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Developing efficient single-atom catalysts (SACs) for nitrogen fixation is of great importance while remaining a great challenge. The lack of an effective strategy to control the polarization electric field of SACs limits their activity and selectivity. Here, using first-principles calculations, we report that a single transition metal (TM) atom sandwiched between hexagonal boron nitride (h-BN) and graphene sheets (namely, BN/TM/G) acts as an efficient SAC for the electrochemical nitrogen reduction reaction (NRR). These sandwich structures realize stable and tunable interfacial polarization fields that enable the TM atom to donate electrons to a neighboring B atom as the active site. As a result, the partially occupied p z orbital of a B atom can form B-to-N π-back bonding with the antibonding state of N 2 , thus weakening the NN bond. The not-strong-not-weak electric field on the h-BN surface further promotes N 2 adsorption and activation. The NRR catalytic activity of the BN/TM/G system is highly correlated with the degree of positively polarized charges on the TM atom. In particular, BN/Ti/G and BN/V/G are identified as promising NRR catalysts with high stability, offering excellent energy efficiency and suppression of the competing hydrogen evolution reaction.

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Age-related changes in levels of melatonin and 6-hydroxymelatonin sulfate and effects of dietary melatonin on their levels in different tissues were determined in mice. Levels of melatonin were highest in the serum followed by liver, kidney, cerebral cortex and heart as measured by a quantitative and sensitive enzyme-labeled immunosorbent assay (ELISA). Serum melatonin levels decreased with age, and were reduced by 80% in 27-month old mice relative to 12-month old mice. Levels of 6-hydroxymelatonin sulfate were measured independently in various tissues. Levels of the melatonin metabolite, 6-hydroxymelatonin sulfate were significantly higher than free melatonin in all tissues tested. Levels of 6-hydroxymelatonin sulfate were highest in the cerebral cortex followed by the serum, heart, kidney, and liver. In 12-month old mice 6-hydroxymelatonin sulfate concentration was approximately 1000-fold greater than that of melatonin in the cerebral cortex, it was only 3-fold greater than melatonin levels in the serum. Thus only 0.1% of total melatonin in the brain was present in the free and unconjugated form but the corresponding value for serum was 27.4%. The cerebral cortex had the highest levels of combined melatonin and 6-hydroxymelatonin sulfate than other tissue tested in control mice. There was no significant change in 6-hydroxymelatonin sulfate levels between young and old mice. There was also no age-dependent change in levels of serotonin or cortisol in the serum samples. Dietary supplementation with melatonin resulted in a significant increase in levels of melatonin in the serum and all other tissue samples tested. Thus, any age-related decline of tissue melatonin can be reversed by supplementation with dietary melatonin.

Melatonin levels decrease with aging in mice. Dietary supplementation with melatonin has recently been shown to result in a significant rise in levels of endogenous melatonin in the serum and all other tissue samples tested. Herein, the effects of dietary melatonin on brain levels of nitric oxide synthase, synaptic proteins and amyloid beta-peptides (Abeta) were determined in mice. Melatonin supplementation did not significantly change cerebral cortical levels of nitric oxide synthase or synaptic proteins such as synaptophysin and SNAP-25. Increased brain melatonin concentrations however, led to a significant reduction in levels of toxic cortical Abeta of both short and long forms which are involved in amyloid depositions and plaque formation in Alzheimer's diseases. Thus, melatonin supplementation may retard neurodegenerative changes associated with brain aging. Depletion of melatonin in the brain of aging mice may in part account for this adverse change.

The production of ammonia (NH 3 ) from molecular dinitrogen (N 2 ) under ambient conditions is of great significance but remains as a great challenge. Using first-principles calculations, we have investigated the potential of using a transition metal (TM) atom embedded on defective MXene nanosheets (Ti 3−x C 2 O y and Ti 2−x CO y with a Ti vacancy) as a single-atom electrocatalyst (SAC) for the nitrogen reduction reaction (NRR). The Ti 3−x C 2 O y nanosheet with Mo and W embedded, and the Ti 2−x C 2 O y nanosheet with Cr, Mo, and W embedded, can significantly promote the NRR while suppressing the competitive hydrogen evolution reaction, with the low limiting potential of −0.11 V for W/Ti 2−x C 2 O y . The outstanding performance is attributed to the synergistic effect of the exposed Ti atom and the TM atom around an extra oxygen vacancy. The polarization charges of the active center are reasonably tuned by the embedded TM atoms, which can optimize the binding strength of key intermediate *N 2 H. The good feasibility of preparing such TM SACs on defective MXenes and the high NRR selectivity with regard to the competitive HER suggest new opportunities for driving NH 3 production by MXene-based SAC electrocatalysts under ambient conditions.

The challenge of evaluating catalyst surface–molecular adsorbate interactions holds the key for rational design of catalysts. Finding an experimentally measurable and theoretically computable descriptor for evaluating surface–adsorbate interactions is a significant step toward achieving this goal. Here we show that the electric dipole moment can serve as a convenient yet accurate descriptor for establishing structure–property relationships for molecular adsorbates on metal catalyst surfaces. By training a machine learning neural network with a large data set of first-principles calculations, we achieve quick and accurate predictions of molecular adsorption energy and transferred charge. The training model using NO/CO@Au(111) can be extended to study additional substrates such as Au(001) or Ag(111), thus exhibiting extraordinary transferability. These findings validate the effectiveness of the electric dipole descriptor, providing an efficient modality for future catalyst design.

The development of type 2 diabetes is accompanied by decreased immune function and the mechanisms are unclear. We hypothesize that oxidative damage and mitochondrial dysfunction may play an important role in the immune dysfunction in diabetes. In the present study, we investigated this hypothesis in diabetic Goto-Kakizaki rats by treatment with a combination of four mitochondrial-targeting nutrients, namely, R-α-lipoic acid, acetyl-L-carnitine, nicotinamide and biotin. We first studied the effects of the combination of these four nutrients on immune function by examining cell proliferation in immune organs (spleen and thymus) and immunomodulating factors in the plasma. We then examined, in the plasma and thymus, oxidative damage biomarkers, including lipid peroxidation, protein oxidation, reactive oxygen species, calcium and antioxidant defence systems, mitochondrial potential and apoptosis-inducing factors (caspase 3, p53 and p21). We found that immune dysfunction in these animals is associated with increased oxidative damage and mitochondrial dysfunction and that the nutrient treatment effectively elevated immune function, decreased oxidative damage, enhanced mitochondrial function and inhibited the elevation of apoptosis factors. These effects are comparable to, or greater than, those of the anti-diabetic drug pioglitazone. These data suggest that a rational combination of mitochondrial-targeting nutrients may be effective in improving immune function in type 2 diabetes through enhancement of mitochondrial function, decreased oxidative damage, and delayed cell death in the immune organs and blood.