Samantha J. O. Hardman

Electronic and surface properties of PbS nanoparticles exhibiting efficient multiple exciton generation

Ultrafast transient absorption measurements have been used to study multiple exciton generation in solutions of PbS nanoparticles vigorously stirred to avoid the effects of photocharging. The threshold and slope efficiency of multiple exciton generation are found to be 2.5 ± 0.2 ×E(g) and 0.34 ± 0.08, respectively. Photoemission measurements as a function of nanoparticle size and ageing show that the position of the valence band maximum is pinned by surface effects, and that a thick layer of surface oxide is rapidly formed at the nanoparticle surfaces on exposure to air.

Controlled Synthesis of Tuned Bandgap Nanodimensional Alloys of PbSxSe1−x

Truly alloyed PbS(x)Se(1-x) (x = 0-1) nanocrystals (∼5 nm in size) have been prepared, and their resulting optical properties are red-shifted systematically as the sulfur content of the materials increases. Their optical properties are discussed using a modified Vegards approach and the bowing parameter for these nanoalloys is reported for the first time. The alloyed structure of the nanocrystals is supported by the energy-filtered transmission electron microscope images of the samples, which show a homogeneous distribution of sulfur and selenium within the nanocrystals. X-ray photoelectron spectroscopy studies on ligand-exchanged nanocrystals confirmed the expected stoichiometry and various oxidized species.

Reaction of a Phospholipid Monolayer with Gas-Phase Ozone at the Air—Water Interface: Measurement of Surface Excess and Surface Pressure in Real Time

The reaction between gas-phase ozone and monolayers of the unsaturated lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC, on aqueous solutions has been studied in real time using neutron reflection and surface pressure measurements. The reaction between ozone and lung surfactant, which contains POPC, leads to decreased pulmonary function, but little is known about the changes that occur to the interfacial material as a result of oxidation. The results reveal that the initial reaction of ozone with POPC leads to a rapid increase in surface pressure followed by a slow decrease to very low values. The neutron reflection measurements, performed on an isotopologue of POPC with a selectively deuterated palmitoyl strand, reveal that the reaction leads to loss of this strand from the air-water interface, suggesting either solubilization of the product lipid or degradation of the palmitoyl strand by a reactive species. Reactions of (1)H-POPC on D(2)O reveal that the headgroup region of the lipids in aqueous solution is not dramatically perturbed by the reaction of POPC monolayers with ozone supporting degradation of the palmitoyl strand rather than solubilization. The results are consistent with the reaction of ozone with the oleoyl strand of POPC at the air-water interface leading to the formation of OH radicals. The highly reactive OH radicals produced can then go on to react with the saturated palmitoyl strands leading to the formation of oxidized lipids with shorter alkyl tails.

The photochemical mechanism of a B12-dependent photoreceptor protein.

The coenzyme B12-dependent photoreceptor protein, CarH, is a bacterial transcriptional regulator that controls the biosynthesis of carotenoids in response to light. On binding of coenzyme B12 the monomeric apoprotein forms tetramers in the dark, which bind operator DNA thus blocking transcription. Under illumination the CarH tetramer dissociates, weakening its affinity for DNA and allowing transcription. The mechanism by which this occurs is unknown. Here we describe the photochemistry in CarH that ultimately triggers tetramer dissociation; it proceeds via a cob(III)alamin intermediate, which then forms a stable adduct with the protein. This pathway is without precedent and our data suggest it is independent of the radical chemistry common to both coenzyme B12 enzymology and its known photochemistry. It provides a mechanistic foundation for the emerging field of B12 photobiology and will serve to inform the development of a new class of optogenetic tool for the control of gene expression.

Is there a dynamic protein contribution to the substrate trigger in coenzyme B12-dependent ethanolamine ammonia lyase?

Coenzyme B12, or 5’-deoxyadenosylcobalamin (AdoCbl), acts as cofactor to a number of enzymes from a range of organisms. 2] In all cases, the Co C bond in the cofactor undergoes homolysis upon substrate binding, generating a singlet-born, Cbl/adenosyl radical pair (RP) and thus initiating radical-mediated catalysis. When compared to thermal homolysis of the free cofactor in solution, rate increases achieved by these enzymes are in the region of 10– 10, the precise origin of which is not yet fully understood. To date, the protein contribution to this catalytic power has been discussed either in terms of ground-state destabilization and a “strain” hypothesis, or transition state stabilization by electrostatic factors. However, there may be another contribution to consider—protein dynamics. Using a unique combination of spin-chemical and photochemical techniques we present evidence for coupling between RP reaction dynamics and protein dynamics in AdoCbl-dependent ethanolamine ammonia lyase (EAL). The adenosyl radical has never been observed directly during turnover in an AdoCbl-dependent enzyme under ambient conditions. In EAL it is rapidly quenched by Habstraction from the substrate to give the more stable substrate radical. 9, 10] The Co C bond can be photolyzed, however, enabling investigation of the singlet-born geminate pair dynamics at room temperature both in the free and protein-bound cofactor. The spin-state of this RP can coherently interconvert between the singlet and the triplet sublevels (Scheme 1). If the geminate RP re-encounter, only those in the singlet state will recombine, whereas triplet pairs will separate again. The extent to which the spin-states mix can be altered by the application of external magnetic fields (MFs). For increasing MFs of moderate strength (tens to hundreds of mT) the T 1 levels are gradually removed in energy and ultimately only S and T0 interconvert. With a singlet-born RP, this process increases the relative S population and hence the probability of recombination. Such magnetic field effects (MFEs) have been observed in the rate of anaerobic, continuous wave (cw) photolysis of both free and EAL-bound AdoCbl. 16] Under continuous illumination the reactive adenosyl radicals are ultimately and irreversibly quenched to yield an accumulated Cbl signal, and the MFE manifests as a decrease in the apparent rate of this accumulation. The magnitude of the MFE was viscositydependent for unbound AdoCbl, the viscogen acting as a RP “cage”. Likewise, the protein limits RP diffusion, thus enhancing the MFE over that observed in buffered water. The magnetic sensitivity of homolysis is removed in EAL, however, when the Co C bond is broken thermally by substrate binding. The observation of a significant kinetic isotope effect in the pre-steady-state signal representing the conversion of AdoCbl to Cbl suggests kinetic coupling of homolysis to subsequent H-abstraction from the substrate. The effect of this coupling is to rapidly quench the adenosyl radical, generating the substrate radical (which accumulates during turnover), thus stabilizing against recombination of the geminate pair and removing the MFE. However, this does not preclude the possibility of MF-sensitivity in the recombination step after product release. While the chemistry that immediately follows homolysis in the EAL-catalyzed reaction appears to favor RP dissociation, what of the protein contribution? The role of protein dynamics in enzyme function 19] is commonly probed by varying solvent viscosity (see, e.g. Ref. [20,21]) and assessing the extent to which this variation at the protein surface is transmitted to the active site. We therefore investigated the influence of viscosity on the cw-photolysis rate, and its MFE, of both free and EAL-bound AdoCbl at 298 K, using a specially configured MFE stopped-flow spectrophotometer. 22] The aim was to isolate the effect of protein dynamics on the geminate RP. Typical traces acquired at 525 nm are shown in the Supporting Information (Figure S1a and b). Scheme 1. The reaction and spin dynamics of the separated Cbl/ adenosyl radical pair following anaerobic photolysis of AdoCbl. During cw-photolysis, the adenosyl radicals are irreversibly quenched yielding an accumulated Cbl signal.

Evidence for a Nonbase Stacking Effect for the Environment‐sensitive Fluorescent Base Pyrrolocytosine—Comparison with 2‐Aminopurine

Pyrrolocytosine (PC), is a highly fluorescent analog of the natural nucleobase cytosine. The fluorescence of PC is quenched upon helix formation but the origin of the quenching is not known. We investigated the effects of base stacking in the aqueous phase by following the fluorescence of dinucleotides and trinucleotides containing PC. The quantum yields and lifetimes (ns) (in parenthesis) obtained at 25°C were: PC‐T, 0.026 (2.0), PC‐C, 0.033 (2.5), PC‐A, 0.032 (2.7), PC‐G, 0.021 (2.0), T‐PC‐T, 0.044 (3.0) and G‐PC‐G, 0.036 (0.65 and 2.6), compared with 0.038 (2.9) for PC and 0.028 (2.1) for the nucleoside triphosphate. The results show that base stacking does not, except in the case of guanine, quench the fluorescence of PC; indeed the increased solvent shielding can enhance the emitted fluorescence. In the case of G‐PC‐G the guanines do shield the fluorescent base from the solvent but a particular environment of PC between two guanines also appears to allow a rapid nonradiative pathway, suggested to be electron transfer to the excited PC, to depopulate the excited state leading to the shorter fluorescence lifetime.

Excited‐State Charge Separation in the Photochemical Mechanism of the Light‐Driven Enzyme Protochlorophyllide Oxidoreductase

The unique light-driven enzyme protochlorophyllide oxidoreductase (POR) is an important model system for understanding how light energy can be harnessed to power enzyme reactions. The ultrafast photochemical processes, essential for capturing the excitation energy to drive the subsequent hydride- and proton-transfer chemistry, have so far proven difficult to detect. We have used a combination of time-resolved visible and IR spectroscopy, providing complete temporal resolution over the picosecond–microsecond time range, to propose a new mechanism for the photochemistry. Excited-state interactions between active site residues and a carboxyl group on the Pchlide molecule result in a polarized and highly reactive double bond. This so-called “reactive” intramolecular charge-transfer state creates an electron-deficient site across the double bond to trigger the subsequent nucleophilic attack of NADPH, by the negatively charged hydride from nicotinamide adenine dinucleotide phosphate. This work provides the crucial, missing link between excited-state processes and chemistry in POR. Moreover, it provides important insight into how light energy can be harnessed to drive enzyme catalysis with implications for the design of light-activated chemical and biological catalysts.

Ultrafast exciton dynamics in InAs/ZnSe nanocrystal quantum dots

Colloidal nanocrystal quantum dots with a band gap in the near infra-red have potential application as the emitters for telecommunications or in vivo imaging, or as the photo-absorbing species in next generation solar cells or photodetectors. However, electro- and photoluminescence yields and the efficiency with which photo-generated charges can be extracted from quantum dots depend on the total rate of recombination, which can be dominated by surface-mediated processes. In this study, we use ultrafast transient absorption spectroscopy to characterise the recombination dynamics of photo-generated charges in InAs/ZnSe nanocrystal quantum dots. We find that recombination is dominated by rapid, sub-nanosecond transfer of conduction band electrons to surface states. For the size of dots studied, we also find no evidence of significant multiple exciton generation for photon energies up to 3.2 times the band gap, in agreement with our theoretical modelling.

Ultrafast red light activation of Synechocystis phytochrome Cph1 triggers major structural change to form the Pfr signalling-competent state.

Phytochromes are dimeric photoreceptors that regulate a range of responses in plants and microorganisms through interconversion of red light-absorbing (Pr) and far-red light-absorbing (Pfr) states. Photoconversion between these states is initiated by light-driven isomerization of a bilin cofactor, which triggers protein structural change. The extent of this change, and how light-driven structural changes in the N-terminal photosensory region are transmitted to the C-terminal regulatory domain to initiate the signalling cascade, is unknown. We have used pulsed electron-electron double resonance (PELDOR) spectroscopy to identify multiple structural transitions in a phytochrome from Synechocystis sp. PCC6803 (Cph1) by measuring distances between nitroxide labels introduced into the protein. We show that monomers in the Cph1 dimer are aligned in a parallel ‘head-to-head’ arrangement and that photoconversion between the Pr and Pfr forms involves conformational change in both the N- and C-terminal domains of the protein. Cryo-trapping and kinetic measurements were used to probe the extent and temporal properties of protein motions for individual steps during photoconversion of Cph1. Formation of the primary photoproduct Lumi-R is not affected by changes in solvent viscosity and dielectric constant. Lumi-R formation occurs at cryogenic temperatures, consistent with their being no major structural reorganization of Cph1 during primary photoproduct formation. All remaining steps in the formation of the Pfr state are affected by solvent viscosity and dielectric constant and occur only at elevated temperatures, implying involvement of a series of long-range solvent-coupled conformational changes in Cph1. We show that signalling is achieved through ultrafast photoisomerization where localized structural change in the GAF domain is transmitted and amplified to cause larger-scale and slower conformational change in the PHY and histidine kinase domains. This hierarchy of timescales and extent of structural change orientates the histidine kinase domain to elicit the desired light-activated biological response.

Adsorption of Dopamine on Rutile TiO2 (110): A Photoemission and Near-Edge X-ray Absorption Fine Structure Study

Synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the adsorption of dopamine on a rutile TiO2 (110) single crystal. Photoemission results suggest that dopamine bonds through the oxygen molecules in a bidentate fashion. From the data, it is ambiguous whether the oxygens bond to the same 5-fold coordinated surface titanium atom or bridges across two, although based on the bonding of pyrocatechol on rutile TiO2 (110), it is likely that the dopamine bridges two titanium atoms. Using the searchlight effect, the carbon K-edge near-edge X-ray absorption fine structure NEXAFS spectra recorded for dopamine on rutile TiO2 (110) show the phenyl ring to be oriented at 78° ± 5° from the surface and twisted 11 ± 10° relative to the (001) direction.