion of the primary acceptor. For plant PS II, Pheo(D1)(-) was generated under conditions in which the primary plastoquinone was present as an anion (Q(A)(-)) and as a modified species taken to be the neutral doubly reduced hydroquinone (Q(A)H(2)). The bleaches observed upon Pheo(D1)(-) formation in the presence of Q(A)(-) are shifted to the blue compared those in the presence of Q(A)H(2). This is attributed to the influence of the charge on Q(A)(-), and this effect mirrors the well-known electrochromic effect of Q(A)(-) on the neutral pigments. The absorption bleaches induced by Pheo(D1) reduction are species dependent. Structured changes of the CD in the 680-690 nm spectral region are seen upon photoaccumulation of Pheo(D1)(-) in PS II from plant, Synechocystis and Thermosynechococcus vulcanus. These CD changes are shown to be consistent with the overall electronic assignments of Raszewski et al. [Raszewski et al. Biophys. J. 2008, 95, 105], which place the dominant Pheo(D1) excitation near 672 nm. CD changes associated with Pheo(D1) reduction are modeled to arise from the shift and intensity changes of two CD features: one predominately of Chl(D1) character, the other predominately Pheo(D2) in character. The assignments are also shown to account for the Q(Y) absorption changes in samples where the quinone is its charged (Q(A)(-)) and neutral (Q(A)H(2)) states.

D1 protein variants in Photosystem II from Thermosynechococcus elongatus studied by low temperature optical spectroscopy

In Photosystem II (PSII) from Thermosynechococcus elongatus, high-light intensity growth conditions induce the preferential expression of the psbA(3) gene over the psbA(1) gene. These genes encode for the D1 protein variants labeled D1:3 and D1:1, respectively. We have compared steady state absorption and photo-induced difference spectra at <10 K of PSII containing either D1:1 or D1:3. The following differences were observed. (i) The pheophytin Q(x) band was red-shifted in D1:3 (547.3 nm) compared to D1:1 (544.3 nm). (ii) The electrochromism on the Pheo(D1) Q(x) band induced by Q(A)(-) (the C550 shift) was more asymmetric in D1:3. (iii) The two variants differed in their responses to excitation with far red (704 nm) light. When green light was used there was little difference between the two variants. With far red light the stable (t(1/2)>50 ms) Q(A)(-) yield was approximately 95% in D1:3, and approximately 60% in D1:1, relative to green light excitation. (iv) For the D1:1 variant, the quantum efficiency of photo-induced oxidation of side-pathway donors was lower. These effects can be correlated with amino acid changes between the two D1 variants. The effects on the pheophytin Q(x) band can be attributed to the hydrogen bond from Glu130 in D1:3 to the 13(1)-keto of Pheo(D1), which is absent for Gln130 in D1:1. The reduced yield with red light in the D1:1 variant could be associated with either the Glu130Gln change, and/or the four changes near the binding site of P(D1), in particular Ser153Ala. Photo-induced Q(A)(-) formation with far red light is assigned to the direct optical excitation of a weakly absorbing charge transfer state of the reaction centre. We suggest that this state is blue-shifted in the D1:1 variant. A reduced efficiency for the oxidation of side-pathway donors in the D1:1 variant could be explained by a variation in the location and/or redox potential of P+.

Quantum efficiency distributions of photo-induced side-pathway donor oxidation at cryogenic temperature in photosystem II

We monitored illuminated-minus-dark absorption difference spectra in the range of 450–1100 nm induced by continuous illumination at 8 K of photosystem II (PSII) core complexes from Thermosynechococcus elongatus. The photo-induced oxidation of the side-path donors Cytb559, β-carotene and chlorophyll Z, as well as the concomitant stable (t1/2 > 1 s) reduction of the first plastoquinone electron acceptor, QA (monitored by the well-known ‘C550’ shift), were quantified as a function of the absorbed photons per PSII. The QA photo-induced reduction data can be described by three distinct quantum efficiency distributions: (i) a very high efficiency of ~0.5–1, (ii) a middle efficiency with a very large range of ~0.014–0.2, and (iii) a low efficiency of ~0.002. Each of the observed side-path donors exhibited similar quantum efficiency distributions, which supports a branched pathway model for side-path oxidation where β-carotene is the immediate electron donor to the photo-oxidized chlorophyll (P680+). The yields of the observed side-path donors account quantitatively for the wide middle efficiency range of photo-induced QA reduction, but not for the PSII fractions that exhibit the highest and lowest efficiencies. The high-efficiency component may be due to TyrZ oxidation. A donor that does not exhibit an identified absorption in the visible-near-IR region is mainly responsible for the lowest efficiency component.

The Native Reaction Centre of Photosystem II: A New Paradigm for P680

Low-temperature spectra of fully active (oxygen-evolving) Photosystem II (PSII) cores prepared from spinach exhibit well developed structure. Spectra of isolated sub-fragments of PSII cores establish that the native reaction centre is better structured and red-shifted compared to the isolated reaction centre. Laser illumination of PSII cores leads to efficient and deep spectral hole-burning. Measurements of homogeneous hole-widths establish excited-state lifetimes in the 40–300 ps range. The high hole-burning efficiency is attributed to charge separation of P680 in native PSII that follows reaction-centre excitation via ‘slow transfer’ states in the inner light-harvesting assemblies CP43 and CP47. The ‘slow transfer’ state in CP47 and that in CP43 can be distinguished in the hole-burning action spectrum and high-resolution hole-burning spectra. An important observation is that 685–700 nm illumination gives rise to efficient P680 charge separation, as established by QA− formation. This leads to a new paradigm for P680. The charge-separating state has surprisingly weak absorption and extends to 700 nm.

The Chemical Problem of Energy Change: Multi-Electron Processes

This special issue is focussed on arguably the most important fundamental question in contemporary chemical research: how to efficiently and economically convert abundant and thermodynamically stable molecules, such as H2O, CO2, and N2 into useable fuel and food sources. The 3 billion year evolutionary experiment of nature has provided a blueprint for the answer: multi-electron catalysis. However, unlike one-electron transfer, we have no refined theories for multi-electron processes. This is despite its centrality to much of chemistry, particularly in catalysis and biology. In this article we highlight recent research developments relevant to this theme with emphasis on the key physical concepts and premises: (i) multi-electron processes as stepwise single-electron transfer events; (ii) proton-coupled electron transfer; (iii) stimulated, concerted, and co-operative phenomena; (iv) feedback mechanisms that may enhance electron transfer rates by minimizing activation barriers; and (v) non-linearity and far-from-equilibrium considerations. The aim of our discussion is to provide inspiration for new directions in chemical research, in the context of an urgent contemporary issue.

What Is the Origin of the Highly Dispersive Quantum Efficiencies for Secondary Donor Oxidation at Low Temperature in Photosystem II

Optical and EPR illumination-induced difference spectroscopy was used to study secondary electron donation at low temperature in photosystem II (PSII) from Thermosynechococcus elongatus. Green or white light illumination led to the same quantum efficiency (QE) distribution for stable (t½ > 60 s) QA reduction and side-path donor oxidation. Selective excitation in the 700–730 nm region also resulted in stable QA reduction and secondary donor oxidation, although with a QE decreasing by orders of magnitude. Excitation in the 700–730 nm region did however lead to high yield, high QE formation of the EPR S1-state split signal associated with TyrZ oxidation. These results indicate that the characteristic distribution and decrease of QEs is due to side-pathway processes, and that excitation of the weak long-wavelength (700–730 nm) band promotes primary photochemistry with high QE.

Exploring action learning for academic development in research intensive settings

Abstract The potential of action learning (AL) for academic development has not received a lot of attention. Building from two case studies in which AL has been used in different ways in research-intensive universities in Australia and the UK, we suggest that the approach may be of benefit to developers in the changing landscape in which they are expected to function. The opportunities for and challenges of leadership for AL in educational development, particularly involving non-academic staff, are also briefly explored. We argue that AL offers a way to engage academic and related staff groups that fits with their institutional culture and is therefore likely to lead to the kind of continual professional learning and positive change that will be both valued and valuable in academia. Furthermore, we believe that AL might offer productive ways forward for the further evolution of academic development work, especially, perhaps, in research-intensive settings.

The Primary Electron Acceptor of Photosystem II Is Weakly Coupled to the Accessory Chlorophyll

Optical spectra of chemically reduced PSII core complexes isolated from spinach are presented. In these samples, QA is pre-reduced in darkness, allowing the photo-accumulation of its electron transfer pathway precursor, PheoD1 —. We report low-temperature (2200 K) spectral changes in circular dichroism (CD) and absorption spectra associated with PheoD1 photo-reduction. The area of the narrow (2 nm FWHM) bleach at 683.8 nm is fully commensurate with that of an isolated Pheoa, indicating weak coupling to its neighboring pigment, the accessory chlorophyll ChlD1. Also, a highly structured, second-derivative-like pattern is seen in the change in the CD at 683.8 nm upon photoreduction. This can be interpreted as indicative of a weak PheoD1-ChlD1interaction.

Low Temperature Secondary Pathway Donation in Photosystem II of Spinach

At low temperature (5 K) light-induced charge separation in photosystem II (PS II) is followed by charge stabilization due to reduction of P680+ by secondary donors such as carotenoid (Car) or chlorophyll (Chl) that compete with P680+ QA − charge recombination.