Analytical one-dimensional model for laser-induced ultrasound in planar optically absorbing layer.

Ultrasound generated by means of laser-based photoacoustic principles are in common use today and applications can be found both in biomedical diagnostics, non-destructive testing and materials characterisation. For certain measurement applications it could be beneficial to shape the generated ultrasound regarding spectral properties and temporal profile. To address this, we studied the generation and propagation of laser-induced ultrasound in a planar, layered structure. We derived an analytical expression for the induced pressure wave, including different physical and optical properties of each layer. A Laplace transform approach was employed in analytically solving the resulting set of photoacoustic wave equations. The results correspond to simulations and were compared to experimental results. To enable the comparison between recorded voltage from the experiments and the calculated pressure we employed a system identification procedure based on physical properties of the ultrasonic transducer to convert the calculated acoustic pressure to voltages. We found reasonable agreement between experimentally obtained voltages and the voltages determined from the calculated acoustic pressure, for the samples studied. The system identification procedure was found to be unstable, however, possibly from violations of material isotropy assumptions by film adhesives and coatings in the experiment. The presented analytical model can serve as a basis when addressing the inverse problem of shaping an acoustic pulse from absorption of a laser pulse in a planar layered structure of elastic materials.

Microwave-assisted synthesis of SnO2 nanorods for oxygen gas sensing at room temperature.

High-quality single-crystalline SnO2 nanorods were synthesized using a microwave-assisted solution method. The nanorods were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), ultraviolet-visible and Raman spectroscopy, Brunauer-Emmett-Teller (BET), and electrical resistance measurements. The XRD pattern indicated the formation of single-phase SnO2 nanorods with rutile structure. FE-SEM and TEM images revealed tetragonal nanorods of about 450-500 nm in length and 60-80 nm in diameter. The nanorods showed a higher BET surface area of 288 m²/g, much higher than that of previously reported work. The Raman scattering spectra indicated a typical rutile phase of the SnO2. The absorption spectrum showed an absorption peak centered at 340 nm, and the band-gap value was found to be 3.64 eV. The gas-sensing properties of the SnO2 nanorods for oxygen gas with different concentrations were measured at room temperature. It was found that the value of resistance increased with the increase in oxygen gas concentration in the test chamber. The SnO2 nanorods exhibited high sensitivity and rapid response-recovery characteristics to oxygen gas, and could detect oxygen concentration as low as 1, 3, 5, and 10 ppm.

Intense ultra-broadband down-conversion in co-doped oxide glass by multipolar interaction process.

We report that Eu(2+) can be an efficient sensitizer for Yb(3+) and a broadband absorber for blue solar spectra in the host of oxide glass. The greenish 4f → 5d transition of Eu(2+) and the characteristic near-infrared emission of Yb(3+) were observed, with the blue-light of xenon lamp excitation. The 5d energy can be adjusted by the host and the energy transfer efficiency can be enhanced. The quantum efficiency is up to 163.8%. Given the broad excitation band, high absorption coefficient and excellent mechanical, thermal and chemical stability, this system can be useful as down-conversion layer for solar cells.

Breaking the carboxyl rule: lysine 96 facilitates reprotonation of the Schiff base in the photocycle of a retinal protein from Exiguobacterium sibiricum.

A lysine instead of the usual carboxyl group is in place of the internal proton donor to the retinal Schiff base in the light-driven proton pump of Exiguobacterium sibiricum (ESR). The involvement of this lysine in proton transfer is indicated by the finding that its substitution with alanine or other residues slows reprotonation of the Schiff base (decay of the M intermediate) by more than 2 orders of magnitude. In these mutants, the rate constant of the M decay linearly decreases with a decrease in proton concentration, as expected if reprotonation is limited by the uptake of a proton from the bulk. In wild type ESR, M decay is biphasic, and the rate constants are nearly pH-independent between pH 6 and 9. Proton uptake occurs after M formation but before M decay, which is especially evident in D2O and at high pH. Proton uptake is biphasic; the amplitude of the fast phase decreases with a pKa of 8.5 ± 0.3, which reflects the pKa of the donor during proton uptake. Similarly, the fraction of the faster component of M decay decreases and the slower one increases, with a pKa of 8.1 ± 0.2. The data therefore suggest that the reprotonation of the Schiff base in ESR is preceded by transient protonation of an initially unprotonated donor, which is probably the ε-amino group of Lys-96 or a water molecule in its vicinity, and it facilitates proton delivery from the bulk to the reaction center of the protein.

Feedback-controlled LED photobioreactor for photophysiological studies of cyanobacteria.

A custom photobioreactor was designed to enable automatic light adjustments using computerized feedback control. The system consisted of a 7.5-L cylindrical vessel and an aluminum enclosure housing quantum sensors and light-emitting diode arrays, which provide 630 or 680 nm light to preferentially excite the major cyanobacterial pigments, phycocyanin and/or chlorophyll a, respectively. Custom-developed software rapidly measures light transmission and subsequently adjusts the irradiance to maintain a defined light profile to compensate for culture dynamics, biomass accumulation, and pigment adaptations during physiological transitions, thus ensuring appropriate illumination across batch and continuous growth modes. In addition to chemostat cultivation, the photobioreactor may also operate as a turbidostat, continuously adjusting the media dilution to achieve maximal growth at a fixed culture density. The cultivation system doubles as an analytical device, using real-time monitoring to avoid sampling bias (e.g., in-situ light-saturation response), determine conditions for optimal growth, and observe perturbation responses at high time-resolution.

Gamma irradiation: a method to produce an abiotic control for biological activated carbon.

The aim of this paper was to investigate the feasibility of using gamma irradiation to inhibit the microbial activity of biological powder activated carbon (PAC) without impacting its adsorptive properties. First of all, the range of dose of gamma rays required to produce abiotic PAC was selected on the basis of heterotrophic plate counts (HPC) inactivation and methylene blue (MB) adsorption kinetics. Doses inferior to 10 kGy were not sufficient to inhibit the culture of heterotrophic bacteria. On the other hand, doses superior to 15 kGy were demonstrated to affect the adsorption rate of MB. Consequently, a dose comprised between 10 and 15 kGy was selected for further investigation. In order to validate the adequacy of the range of dose (i.e. 10-15 kGy), adsorption characteristics were tested by monitoring the removal kinetics of refractory dissolved organic carbon (RDOC). No significant differences were observed between irradiated and non-irradiated biological PAC for the adsorption of RDOC. Irradiated, non-irradiated and virgin PAC were also evaluated in terms of abundance of viable (using the LIVE/DEAD BacLight method) bacteria and in terms of heterotrophic biomass activity. The results of the BacLight method demonstrated that attachment of the biofilm on the PAC was not impacted by the irradiation and heterotrophic activity measurements demonstrated that the latter could be radically reduced in the range of dose selected. In conclusion, when using a proper dose, the gamma irradiation of colonized activated carbon drastically reduced the heterotrophic activity on activated carbon without significantly impacting its adsorptive behaviour.

A mechanistic model for the light response of photosynthetic electron transport rate based on light harvesting properties of photosynthetic pigment molecules.

Models describing the light response of photosynthetic electron transport rate (ETR) are routinely used to determine how light absorption influences energy, reducing power and yields of primary productivity; however, no single model is currently able to provide insight into the fundamental processes that implicitly govern the variability of light absorption. Here we present development and application of a new mechanistic model of ETR for photosystem II based on the light harvesting (absorption and transfer to the core 'reaction centres') characteristics of photosynthetic pigment molecules. Within this model a series of equations are used to describe novel biophysical and biochemical characteristics of photosynthetic pigment molecules and in turn light harvesting; specifically, the eigen-absorption cross-section and the minimum average lifetime of photosynthetic pigment molecules in the excited state, which describe the ability of light absorption of photosynthetic pigment molecules and retention time of excitons in the excited state but are difficult to be measured directly. We applied this model to a series of previously collected fluorescence data and demonstrated that our model described well the light response curves of ETR, regardless of whether dynamic down-regulation of PSII occurs, for a range of photosynthetic organisms (Abies alba, Picea abies, Pinus mugo and Emiliania huxleyi). Inherent estimated parameters (e.g. maximum ETR and the saturation irradiance) by our model are in very close agreement with the measured data. Overall, our mechanistic model potentially provides novel insights into the regulation of ETR by light harvesting properties as well as dynamical down-regulation of PSII.

Study of p53 expression and post-transcriptional modifications after GSM-900 radiofrequency exposure of human amniotic cells.

The potential effects of radiofrequency (RF) exposure on the genetic material of cells are very important to determine since genome instability of somatic cells may be linked to cancer development. In response to genetic damage, the p53 protein is activated and can induce cell cycle arrest allowing more time for DNA repair or elimination of damaged cells through apoptosis. The objective of this study was to investigate whether the exposure to RF electromagnetic fields, similar to those emitted by mobile phones of the second generation standard, Global System for Mobile Communications (GSM), may induce expression of the p53 protein and its activation by post-translational modifications in cultured human cells. The potential induction of p53 expression and activation by GSM-900 was investigated after in vitro exposure of human amniotic cells for 24 h to average specific absorption rates (SARs) of 0.25, 1, 2, and 4 W/kg in the temperature range of 36.3-39.7 °C. The exposures were carried out using a wire-patch cell (WPC) under strictly controlled conditions of temperature. Expression and activation of p53 by phosphorylation at serine 15 and 37 were studied using Western blot assay immediately after three independent exposures of cell cultures provided from three different donors. Bleomycin-exposed cells were used as a positive control. According to our results, no significant changes in the expression and activation of the p53 protein by phosphorylation at serine 15 and 37 were found following exposure to GSM-900 for 24 h at average SARs up to 4 W/kg in human embryonic cells.

Concentração plasmática de diclofenaco sódico em cães, submetidos à fonoforese / Plasma concentration of diclofenac sodium in dogs submitted to diclofenac phonophoresis

Com o objetivo de avaliar a concentração plasmática de diclofenaco sódico (DS) emulgel em cães com ou sem o uso de fonoforese e de verificar se a fonoforese induz à maior absorção desse fármaco, foram utilizados cinco cães, e todos eles passaram por oito grupos distintos. Um grupo recebeu, via oral, um comprimido de DS, 40mg, por animal, e sete grupos receberam aplicação transdérmica de diclofenaco sódico emulgel por ultrassom. Pela via transdérmica, a área de aplicação era de 20cm². A frequência do ultrassom foi de 1MHz, modo contínuo, com intensidade de 0,4Wcm-2. Colheram-se amostras de sangue antes de se executarem os protocolos - tempo zero -, após uma hora - tempo 1 - e após quatro horas da aplicação - tempo 2 - em todos os grupos, e realizou-se análise das amostras por cromatografia líquida de alta eficiência. Houve diferença (P<0,05) apenas nas amostras no tempo 1 do grupo que recebeu dose oral de DS em relação às outras amostras. Não foi possível verificar concentração plasmática de diclofenaco sódico com aplicação tópica em cães submetidos ou não à fonoforese, apenas quantificou-se o diclofenaco sódico pela administração via oral. A facilitação da penetração transdérmica pelo ultrassom não foi verificada sob o protocolo especificado nesta pesquisa.

The aim of this study was to evaluate the plasma concentration of diclofenac sodium (DS) in dogs submitted to diclofenaco phonophoresis and to evaluate if phonophoresis induces greater absorption of this drug in dogs. Five dogs were used in eight different groups at different times: One group received oral administration of 40mg of DS per dog and seven groups received topical application of emulgel DS. The topical application area was 20cm². A continuous ultrasound frequency of 1MHz and intensity of 0.4W cm-2 was used. Blood collections were performed before the treatment (T0), and 1h (T1) and 4h (T2) after ultrasound application for all groups. DS concentrations in plasma were measured by high performance liquid choramatohraphy (HPLC). There was significant increase of DS plasma concentration only at T1 in the oral administration group. It was not possible to detect any concentration of DS in the plasma of dogs after topical application of DS, even after DS phonophoresis. The facilitation of transdermal penetration by ultrasound has not been verified under the protocol specified in this research.

Soft x-ray irradiation effects of Li2O2, Li2CO3 and Li2O revealed by absorption spectroscopy.

Li(2)O(2), Li(2)CO(3), and Li(2)O are three critical compounds in lithium-air and lithium-ion energy storage systems. Extensive measurements have been carried out to study the chemical species and their evolutions at difference stages of the device operation. While x-ray spectroscopy has been demonstrated to be one of the most powerful tools for such purpose, no systematic study on the irradiation effects have been reported. Here we carry out extensive time, position, and irradiation dependent Li K-edge soft x-ray absorption spectroscopy on these compounds with so far the best energy resolution. The ultra-high resolution in the current study allows the features in the absorption spectra to be well-resolved. The spectral lineshape thus serves as the fingerprints of these compounds, enabling the tracking of their evolution under x-ray irradiation. We found that both Li(2)O(2) and Li(2)CO(3) evidently evolve towards Li(2)O under the soft x-ray irradiation with Li(2)CO(3) exhibiting a surprisingly higher sensitivity to x-rays than Li(2)O(2). On the other hand, Li(2)O remains the most stable compound despite experiencing substantial irradiation dose. We thus conclude that high resolution soft x-ray spectroscopy could unambiguously fingerprint different chemical species, but special cautions on irradiation effects would be needed in performing the experiments and interpreting the data properly.

SAR measurement due to mobile phone exposure in a simulated biological media.

The specific absorption rate (SAR) measurements are carried out for compliance testing of personal 3G Mobile phone. The accuracy of this experimental setup has been checked by comparing the SAR in 10 gm of simulated tissue and an arbitrary shaped box. This has been carried out using a 3G mobile Phone at 1718.5 MHz, in a medium simulating brain and muscle phantom. The SAR measurement system consists of a stepper motor to move a monopole E-field probe in two dimensions inside an arbitrary shaped box. The phantom is filled with appropriate frequency-specific fluids with measured electrical properties (dielectric constant and conductivity). That is close to the average for gray and white matters of the brain at the frequencies of interest (1718.5 MHz). Induced fields are measured using a specially designed monopole probe in its close vicinity. The probe is immersed in the phantom material. The measured data for induced fields are used to compute SAR values at various locations with respect to the mobile phone location. It is concluded that these SAR values are position dependent and well below the safety criteria prescribed for human exposure.

Photolysis kinetics and influencing factors of bisphenol S in aqueous solutions.

The photodegradation of bisphenol S (BPS) in aqueous solutions was studied under different conditions. Photolysis and kinetics were investigated, as were the photolysis mechanism and the influences of initial pH value, light source, and environmental substances in water. The results showed that the photolysis of BPS occurred under UV light, and the rate increased with light source intensity. The photolysis of 5.0-50.0 mg/L BPS in water followed first-order kinetics: the rate was gamma = 0.0161C(BPS) under a 40-W UV-lamp, and the degradation half-life was 43.1 min. Due to its absorption of light, direct photolysis of BPS was a predominant pathway for BPS but was not obviously affected by reactive oxygen species. The results confirmed that the photolysis rates of BPS in alkaline water solution were faster than those in acidic and neutral water solution because of the ionization of BPS. The photodegradation rate of BPS increased in the presence of chloride and ferric ions, while the rate was inhibited by nitrate and phosphate in aqueous solution.

Salinity induced synthesis of UV-screening compound scytonemin in the cyanobacterium Lyngbya aestuarii.

Lyngbya aestuarii is the dominant cyanobacterium in Chilika lagoon occurring in all the seasons irrespective of variation in the salinity regime ranging from 3 to 28 ppt. The organism possess the UV screening scytonemin pigment, which was maximum when grown at 56 ppt salinity. Three different forms of scytonemin were detected in L. aestuarii with retention time (RT) 1.76, 2.42 and 2.94 min, however, occurrence of these forms was influenced by the salinity. Scytonemin with RT 2.42 was sensitive to higher salinity and its maximum concentration was obtained at 28 ppt salinity correlated with the highest salinity level of Chilika. Formation of multilayer colored sheath around the trichome was prominently observed at the salinity of the culture from 28 to 56 ppt. But at salinity below 7 ppt and also at more than 56 ppt salinity degradation of sheath with corresponding decrease in scytonemin was observed.

Monitoring dynamic reactions of red blood cells to UHF electromagnetic waves radiation using a novel micro-imaging technology.

Multiple state-of-the-art techniques, such as multi-dimensional micro-imaging, fast multi-channel micro-spetrophotometry, and dynamic micro-imaging analysis, were used to dynamically investigate various effects of cell under the 900 MHz electromagnetic radiation. Cell changes in shape, size, and parameters of Hb absorption spectrum under different power density electromagnetic waves radiation were presented in this article. Experimental results indicated that the isolated human red blood cells (RBCs) do not have obviously real-time responses to the ultra-low density (15 µW/cm(2), 31 µW/cm(2)) electromagnetic wave radiation when the radiation time is not more than 30 min; however, the cells do have significant reactions in shape, size, and the like, to the electromagnetic waves radiation with power densities of 1 mW/cm(2) and 5 mW/cm(2). The data also reveal the possible influences and statistical relationships among living human cell functions, radiation amount, and exposure time with high-frequency electromagnetic waves. The results of this study may be significant on protection of human being and other living organisms against possible radiation affections of the high-frequency electromagnetic waves.

Assessment of wavelength-dependent parameters of photosynthetic electron transport with a new type of multi-color PAM chlorophyll fluorometer.

Technical features of a novel multi-color pulse amplitude modulation (PAM) chlorophyll fluorometer as well as the applied methodology and some typical examples of its practical application with suspensions of Chlorella vulgaris and Synechocystis PCC 6803 are presented. The multi-color PAM provides six colors of pulse-modulated measuring light (peak-wavelengths at 400, 440, 480, 540, 590, and 625 nm) and six colors of actinic light (AL), peaking at 440, 480, 540, 590, 625 and 420-640 nm (white). The AL can be used for continuous illumination, maximal intensity single-turnover pulses, high intensity multiple-turnover pulses, and saturation pulses. In addition, far-red light (peaking at 725 nm) is provided for preferential excitation of PS I. Analysis of the fast fluorescence rise kinetics in saturating light allows determination of the wavelength- and sample-specific functional absorption cross section of PS II, Sigma(II)(λ), with which the PS II turnover rate at a given incident photosynthetically active radiation (PAR) can be calculated. Sigma(II)(λ) is defined for a quasi-dark reference state, thus differing from σ(PSII) used in limnology and oceanography. Vastly different light response curves for Chlorella are obtained with light of different colors, when the usual PAR-scale is used. Based on Sigma(II)(λ) the PAR, in units of µmol quanta/(m(2) s), can be converted into PAR(II) (in units of PS II effective quanta/s) and a fluorescence-based electron transport rate ETR(II) = PAR(II) · Y(II)/Y(II)(max) can be defined. ETR(II) in contrast to rel.ETR qualifies for quantifying the absolute rate of electron transport in optically thin suspensions of unicellular algae and cyanobacteria. Plots of ETR(II) versus PAR(II) for Chlorella are almost identical using either 440 or 625 nm light. Photoinhibition data are presented suggesting that a lower value of ETR(II)(max) with 440 nm possibly reflects photodamage via absorption by the Mn-cluster of the oxygen-evolving complex.

Fractional, biodegradable and spectral characteristics of extracted and fractionated sludge extracellular polymeric substances.

Correlation between fractional, biodegradable and spectral characteristics of sludge extracellular polymeric substances (EPS) by different protocols has not been well established. This work extracted sludge EPS using alkaline extractants (NH4OH and formaldehyde + NaOH) and physical protocols (ultrasonication, heating at 80 °C or cation exchange resin (CER)) and then fractionated the extracts using XAD-8/XAD-4 resins. The alkaline extractants yielded more sludge EPS than the physical protocols. However, the physical protocols extracted principally the hydrophilic components which were readily biodegradable by microorganisms. The alkaline extractants dissolved additional humic-like substances from sludge solids which were refractory in nature. Different extraction protocols preferably extracted EPS with distinct fractional, biodegradable and spectral characteristics which could be applied in specific usages.

Freshwater DOM quantity and quality from a two-component model of UV absorbance.

We present a model that considers UV-absorbing dissolved organic matter (DOM) to consist of two components (A and B), each with a distinct and constant spectrum. Component A absorbs UV light strongly, and is therefore presumed to possess aromatic chromophores and hydrophobic character, whereas B absorbs weakly and can be assumed hydrophilic. We parameterised the model with dissolved organic carbon concentrations [DOC] and corresponding UV spectra for c. 1700 filtered surface water samples from North America and the United Kingdom, by optimising extinction coefficients for A and B, together with a small constant concentration of non-absorbing DOM (0.80 mg DOCL⁻¹). Good unbiased predictions of [DOC] from absorbance data at 270 and 350 nm were obtained (r² = 0.98), the sum of squared residuals in [DOC] being reduced by 66% compared to a regression model fitted to absorbance at 270 nm alone. The parameterised model can use measured optical absorbance values at any pair of suitable wavelengths to calculate both [DOC] and the relative amounts of A and B in a water sample, i.e. measures of quantity and quality. Blind prediction of [DOC] was satisfactory for 9 of 11 independent data sets (181 of 213 individual samples).

Opsins in onychophora (velvet worms) suggest a single origin and subsequent diversification of visual pigments in arthropods.

Multiple visual pigments, prerequisites for color vision, are found in arthropods, but the evolutionary origin of their diversity remains obscure. In this study, we explore the opsin genes in five distantly related species of Onychophora, using deep transcriptome sequencing and screening approaches. Surprisingly, our data reveal the presence of only one opsin gene (onychopsin) in each onychophoran species, and our behavioral experiments indicate a maximum sensitivity of onychopsin to blue-green light. In our phylogenetic analyses, the onychopsins represent the sister group to the monophyletic clade of visual r-opsins of arthropods. These results concur with phylogenomic support for the sister-group status of the Onychophora and Arthropoda and provide evidence for monochromatic vision in velvet worms and in the last common ancestor of Onychophora and Arthropoda. We conclude that the diversification of visual pigments and color vision evolved in arthropods, along with the evolution of compound eyes-one of the most sophisticated visual systems known.

Oxygen evolution and chlorophyll fluorescence from multiple turnover light pulses: charge recombination in photosystem II in sunflower leaves.

Oxygen evolution and Chl fluorescence induction were measured during multiple turnover light pulses (MTP) of 630-nm wavelength, intensities from 250 to 8,000 µmol quanta m(-2) s(-1) and duration from 0.3 to 200 ms in sunflower leaves at 22 °C. The ambient O(2) concentration was 10-30 ppm and MTP were applied after pre-illumination under far-red light (FRL), which oxidized plastoquinone (PQ) and randomized S-states because of the partial excitation of PSII. Electron (e ( - )) flow was calculated as 4·O(2) evolution. Illumination with MTP of increasing length resulted in increasing O(2) evolution per pulse, which was differentiated against pulse length to find the time course of O(2) evolution rate with sub-millisecond resolution. Comparison of the quantum yields, Y (IIO) = e ( - )/hν from O(2) evolution and Y (IIF) = (F (m) - F)/F (m) from Chl fluorescence, detected significant losses not accompanied by fluorescence emission. These quantum losses are discussed to be caused by charge recombination between Q (A) (-) and oxidized TyrZ at a rate of about 1,000 s(-1), either directly or via the donor side equilibrium complex Q(A) â†’ P (D1) (+)  â†” TyrZ(ox), or because of cycling facilitated by Cyt b (559). Predicted from the suggested mechanism, charge recombination is enhanced by damage to the water-oxidizing complex and by restricted PSII acceptor side oxidation. The rate of PSII charge recombination/cycling is fast enough for being important in photoprotection.

Structural backgrounds for the formation of a catalytically competent complex with NADP(H) during hydride transfer in ferredoxin-NADP(+) reductases.

The role of the highly conserved C266 and L268 of pea ferredoxin-NADP(+) reductase (FNR) in formation of the catalytically competent complex of the enzyme with NADP(H) was investigated. Previous studies suggest that the volume of these side-chains, situated facing the side of the C-terminal Y308 catalytic residue not stacking the flavin isoalloxazine ring, may be directly involved in the fine-tuning of the catalytic efficiency of the enzyme. Wild-type pea FNR as well as single and double mutants of C266 and L268 residues were analysed by fast transient-kinetic techniques and their midpoint reduction potentials were determined. For the C266A, C266M and C266A/L268A mutants a significant reduction in the overall hydride transfer (HT) rates was observed along with the absence of charge-transfer complex formation. The HT rate constants for NADPH oxidation were lower than those for NADP(+) reduction, reaching a 30-fold decrease in the double mutant. In agreement, these variants exhibited more negative midpoint potentials with respect to the wild-type enzyme. The three-dimensional structures of C266M and L268V variants were solved. The C266M mutant shows a displacement of E306 away from the relevant residue S90 to accommodate the bulky methionine introduced. The overall findings indicate that in FNR the volume of the residue at position 266 is essential to attain the catalytic architecture between the nicotinamide and isoalloxazine rings at the active site and, therefore, for an efficient HT process. In addition, flexibility of the 268-270 loop appears to be critical for FNR to achieve catalytically competent complexes with NADP(H).