Graphene‐Tailored Thermodynamics and Kinetics to Fabricate Metal Borohydride Nanoparticles with High Purity and Enhanced Reversibility

Due to their ultrahigh theoretical capacity, metal borohydrides are considered to be one of the most promising candidate hydrogen storage materials. Their application still suffers, however, from high operating temperature, sluggish kinetics, and poor reversibility. Designing nanostructures is an effective way of addressing these issues, but seeking suitable approaches remains a big challenge. Here, a space‐confined solid‐gas reaction to synthesize Mg(BH₄)₂ nanoparticles supported on grapheme is reported, which serves as the structural support for the dispersed Mg(BH₄)₂ nanoparticles. More notably, density functional theory calculations reveal that graphene could weaken both the Mg? H bonds of MgH₂ and B? B bonds of B₂H₆, which could thermodynamically and kinetically facilitate the chemical transformation to synthesize Mg(BH₄)₂ with high purity. Because of the synergistic effects of both the significant reduction in particle size and the catalytic effect of graphene, an onset dehydrogenation temperature of ≈154 °C is observed for Mg(BH₄)₂ nanoparticles, and a complete dehydrogenation could be achieved at a temperature as low as 225 °C, with the formation of MgB₂ as the by‐product. This work provides a new perspective to tailoring the thermodynamics and kinetics of chemical reactions toward the favorable synthesis of functional inorganic materials.     

Lithium Fast‐Ionic Conduction in Complex Hydrides: Review and Prospects

Complex hydrides exhibit various energy‐related functions such as hydrogen storage, microwave absorption, and neutron shielding. Furthermore, another novel energy‐related function was recently reported by the authors; lithium fast‐ionic conduction, which suggests that complex hydrides may be a potential candidate for solid electrolytes in lithium‐ion batteries. This review presents the recent progress in the development of lithium fast‐ionic conductors of complex hydrides. First, the fast‐ionic conduction in LiBH₄ as a result of clarifying the mechanism of microwave absorption is presented, and then the conceptual development of complex hydrides as a new type of solid‐state lithium fast‐ionic conductors in LiBH₄–, LiNH₂–, and LiAlH₄‐based complex hydrides is discussed. Finally, the future prospects of this study from both application and fundamental viewpoints are described: possible use as solid electrolytes for batteries, formation of ionic liquids in complex hydrides, and similarity between complex hydrides and Laves‐phase metal hydrides.     

Near Ambient Condition Hydrogen Storage in a Synergized Tricomponent Hydride System

Reversible hydrogen storage over hydrides of light elements (HLEs) under ambient condition has been pursued actively for nearly two decades. However, because of unfavorable thermodynamics and/or severe kinetic barrier of HLEs, limited progress has been made. Here, it is demonstrated that the interaction of LiBH₄ with Mg(NH₂)₂ and LiH, three of the most investigated HLEs, can lead to a fully reversible dehydrogenation/rehydrogenation cycle at temperatures below 373 K. More importantly, with the desorption enthalpy of 24 kJ (mol H₂)^?1 the dehydrogenation process at 1.0 bar H₂ is theoretically possible to be as low as 266 K. Characterization of this combination of HLEs shows that LiBH₄ serves as a reagent complexing with intermediates and products of the dehydrogenation of Mg(NH₂)₂‐LiH, and significantly alters the overall thermodynamic and kinetic properties of the system.     

Catalytic De/Hydrogenation in Mg by Co‐Doped Ni and VOx on Active Carbon: Extremely Fast Kinetics at Low Temperatures and High Hydrogen Capacity

A multi‐component catalyst Ni‐VO_x/AC (VO_x is comprised of V₂O₅ and VO₂, x = 2.18) was synthesized by a wet impregnation method. The synthesized Ni‐VO_x/AC shows a superior catalytic effect on de/hydrogenation of Mg. The MgH₂+Ni‐VO_x/AC composites can absorb 6.2 wt.‐% hydrogen within only 1 min at 150 °C under a hydrogen pressure of 2 MPa and desorb 6.5 wt.‐% hydrogen within 10 min at 300 °C under an initial hydrogen pressure of 1 KPa, which overcomes a critical barrier for practical use of Mg as a hydrogen storage material. A significant decrease of activation energy (E_a) indicates that Ni‐VO_x/AC catalyst is highly efficient for Mg de/hydrogenation, which may be ascribed to the synergistic effect of bimetals (metal oxides) and nanocarbon.     

Prediction of thermodynamically reversible hydrogen storage reactions utilizing Ca–M(M = Li, Na, K)–B–H systems: a first-principles study

Calcium borohydride is a potential candidate for onboard hydrogen storage because it has a high gravimetric capacity (11.5 wt.%) and a high volumetric hydrogen content (～130 kg m^?3). Unfortunately, calcium borohydride suffers from the drawback of having very strongly bound hydrogen. In this study, Ca(BH₄)₂ was predicted to form a destabilized system when it was mixed with LiBH₄, NaBH₄, or KBH₄. The release of hydrogen from Ca(BH₄)₂ was predicted to proceed via two competing reaction pathways (leading to CaB₆ and CaH₂ or CaB₁₂H₁₂ and CaH₂) that were found to have almost equal free energies. Using a set of recently developed theoretical methods derived from first principles, we predicted five new hydrogen storage reactions that are among the most attractive of those presently known. These combine high gravimetric densities (>6.0 wt.% H₂) with have low enthalpies [approximately 35 kJ/(mol^?1 H₂)] and are thermodynamically reversible at low pressure within the target window for onboard storage that is actively being considered for hydrogen storage applications. Thus, the first-principles theoretical design of new materials for energy storage in future research appears to be possible.

Density functional theory study on (LiNH<Subscript>2</Subscript>)<Subscript>n</Subscript> (<Emphasis Type="Italic">n</Emphasis> = 1–5) clusters

Geometrical structures and relative stabilities of (LiNH₂)_n (n = 1–5) clusters were studied using density functional theory (DFT) at the B3LYP/6-31G* and B3LYP/6-31++G* levels. The electronic structures, vibrational properties, N–H bond dissociation energies (BDE), thermodynamic properties, bond properties and ionization potentials were analyzed for the most stable isomers. The calculated results show that the Li–N and Li–Li bonds can be formed more easily than those of the Li–H or N–H bonds in the clusters, in which NH₂ is bound to the framework of Li atomic clusters with fused rings. The average binding energies for each LiNH₂ unit increase gradually from 142 kJ mol⁻¹ up to about 180 kJ mol⁻¹ with increasing n. Natural bond orbital (NBO) analysis suggests that the bonds between Li and NH₂ are of strong ionicity. Three-center–two-electron Li–N–Li bonding exists in the (LiNH₂)₂ dimer. The N–H BDE values indicate that the change in N–H BDE values from the monomer a1 to the singlet-state clusters is small. The N–H bonds in singlet state clusters are stable, while the N–H bonds in triplet clusters dissociate easily. A study of their thermodynamic properties suggests that monomer a1 forms clusters (b1, c1, d2 and e1) easily at low temperature, and clusters with fewer numbers of rings tend to transfer to ones with more rings at low temperature. E _g, E _HOMO and E _av decrease gradually, and become constant. Ring-like (LiNH₂)_3,4 clusters possess higher ionization energy (VIE) and E _g, but lower values of E _HOMO. Ring-like (LiNH₂)_3,4 clusters are more stable than other types. A comparison of structures and spectra between clusters and crystal showed that the NH₂ moiety in clusters has a structure and spectral features similar to those of the crystal.     

A new interaction mechanism of LiNH2 with MgH2: magnesium bond

Quantum chemical calculations were performed for LiNH₂–HMgX (X?=?H, F, Cl, Br, CH₃, OH, and NH₂) complexes to propose a new interaction mechanism between them. This theoretical survey showed that the complexes are stabilized through the combinative interaction of magnesium and lithium bonds. The binding energies are in the range of 63.2–66.5 kcal mol^?1, i.e., much larger than that of the lithium bond. Upon complexation, both Mg–H and Li–N bonds are lengthened. Substituents increase Mg-H bond elongation and at the same time decrease Li-N bond elongation. These cyclic complexes were characterized with the presence of a ring critical point and natural population analysis charges.

Biopterin

Intraperitoneally injected [¹⁴C]biopterin (B), dihydrobiopterin (BH₂), and tetrahydrobiopterin (BH₄) penetrated the brain rapidly, but in amounts sufficient to represent only a minor source of supply. Unlike isobiopterin, B was rapidly reduced to BH₂ in brain. The distribution of BH₄ and BH₂, but not of B, could be correlated with the tryptophan and tyrosine hydroxylase activity of various cerebral areas. In the whole brain, the sizes of pools were 0.117 for B, 0.204 for BH₂, and 0.341 g/g for BH₄, while the cerebral turnover rates of B, BH₂, and BH₄ were 0.25, 0.43, and 0.71 nmol/g per h, respectively. From birth through development, the cerebral levels of B remained constant, whereas the levels of the reduced biopterins increased. After subcellular fractionation, 65% of the biopterins (B, BH₂, and BH₄) were recovered from the supernatant. Of the organelles, microsomes contained the largest concentration of pterins. About 1/6 of all pterins in the brain was present in the synaptosomes.     

Singlet and triplet excited state properties of natural chlorophylls and bacteriochlorophylls

Ten naturally occurring chlorophylls (a, b, c ₂, d) and bacteriochlorophylls (a, b, c, d, e, g) were purified and studied using the optical spectroscopic techniques of both steady state and time-resolved absorption and fluorescence. The studies were carried out at room temperature in nucleophilic solvents in which the central Mg is hexacoordinated. The comprehensive studies of singlet excited state lifetimes show a clear dependency on the structural features of the macrocycle and terminal substituents. The wide-ranging studies of triplet state lifetime demonstrate the existence of an energy gap law for these molecules. The knowledge of the dynamics and the energies of the triplet state that were obtained in other studies allowed us to construct an energy gap law expression that can be used to estimate the triplet state energies of any (B)chlorophyll molecule from its triplet lifetime obtained in a liquid environment.     

Oxidation of tetrahydrobiopterin by peroxynitrite or oxoferryl species occurs by a radical pathway

The molecular mechanisms of tetrahydrobiopterin (BH₄) oxidation by peroxynitrite (ONOO^-) was studied using ultra-weak chemiluminescence, electron paramagnetic resonance (EPR) and UV-visible diodearray spectrophotometry, and compared to BH₄ oxidation by oxoferryl species produced by the myoglobin/hydrogen peroxide (Mb/H₂O₂) system. The oxidation of BH₄ by ONOO^- produced a weak chemiluminescence, which was altered by addition of 50 mM of the spin trap α-(4-pyridyl-1-oxide)-N-tert butylnitrone (POBN). EPR spin trapping demonstrated that the reaction occurred at least in part by a radical pathway. A mixture of two spectra composed by an intense six-line spectrum and a fleeting weak nine-line one was observed when using ONOO^-. Mb/H₂O₂ produced a short-living light emission that was suppressed by the addition of BH₄. Simultaneous addition of POBN, BH₄ and Mb/H₂O₂ produced the same six-line EPR spectrum, with a signal intensity depending on BH₄ concentration. Spectrophotometric studies confirmed the rapid disappearance of the characteristic peak of ONOO^- (302 nm) as well as substantial modifications of the initial BH₄ spectrum with both oxidant systems. These data demonstrated that BH₄ oxidation, either by ONOO^- or by Mb/H₂O₂, occurred with the production of activated species and by radical pathways.     

The electronic structure of Ti(BH4)3: Photoelectron spectra and calculation of vertical ionization energies

He I and He II PE spectra of Ti(BH₄)₃ are reported and assigned by reference to density functional calculations on the molecule and cation. The performance of different functionals in predicting the first vertical ionization energy is assessed. Calculations based on hybrid functionals are found to give ionisation energies closer to the experimental value than those using pure density functionals. The accuracy of the ΔSCF method and time dependent density functional theory in calculating higher vertical ionization energies is also examined.     

Synthesis and thermal stability of Zr(BH4)4 and Zr(BD4)4 produced by mechanochemical processing

Zirconium tetrahydroborate Zr(BH₄)₄ and its deuteride compound Zr(BD₄)₄ were successfully synthesized by mechanochemical reaction between NaBH₄ or NaBD₄ and ZrCl₄, reaching yields of 55% and 46%, respectively. The influence of the synthesis parameters on the yield of Zr(BH₄)₄ was analyzed. The composition of the ZrCl₄:NaBH₄ starting mixture and the use of LiBH₄ instead of NaBH₄ as reactive show a clear effect on the Zr(BH₄)₄ yield. Instead, milling atmosphere does not affect the amount of the obtained product. FTIR analysis of atmosphere inside of milling vial allows to determine the formation of diborane during milling from Zr(BH₄)₄ decomposition. Thermal stability of pure Zr(BH₄)₄ and its deuterated compound was studied by combined gas-phase FTIR and DSC measurements under flowing Ar. We found that Zr(BH₄)₄/Zr(BD₄)₄ melt at about 305/303 K, decompose at about 430 K from the gas-phase and show evolution of B₂H₆/B₂D₆ under heating.     

Catalytic Influence of Various Cerium Precursors on the Hydrogen Sorption Properties of NaAlH4

Sodium alanate (NaAlH₄) is one of the metal complex hydrides most often investigated for use as a hydrogen‐storage material. Doped with transition or rare earth metal compounds, NaAlH₄ can absorb and release hydrogen in low and medium temperature ranges with excellent reversibility and cycling stability. The properties of NaAlH₄ doped with CeCl₃ differ from materials with other dopants, with faster sorption kinetics and a more stable capacity. In this paper, various precursors of Ce are applied to investigate their catalytic effects on the sorption performance of this material. The re‐hydrogenation is found to be completed in approximately 10 min. Although all the Ce precursors investigated in this work result in reversible hydrogen storage materials, desorption kinetics are enhanced upon formation of cerium aluminide (CeAl₄) in the composites. While the use of CeAl₄ instead of CeCl₃ can increase the hydrogen capacity by bypassing the formation of the ineffective NaCl, the highest capacity of 4.9 wt%—close to the theoretical value—is obtained from NaAlH₄ doped directly with metallic cerium. Furthermore, dehydriding under back pressures is also investigated to evaluate the H₂ desorption rates under practical conditions. At 3 bar H₂ pressure, the second desorption step of NaAlH₄ is fully suppressed at 150 °C and only 2.5 wt% H was released, whereas at 180 °C the capacity is not much affected, which is interesting for combination in a system with a high‐temperature PEM fuel cell.     

Enhancing the Regeneration Process of Consumed NaBH4 for Hydrogen Storage

Sodium borohydride (NaBH₄) is regarded as an excellent hydrogen‐generated material, but its irreversibility of hydrolysis and high cost of regeneration restrict its large‐scale application. In this study a convenient and economical method for NaBH₄ regeneration is developed for the first time without hydrides used as starting materials for the reduction process. The real hydrolysis by‐products (NaBO₂ · 2H₂O and NaBO₂ · 4H₂O), instead of dehydrated sodium metaborate (NaBO₂), are applied for the regeneration of NaBH₄ with Mg at room temperature and atmospheric pressure. Therefore, the troublesome heat‐wasting process to obtain NaBO₂ using a drying procedure at over 350 °C from NaBO₂ · xH₂O is omitted. Moreover, the highest regeneration yields of NaBH₄ are achieved to date with 68.55% and 64.06% from reaction with NaBO₂ · 2H₂O and NaBO₂ · 4H₂O, respectively. The cost of NaBH₄ regeneration shows a 34‐fold reduction compared to the previous study that uses MgH₂ as the reduction agent, where H₂ is obtained from a separate process. Furthermore, the regeneration mechanism of NaBH₄ is clarified and the intermediate compound, NaBH₃(OH), is successfully observed for the first time during the regeneration process.     

Modeling of biopterin-dependent pathways of eNOS for nitric oxide and superoxide production

Endothelial dysfunction is associated with increase in oxidative stress and low NO bioavailability. The endothelial NO synthase (eNOS) uncoupling is considered an important factor in endothelial cell oxidative stress. Under increased oxidative stress, the eNOS cofactor tetrahydrobiopterin (BH₄) is oxidized to dihydrobiopterin, which competes with BH₄ for binding to eNOS, resulting in eNOS uncoupling and reduction in NO production. The importance of the ratio of BH₄ to oxidized biopterins versus absolute levels of total biopterin in determining the extent of eNOS uncoupling remains to be determined. We have developed a computational model to simulate the kinetics of the biochemical pathways of eNOS for both NO and O₂^•− production to understand the roles of BH₄ availability and total biopterin (TBP) concentration in eNOS uncoupling. The downstream reactions of NO, O₂^•−, ONOO⁻, O₂, CO₂, and BH₄ were also modeled. The model predicted that a lower [BH₄]/[TBP] ratio decreased NO production but increased O₂^•− production from eNOS. The NO and O₂^•− production rates were independent above 1.5 μM [TBP]. The results indicate that eNOS uncoupling is a result of a decrease in [BH₄]/[TBP] ratio, and a supplementation of BH₄ might be effective only when the [BH₄]/[TBP] ratio increases. The results from this study will help us understand the mechanism of endothelial dysfunction.     

Biopterin Metabolism and eNOS Expression during Hypoxic Pulmonary Hypertension in Mice

Tetrahydrobiopterin (BH₄), which fosters the formation of and stabilizes endothelial NO synthase (eNOS) as an active dimer, tightly regulates eNOS coupling / uncoupling. Moreover, studies conducted in genetically-modified models demonstrate that BH₄ pulmonary deficiency is a key determinant in the pathogenesis of pulmonary hypertension. The present study thus investigates biopterin metabolism and eNOS expression, as well as the effect of sepiapterin (a precursor of BH₄) and eNOS gene deletion, in a mice model of hypoxic pulmonary hypertension. In lungs, chronic hypoxia increased BH₄ levels and eNOS expression, without modifying dihydrobiopterin (BH₂, the oxidation product of BH₄) levels, GTP cyclohydrolase-1 or dihydrofolate reductase expression (two key enzymes regulating BH₄ availability). In intrapulmonary arteries, chronic hypoxia also increased expression of eNOS, but did not induce destabilisation of eNOS dimers into monomers. In hypoxic mice, sepiapterin prevented increase in right ventricular systolic pressure and right ventricular hypertrophy, whereas it modified neither remodelling nor alteration in vasomotor responses (hyper-responsiveness to phenylephrine, decrease in endothelium-dependent relaxation to acetylcholine) in intrapulmonary arteries. Finally, deletion of eNOS gene partially prevented hypoxia-induced increase in right ventricular systolic pressure, right ventricular hypertrophy and remodelling of intrapulmonary arteries. Collectively, these data demonstrate the absence of BH₄/BH₂ changes and eNOS dimer destabilisation, which may induce eNOS uncoupling during hypoxia-induced pulmonary hypertension. Thus, even though eNOS gene deletion and sepiapterin treatment exert protective effects on hypoxia-induced pulmonary vascular remodelling, increase on right ventricular pressure and / or right ventricular hypertrophy, these effects appear unrelated to biopterin-dependent eNOS uncoupling within pulmonary vasculature of hypoxic wild-type mice.     

Reversed micelles to mimic the active site of metalloenzymes

The structure and reactivity of cobalt(II), nickel(II), and copper(II) halides have been investigated in 0.20 M CTAX (X = Cl, Br) |CHCl₃ reversed micelles. The former two metal ions adopt a tetrahedral configuration at low water concentrations in the micelle. The tetrahedral complexes are converted to octahedral aqua complexes by increasing the water concentration (solvochromism) or by lowering the temperature (thermochromism). Upon reaction with imidazole, the tetrahedral cobalt and nickel halide complexes also undergo a structural transformation into an octahedral configuration with imidazole coordination. At low water concentrations, copper halides form a polynuclear complex bridged by halide ions and these halogen bridges are easily broken upon addition of water or imidazole. The copper complexes produced by reaction with imidazole were deduced to be CuIm₂X₂ and CuIm₄X₂ at intermediate and high ligand concentrations, respectively. It was also found that the cupric ion in reversed micelles is readily reduced to the cuprous ion with 2-mercaptoethanol, and the cuprous ion is oxidized to the cupric ion by reaction with hydrogen peroxide.     

BIOPTERIN V. DE NOVO SYNTHESIS OF DIHYDROBIOPTERIN: EVIDENCE FOR ITS QUINONOID STRUCTURE AND LACK OF DEPENDENCE OF ITS REDUCTION TO TETRAHYDROBIOPTERIN ON DIHYDROFOLATE REDUCTASE

Samples of quinonoid-l -erythrodihydrobiopterin (q-BH₂) and quinonoid-6-methyl-dihydro-pterin (q-6-MPH₂) were prepared by oxidation of l -erythro-5,6,7,8-tetrahydrobiopterin (BH₄) and 5,6,7,8-tetrahydro-6-methylpterin (6-MPH₄) and separated from ^D-erythro-7,8-dihydrobiopterin (7,8-BH₂) and 6-methyl-7,8-dihydropterin (7,8-6-MPH₂) as well as from the tetrahydropterins on phosphocellulose column by high-pressure liquid chromatography. The quinonoid dihydropterins were identified and quantitated by scan of their ultraviolet absorption and fluorescence emission spectra through their rearrangement to their 7,8-tautomer and also by gas chromatography of their rapidly synthesized trimethylsilyl derivative. Identification was also achieved by the enzymatic reduction of [³H]q-BH²to [³H]BH₄ by dihydrofolate reductase (DHFR). Direct proof for the enzymatic synthesis of the q-BH² from GTP or from 2-amino-6-(5′-triphosphoribosyl)-amino-5- or -6-formamido-6-hydroxypyrimi-dine (FPyd-P₃) was obtained by isolation of the compound which was identical in all respects to the q-BH₂ obtained by chemical synthesis from BH₄. The reduction of enzymatically synthesized q-BH² by dihydropteridine reductase (DHPR) to BH⁴ was not inhibited by methotrexate (MTX). When the enzymatically synthesized q-BH² was converted to 7,8-BH₂, it was reduced only by DHFR. This reduction, however, was inhibited by MTX. On the biosynthetic pathway from GTP to dihydrobiopterin, the enzyme responsible for the appearance of the quinonoid structure is the d -erythro-dihydroneopterin triphosphate synthetase, the product of which (quinonoid d -erythro-dihydroneopterin triphosphate) is converted to quinonoid dihydrobiopterin by l -erythro-dihydrobiopterin synthetase. Experiments in vivo established that DHFR does not participate in the reduction of dihydrobiopterin to tetra-hydrobiopterin when the former is synthesized from GTP de novo. MTX at 5 × 10^?6M exerted no inhibition on the reduction of the biosynthetic dihydrobiopterin to tetrahydrobiopterin in vivo, yet completely inhibited the reduction of intraventricularly injected tritiated dihydrofolate ([³H]FH₂) to tritiated tetrahydrofolate ([³H]FH₄).     

Synthesis and stereochemistry of new cobalt azo-nitroso complexes

Synthesis and characterization of new cobalt-substituted phenylazo 2,4-dinitrosoresorcinol complexes have been carried out. The analytical data depict the formation of complex compounds with the stoichiometry 2:3 (o-COOH, m-NO₂) and 1:2 (o-Cl, o-CH₃, m-Cl, m-CH₃). All the complexes are of low spin in octahedral and square planar or distorted tetrahedral environments. The octahedral ⇄ tetrahedral equilibria are evident. The complexes in the presence of basic compounds gave some addition products. The electronic transitions and the ligand field parameters are assigned and calculated. The complex formation occurred through the azo group and the phenolic oxygen atom in most complexes. In the o-carboxy ligand, the COO⁻, NN and the oximic groups participate through complexation.     

Impaired Systemic Tetrahydrobiopterin Bioavailability and Increased Dihydrobiopterin in Adult Falciparum Malaria: Association with Disease Severity,Impaired Microvascular Function and Increased Endothelial Activation

Tetrahydrobiopterin (BH₄) is a co-factor required for catalytic activity of nitric oxide synthase (NOS) and amino acid-monooxygenases, including phenylalanine hydroxylase. BH₄ is unstable: during oxidative stress it is non-enzymatically oxidized to dihydrobiopterin (BH₂), which inhibits NOS. Depending on BH₄ availability, NOS oscillates between NO synthase and NADPH oxidase: as the BH₄/BH₂ ratio decreases, NO production falls and is replaced by superoxide. In African children and Asian adults with severe malaria, NO bioavailability decreases and plasma phenylalanine increases, together suggesting possible BH₄ deficiency. The primary three biopterin metabolites (BH₄, BH₂ and B₀ [biopterin]) and their association with disease severity have not been assessed in falciparum malaria. We measured pterin metabolites in urine of adults with severe falciparum malaria (SM; n=12), moderately-severe malaria (MSM, n=17), severe sepsis (SS; n=5) and healthy subjects (HC; n=20) as controls. In SM, urinary BH₄ was decreased (median 0.16 ¼mol/mmol creatinine) compared to MSM (median 0.27), SS (median 0.54), and HC (median 0.34)]; p<0.001. Conversely, BH₂ was increased in SM (median 0.91 ¼mol/mmol creatinine), compared to MSM (median 0.67), SS (median 0.39), and HC (median 0.52); p<0.001, suggesting increased oxidative stress and insufficient recycling of BH2 back to BH4 in severe malaria. Overall, the median BH₄/BH₂ ratio was lowest in SM [0.18 (IQR: 0.04-0.32)] compared to MSM (0.45, IQR 0.27-61), SS (1.03; IQR 0.54-2.38) and controls (0.66; IQR 0.43-1.07); p<0.001. In malaria, a lower BH₄/BH₂ ratio correlated with decreased microvascular reactivity (r=0.41; p=0.03) and increased ICAM-1 (r=-0.52; p=0.005). Decreased BH4 and increased BH₂ in severe malaria (but not in severe sepsis) uncouples NOS, leading to impaired NO bioavailability and potentially increased oxidative stress. Adjunctive therapy to regenerate BH4 may have a role in improving NO bioavailability and microvascular perfusion in severe falciparum malaria.