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Abstract Background Riboflavin is the precursor of several cofactors essential for normal physical and cognitive development, but only plants and some microorganisms can produce it. Humans thus rely on their dietary intake, which at a global level is mainly constituted by cereals (> 50%). Understanding the riboflavin biosynthesis players is key for advancing our knowledge on this essential pathway and can hold promise for biofortification strategies in major crop species. In some bacteria and in Arabidopsis, it is known that RibA1 is a bifunctional protein with distinct GTP cyclohydrolase II (GTPCHII) and 3,4-dihydroxy-2-butanone-4-phosphate synthase (DHBPS) domains. Arabidopsis harbors three RibA isoforms, but only one retained its bifunctionality. In rice, however, the identification and characterization of RibA has not yet been described. Results Through mathematical kinetic modeling, we identified RibA as the rate-limiting step of riboflavin pathway and by bioinformatic analysis we confirmed that rice RibA proteins carry both domains, DHBPS and GTPCHII. Phylogenetic analysis revealed that OsRibA isoforms 1 and 2 are similar to Arabidopsis bifunctional RibA1. Heterologous expression of OsRibA1 completely restored the growth of the rib3∆ yeast mutant, lacking DHBPS expression, while causing a 60% growth improvement of the rib1∆ mutant, lacking GTPCHII activity. Regarding OsRibA2, its heterologous expression fully complemented GTPCHII activity, and improved rib3∆ growth by 30%. In vitro activity assays confirmed that both OsRibA1 and OsRibA2 proteins carry GTPCHII/DHBPS activities, but that OsRibA1 has higher DHBPS activity. The overexpression of OsRibA1 in rice callus resulted in a 28% increase in riboflavin content. Conclusions Our study elucidates the critical role of RibA in rice riboflavin biosynthesis pathway, establishing it as the rate-limiting step in the pathway. By identifying and characterizing OsRibA1 and OsRibA2, showcasing their GTPCHII and DHBPS activities, we have advanced the understanding of riboflavin biosynthesis in this staple crop. We further demonstrated that OsRibA1 overexpression in rice callus increases its riboflavin content, providing supporting information for bioengineering efforts.

Background: Riboflavin is the precursor of several cofactors essential for normal physical and cognitive development, but only plants and some microorganisms can produce it. Humans thus rely on their dietary intake, which at a global level is mainly constituted by cereals (> 50%). Understanding the riboflavin biosynthesis players is key for advancing our knowledge on this essential pathway and can hold promise for biofortification strategies in major crop species. In some bacteria and in Arabidopsis, it is known that RibA1 is a bifunctional protein with distinct GTP cyclohydrolase II (GTPCHII) and 3,4-dihydroxy-2-butanone-4-phosphate synthase (DHBPS) domains. Arabidopsis harbors three RibA isoforms, but only one retained its bifunctionality. In rice, however, the identification and characterization of RibA has not yet been described. Results: Through mathematical kinetic modeling, we identified RibA as the rate-limiting step of riboflavin pathway and by bioinformatic analysis we confirmed that rice RibA proteins carry both domains, DHBPS and GTPCHII. Phylogenetic analysis revealed that OsRibA isoforms 1 and 2 are similar to Arabidopsis bifunctional RibA1. Heterologous expression of OsRibA1 completely restored the growth of the rib3∆ yeast mutant, lacking DHBPS expression, while causing a 60% growth improvement of the rib1∆ mutant, lacking GTPCHII activity. Regarding OsRibA2, its heterologous expression fully complemented GTPCHII activity, and improved rib3∆ growth by 30%. In vitro activity assays confirmed that both OsRibA1 and OsRibA2 proteins carry GTPCHII/DHBPS activities, but that OsRibA1 has higher DHBPS activity. The overexpression of OsRibA1 in rice callus resulted in a 28% increase in riboflavin content. Conclusions: Our study elucidates the critical role of RibA in rice riboflavin biosynthesis pathway, establishing it as the rate-limiting step in the pathway. By identifying and characterizing OsRibA1 and OsRibA2, showcasing their GTPCHII and DHBPS activities, we have advanced the understanding of riboflavin biosynthesis in this staple crop. We further demonstrated that OsRibA1 overexpression in rice callus increases its riboflavin content, providing supporting information for bioengineering efforts. [ABSTRACT FROM AUTHOR]

Double‐base propellants are widely used in missiles and rocket engines. Waste double‐base propellants must be safely disposed of because their safety and reliability greatly deteriorate during storage and use. Alkaline hydrolysis is a promising alternative to the open burning and open detonation disposal methods of waste propellants. To promote the industrialization of the alkaline hydrolysis treatment of waste double‐base propellants, a reaction model of the alkaline hydrolysis of SC double‐base propellants was studied. First, the relationship between the concentration of nitrite‐nitrogen and nitrate‐nitrogen in the alkaline hydrolysate was studied. The results revealed that a good linear relationship between the concentration of nitrite‐nitrogen and nitrate‐nitrogen (slope is 1.771, R2 is 0.9925). The waste SC granules produce fewer nitrite ions than fresh nitrocellulose and nitroglycerin. Secondly, based on the analysis results of scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and the thickness of SC granules before and after alkaline hydrolysis, it was assumed that the reaction model was a shrinking particle reaction model, and the limiting step was the alkaline hydrolysis rate on the surface of granules. Then, the relationship between the alkaline hydrolysis degree xSC of SC granules and reaction time t was xSC=((α+2)/(2te))t‐(α/(2te2))t2 by theoretical reaction model derivation. Additionally, the reaction model and limiting step were experimentally validated. Thus, this study provides theoretical and technical support for the safe industrial deposal and resource utilization of waste SC and other double‐base propellants with nitrocellulose and nitroglycerin as basic energetic components, as well as a basis for its fundamental research. [ABSTRACT FROM AUTHOR]

In this study, the difference of Se content in brown rice of different rice genotypes was evaluated on natural seleniferous soil. Firstly, the Se content of brown rice in 80 rice germplasm resources was determined, which ranged from 0.0249-0.1426 mg/kg, showing obvious differences. Next, two cultivars with a significant difference in Se content in brown rice, i.e., cv. Wuyangeng (high) and cv. IR68144 (low), were used to study the distribution pattern of Se in different organs. Moreover, the physiological mechanism of the Se content diversity in brown rice of the two cultivars was explored preliminarily. The results showed that the Se content of cv. Wuyangeng was 2-3 times higher compared to cv. IR68144. However, the Se contents of cv. IR68144 root and leaf were significantly higher than that of cv. Wuyangeng under both natural soil environment and artificial nutrient solution culture. Cv. IR68144 also had a stronger root Se accumulation coefficient and shoot Se transfer coefficient. Consequently, it can be inferred that the stronger Se transfer ability from stem to grain was the key reason for the higher Se content in brown rice cv. Wuyangeng than in cv. IR68144.

Abstract In battery research, the development of analytical techniques is of key importance for determining intrinsic properties of active materials ultimately dictating the battery performance. We report the application of nano-impact electrochemistry to gain insight into the intrinsic properties of commercial battery materials i.e. TiO 2 particles in non-aqueous media. Potentiostatic lithiation measurements do not only provide qualitative information about the rate-limiting step in the lithiation process, but also demonstrate that nano-impact electrochemistry is a suitable technique in non-aqueous media in complete absence of oxygen and water. Our results reveal that the intrinsic lithiation rate of individual TiO 2 particles is not – as generally assumed – determined by interfacial ion transfer kinetics, mobility of ion and/or electrons in the bulk of the particle, but by the solid-solid electron transfer. These findings have important implications for future studies of fundamental properties of battery materials considering that charge transfer in battery electrodes does not always obey Butler-Volmer kinetics. Graphical abstract fx1 Highlights • Nano-impact electrochemistry provides insight into the intrinsic properties of commercial battery materials. • Nanoimpact electrochemistry is a suitable technique also for non-aqueous media in absence of oxygen and water. • The intrinsic lithiation rate of individual TiO 2 particles is not determined by ion transfer kinetics or bulk diffusivity. • The intrinsic lithiation rate of individual TiO 2 particles is determined by solid-solid electron transfer. [ABSTRACT FROM AUTHOR]

In this study, eight different submerged aquatic species were screened by batch biochemical methane potential and anaerobic degradability tests to identify a promising/suitable feedstock. Kinetics of the best-screened substrate were studied in a mesophilic semi-continuous experiment. The aquatic species Myriophyllum aquaticum, Egeria densa and Potamogeton perfoliatus showed relatively higher methane yields of over 400 NmL/g-VS (volatile solids). Semi-continuous operation was carried out by feeding E. densa for over 400 days. The achieved results were 33%–53% chemical oxygen demand (COD) reduction and methane yield of 126–231 NmL/g-VS with a short hydraulic retention time (HRT). Additionally, the NH4+ and PO43− releases from the biomass to water were found to be low (18%–27% and 2.5%–3.9%) throughout the experiment. Hydrolysis was the limiting step in the digestion of E. densa, regardless of changes in HRT (15–45 days). The acid-phase model indicated that the hydrolysis rate constant (kh) of E. densa was 0.058 one/day, which was one third lower the kh value of food waste, but quite similar to cow manure.

The kinetics of the 99Mo sorption onto T-5 sorbent at different pH values of the nitric acid solution was studied. The previously suggested limiting steps of the 99Mo sorption onto T-5 sorbent were confirmed. The effective rate constant of the molybdenum sorption onto titanium hydroxide (fast step) as a function of pH passes through a maximum at pH from 4.5 to 5.5, which does not correspond to the maximum of the pH dependence of the distribution coefficient but correlates with the maximal content of one of the sorbable species, HMoO4-, in the solution. Thus, the rate of Mo sorption onto titanium hydroxide as a function of pH passes through a maximum at a minimal negative charge of the sorbate near pH of the isoelectric point of the sorbent. The kinetic studies confirm the scheme of the sorption of molybdenum hydroxo complexes onto titanium hydroxide, suggested previously on the basis of the logkd-pH and log e-log[m] sorption isotherms. According to this scheme, simultaneous redistribution and sorption of sorbable molybdenum species, MoO2OH+, H2MoO4, and HMoO4-, are possible. [ABSTRACT FROM AUTHOR]

The kinetics of Мо sorption onto T-5 sorbent in the batch mode at different values of the stirring rate, temperature, and sorbent granule size was studied. All the experimental kinetic curves are described by a sum of two exponents. In the first time interval of phase contact, from 0 to 5-10 min, the sorption is controlled by external diffusion, and in the interval from 10 to 40 min the step of transformation of unsorbable Mo species into sorbable hydroxo species also becomes a rate-determining step (external kinetic control). In the final stage (time interval from 40 min to 2-3 h), the sorption occurs at the lowest rate, and the Mo diffusion in micropores and overcoming of electrostatic hindrance on the surface of titanium hydroxide pores become the most probable limiting steps. In sulfate solutions, the Mo sorption rate considerably decreases already in the first stage, and the external kinetic control becomes prevalent because of hindered redistribution of unsorbable sulfate complexes into sorbable hydroxo complexes. The controlling factor in the sulfate and nitrate solutions in the second stage is presumably the same. [ABSTRACT FROM AUTHOR]

Bioanodes were formed with electrodes made of carbon felt and equipped with a titanium electrical collector, as commonly used in microbial fuel cells. Electrochemical impedance spectroscopy (EIS) performed on the abiotic electrode system evidenced two time constants, one corresponding to the “collector/carbon felt” contact, the other to the “carbon felt/solution” interface. Such a two time constant system was characteristics of the two-material electrode, independent of biofilm presence. EIS was then performed during the bioanode formation around the constant applied potential of 0.1 V/SCE. The equivalent electrical model was similar to that of the abiotic system. Due to the high salinity of the electrolyte (45 g·L − 1 NaCl) the electrolyte resistance was always very low. The bioanode development induced kinetic heterogeneities that were taken into account by replacing the pure capacitance of the abiotic system by a constant phase element for the “carbon felt/solution” interface. The current increase from 0 to 20.6 A·m − 2 was correlated to the considerable decrease of the charge transfer resistance of the “carbon felt/solution” interface from 2.4 10 4 to 92 Ω·cm 2 . Finally, EIS implemented at 0.4 V/SCE showed that the limitation observed at high potential values was not related to mass transfer but to a biofilm-linked kinetics. [ABSTRACT FROM AUTHOR]

In this study, eight different submerged aquatic species were screened by batch biochemical methane potential and anaerobic degradability tests to identify a promising/suitable feedstock. Kinetics of the best-screened substrate were studied in a mesophilic semi-continuous experiment. The aquatic species Myriophyllum aquaticum, Egeria densa and Potamogeton perfoliatus showed relatively higher methane yields of over 400 NmL/g-VS (volatile solids). Semi-continuous operation was carried out by feeding E. densa for over 400 days. The achieved results were 33%-53% chemical oxygen demand (COD) reduction and methane yield of 126-231 NmL/g-VS with a short hydraulic retention time (HRT). Additionally, the NH4+ and PO43- releases from the biomass to water were found to be low (18%-27% and 2.5%-3.9%) throughout the experiment. Hydrolysis was the limiting step in the digestion of E. densa, regardless of changes in HRT (15-45 days). The acid-phase model indicated that the hydrolysis rate constant (kh) of E. densa was 0.058 one/day, which was one third lower the kh value of food waste, but quite similar to cow manure. [ABSTRACT FROM AUTHOR]

A detailed analysis of chain mechanisms of stratospheric ozone destruction in the O, HO, NO, ClO, and BrO cycles is performed and an algorithm for calculating the characteristics of the limiting stages of chain propagation and chain terminations and the chain length for these cycles is proposed. It is shown that a proper description of ozone destruction in these chain processes is only possible through the determination of the rates of limiting steps, while the simple addition of the rates of the reactions in which odd oxygen is lost leads to a significant overestimation of the calculation results. The role of the zero chain processes in these cycles. It is shown that these processes play a decisive role in the formation of the families of odd oxygen, nitrogen, chlorine, and bromine. Based on the methods developed and the results of calculations within the framework of the Socrates two-dimensional model, the rate of ozone loss and the length of the chain in these cycles for the model conditions of June 2000 at a latitude of 65° N are determined. The results make it possible to compare the effectiveness of ozone cycles and more fully assess their contribution to the destruction of the ozone layer, both in the past and in the future. [ABSTRACT FROM AUTHOR]

The characteristics of the chain processes of stratospheric ozone depletion are considered. It is shown that the basic feature of these processes is the presence of a limiting stage, which determines the rate of chain propagation in this process and, accordingly, the rate of ozone depletion. It is also shown that the practice, so far widespread in the stratospheric chemistry, of defining the rate-limiting step of the chain process as a single reaction with the lowest rate throughout the stratosphere does not enable to correctly determine the rare of the chain process and leads to a significant overestimation of the latter. Methods for correctly calculating the rate of the limiting step for an arbitrary number of chain propagation reactions and for determining the termination rate and the chain length are for the first time proposed. Application of the developed method is demonstrated by the example of the nitrogen oxide cycle. [ABSTRACT FROM AUTHOR]

We studied the pH-dependence of ribosome catalyzed peptidyl transfer from fMet-tRNAfMet to the aa-tRNAs Phe-tRNAPhe, Ala- tRNAAla, Gly-tRNAGly, Pro-tRNAPro, Asn-tRNAAsn, and lle-tRNAlle, selected to cover a large range of intrinsic pKa-values for the α-amino group of their amino acids. The peptidyl transfer rates were different at pH 7.5 and displayed different pH-dependence, quantified as the pH-value, pKobsa at which the rate was half maximal. The pKaobsa-values were downshifted relative to the intrinsic pKa-value of aa-tRNAs in bulk solution. Gly-tRNAGly had the smallest downshift. while lle-tRNAlle and Ala-tRNAAlahad the largest downshifts. These downshifts correlate strongly with molecular dynamics (MD) estimates of the downshifts in pKa-values of these aa-tRNAs upon A-site binding. Our data show the chemistry of peptide bond formation to be rate limiting for peptidyl transfer at pH 7.5 in the Gly and Pro cases and indicate rate limiting chemistry for all six aa-tRNAs. [ABSTRACT FROM AUTHOR]

A mathematical model was developed to assess limiting step of mass transfer in the n-hexadecane (HXD) biodegradation by a microbial consortium. A double Monod kinetic (oxygen and HXD) for biomass production was successfully used to describe the experimental data. Good fitting (r 2 = 0.92) between the model solution and experimental data was obtained. The overall mass transfer coefficients for HXD, kLaHXD, and oxygen, kLaO2 , were experimentally determined and biosurfactant production was indirectly determined through surface tension measurements in the aqueous phase. It was observed that a surface tension reduction from 65 (0 h of culture) to 47mNm-1 (240 h of culture) was related to a decrease of 52% in the HXD droplet diameter and to an increase of 63% in kLaHXD, respect the initial values. Conversely, kLaO2 was repressed up to 37% compared to the initial value. The maximum rate analysis based on the mathematical model showed that HXD transfer was up to 5-fold lower than its consumption. On the contrary, oxygen transfer was always higher than its consumption. Thus, the limiting step under the working conditions was the HXD transfer to the aqueous phase. However, slight reductions in kLaO2 could result in oxygen transfer limitations during the last 60 h of the cultures. [ABSTRACT FROM AUTHOR]

Abstract: The concept of the limiting step is extended to the asymptotology of multiscale reaction networks. Complete theory for linear networks with well separated reaction rate constants is developed. We present algorithms for explicit approximations of eigenvalues and eigenvectors of kinetic matrix. Accuracy of estimates is proven. Performance of the algorithms is demonstrated on simple examples. Application of algorithms to nonlinear systems is discussed. [Copyright &y& Elsevier]

Effects of growth light intensity on the temperature dependence of CO2 assimilation rate were studied in tobacco ( Nicotiana tabacum) because growth light intensity alters nitrogen allocation between photosynthetic components. Leaf nitrogen, ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) contents increased with increasing growth light intensity, but the cyt f/Rubisco ratio was unaltered. Mesophyll conductance to CO2 diffusion ( gm) measured with carbon isotope discrimination increased with growth light intensity but not with measuring light intensity. The responses of CO2 assimilation rate to chloroplast CO2 concentration ( Cc) at different light intensities and temperatures were used to estimate the maximum carboxylation rate of Rubisco ( Vcmax) and the chloroplast electron transport rate ( J). Maximum electron transport rates were linearly related to cyt f content at any given temperature (e.g. 115 and 179 µmol electrons mol−1 cyt f s−1 at 25 and 40 °C, respectively). The chloroplast CO2 concentration ( Ctrans) at which the transition from RuBP carboxylation to RuBP regeneration limitation occurred increased with leaf temperature and was independent of growth light intensity, consistent with the constant ratio of cyt f/Rubisco. In tobacco, CO2 assimilation rate at 380 µmol mol−1 CO2 concentration and high light was limited by RuBP carboxylation above 32 °C and by RuBP regeneration below 32 °C. [ABSTRACT FROM AUTHOR]

• Background and Aims Influences of rising global CO2 concentration and temperature on plant growth and ecosystem function have become major concerns, but how photosynthesis changes with CO2 and temperature in the field is poorly understood. Therefore, studies were made of the effect of elevated CO2 on temperature dependence of photosynthetic rates in rice (Oryza sativa) grown in a paddy field, in relation to seasons in two years.• Methods Photosynthetic rates were determined monthly for rice grown under free-air CO2 enrichment (FACE) compared to the normal atmosphere (570 vs 370 µmol mol−1). Temperature dependence of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (Vcmax) and the maximum rate of electron transport (Jmax) were analysed with the Arrhenius equation. The photosynthesis–temperature response was reconstructed to determine the optimal temperature (Topt) that maximizes the photosynthetic rate.• Key Results and Conclusions There was both an increase in the absolute value of the light-saturated photosynthetic rate at growth CO2 (Pgrowth) and an increase in Topt for Pgrowth caused by elevated CO2 in FACE conditions. Seasonal decrease in Pgrowth was associated with a decrease in nitrogen content per unit leaf area (Narea) and thus in the maximum rate of electron transport (Jmax) and the maximum rate of RuBP carboxylation (Vcmax). At ambient CO2, Topt increased with increasing growth temperature due mainly to increasing activation energy of Vcmax. At elevated CO2, Topt did not show a clear seasonal trend. Temperature dependence of photosynthesis was changed by seasonal climate and plant nitrogen status, which differed between ambient and elevated CO2. [ABSTRACT FROM AUTHOR]

Growth temperature alters temperature dependence of the photosynthetic rate (temperature acclimation). In many species, the optimal temperature that maximizes the photosynthetic rate increases with increasing growth temperature. In this minireview, mechanisms involved in changes in the photosynthesis–temperature curve are discussed. Based on the biochemical model of photosynthesis, change in the photosynthesis–temperature curve is attributable to four factors: intercellular CO2 concentration, activation energy of the maximum rate of RuBP (ribulose-1,5-bisphosphate) carboxylation (Vc max), activation energy of the rate of RuBP regeneration (Jmax), and the ratio of Jmax to Vc max. In the survey, every species increased the activation energy of Vc max with increasing growth temperature. Other factors changed with growth temperature, but their responses were different among species. Among these factors, activation energy of Vc max may be the most important for the shift of optimal temperature of photosynthesis at ambient CO2 concentrations. Physiological and biochemical causes for the change in these parameters are discussed. [ABSTRACT FROM PUBLISHER]

A preface to the "Journal of Experimental Botany" is presented.