An unusual light-sensing function for coenzyme B in bacterial transcription regulator CarH.

Coenzyme B is one of the most complex cofactors found in nature and synthesized de novo by certain groups of bacteria. Although its use in various enzymatic reactions is well characterized, only recently an unusual light-sensing function has been ascribed to coenzyme B. It has been reported that the coenzyme B binding protein CarH, found in the carotenoid biosynthesis pathway of several thermostable bacteria, binds to the promoter region of DNA and suppresses transcription.

Direct Evidence of an Excited-State Triplet Species upon Photoactivation of the Chlorophyll Precursor Protochlorophyllide.

The chlorophyll precursor protochlorophyllide (Pchlide), which is the substrate for the light-driven enzyme protochlorophyllide oxidoreductase, has unique excited-state properties that facilitate photocatalysis. Previous time-resolved spectroscopy measurements have implied that a long-lived triplet state is formed during the excited-state relaxation of Pchlide, although direct evidence of its existence is still lacking. Here we use time-resolved electron paramagnetic resonance (EPR) in combination with time-resolved absorption measurements at a range of temperatures (10-290 K), solvents, and oxygen concentrations to provide a detailed characterization of the triplet state of Pchlide.

Excited-State Properties of Protochlorophyllide Analogues and Implications for Light-Driven Synthesis of Chlorophyll.

Protochlorophyllide (Pchlide), an intermediate in the biosynthesis of chlorophyll, is the substrate for the light-driven enzyme protochlorophyllide oxidoreductase. Pchlide has excited-state properties that allow it to initiate photochemistry in the enzyme active site, which involves reduction of Pchlide by sequential hydride and proton transfer. The basis of this photochemical behavior has been investigated here using a combination of time-resolved spectroscopies and density functional theory calculations of a number of Pchlide analogues with modifications to various substituent groups.

Multiple active site residues are important for photochemical efficiency in the light-activated enzyme protochlorophyllide oxidoreductase (POR).

Protochlorophyllide oxidoreductase (POR) catalyzes the light-driven reduction of protochlorophyllide (Pchlide), an essential, regulatory step in chlorophyll biosynthesis. The unique requirement of the enzyme for light has provided the opportunity to investigate how light energy can be harnessed to power biological catalysis and enzyme dynamics. Excited state interactions between the Pchlide molecule and the protein are known to drive the subsequent reaction chemistry.

Cross-Species Analysis of Protein Dynamics Associated with Hydride and Proton Transfer in the Catalytic Cycle of the Light-Driven Enzyme Protochlorophyllide Oxidoreductase.

Experimental interrogation of the relationship between protein dynamics and enzyme catalysis is challenging. Light-activated protochlorophyllide oxidoreductase (POR) is an excellent model for investigating this relationship because photoinitiation of the reaction cycle enables coordinated turnover in a "dark-assembled" ternary enzyme-substrate complex. The catalytic cycle involves sequential hydride and proton transfers (from NADPH and an active site tyrosine residue, respectively) to the substrate protochlorophyllide.

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.

Comprehensive analysis of the green-to-blue photoconversion of full-length Cyanobacteriochrome Tlr0924.

Cyanobacteriochromes are members of the phytochrome superfamily of photoreceptors and are of central importance in biological light-activated signaling mechanisms. These photoreceptors are known to reversibly convert between two states in a photoinitiated process that involves a basic E/Z isomerization of the bilin chromophore and, in certain cases, the breakage of a thioether linkage to a conserved cysteine residue in the bulk protein structure. The exact details and timescales of the reactions involved in these photoconversions have not been conclusively shown.

Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101[combining macron]0).

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.

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.

The photoinitiated reaction pathway of full-length cyanobacteriochrome Tlr0924 monitored over 12 orders of magnitude.

The coupling of photochemistry to protein chemical and structural change is crucial to biological light-activated signaling mechanisms. This is typified by cyanobacteriochromes (CBCRs), members of the phytochrome superfamily of photoreceptors that exhibit a high degree of spectral diversity, collectively spanning the entire visible spectrum. CBCRs utilize a basic E/Z isomerization of the bilin chromophore as the primary step in their photocycle, which consists of reversible photoconversion between two photostates.

Excited state dynamics can be used to probe donor-acceptor distances for H-tunneling reactions catalyzed by flavoproteins.

In enzyme systems where fast motions are thought to contribute to H-transfer efficiency, the distance between hydrogen donor and acceptor is a very important factor. Sub-ångstrom changes in donor-acceptor distance can have a large effect on the rate of reaction, so a sensitive probe of these changes is a vital tool in our understanding of enzyme function. In this study we use ultrafast transient absorption spectroscopy to investigate the photoinduced electron transfer rates, which are also very sensitive to small changes in distance, between coenzyme analog, NAD(P)H4, and the isoalloxazine center in the model flavoenzymes morphinone reductase (wild-type and selected variants) and pentaerythritol tetranitrate reductase (wild-type).

Modulation of ligand-heme reactivity by binding pocket residues demonstrated in cytochrome c' over the femtosecond-second temporal range.

The ability of hemoproteins to discriminate between diatomic molecules, and the subsequent affinity for their chosen ligand, is fundamental to the existence of life. These processes are often controlled by precise structural arrangements in proteins, with heme pocket residues driving reactivity and specificity. One such protein is cytochrome c', which has the ability to bind nitric oxide (NO) and carbon monoxide (CO) on opposite faces of the heme, a property that is shared with soluble guanylate cycle.

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.

Mechanistic reappraisal of early stage photochemistry in the light-driven enzyme protochlorophyllide oxidoreductase.

The light-driven enzyme protochlorophyllide oxidoreductase (POR) catalyzes the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This reaction is a key step in the biosynthesis of chlorophyll. Ultrafast photochemical processes within the Pchlide molecule are required for catalysis and previous studies have suggested that a short-lived excited-state species, known as I675*, is the first catalytic intermediate in the reaction and is essential for capturing excitation energy to drive subsequent hydride and proton transfers.

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.

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.

Controlled synthesis of tuned bandgap nanodimensional alloys of PbS(x)Se(1-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 Vegard's 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.

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.

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.

Influence of base stacking and hydrogen bonding on the fluorescence of 2-aminopurine and pyrrolocytosine in nucleic acids.

Fluorescent nucleobase analogues are used extensively to probe the structure and dynamics of nucleic acids. The fluorescence of the adenine analogue 2-aminopurine and the cytosine analogue pyrrolocytosine is significantly quenched when the bases are located in regions of double-stranded nucleic acids. To allow more detailed structural information to be obtained from fluorescence studies using these bases, we have studied the excited-state properties of the bases at the CIS and TDB3LYP level in hydrogen-bonded and base-stacked complexes.