Zwitterionic Osmolytes Employ Dual Mechanisms for Resurrection of Surface Charge under Salt-stress

Salt imbalance in cells is a major detrimental abiotic stress which causes ionic toxicity and disrupts important cellular functions. To counter this saline stress, cell often produces low molecular weight cosolutes, known as osmolytes, which have the ability to resuscitate homeostasis. Here we combine atomistic computer simulation, contact angle measurements and Raman spectroscopic analysis to identify the mechanistic role of multiple osmolytes (glycine, TMAO and betaine) in modulating the electrostatic interaction under salt stress, a slowly emerging aspect of osmoprotection. By utilising a pair of negatively charged silica surfaces in a ternary mixture of osmolyte and KCl solution as a proxy of charged surface of biomacromolecule, our investigation reveals that all three osmolytes are able to resurrect the electrostatic interaction between the two surfaces, which had been otherwise charge-screened by excess salt. The joint venture of experiment and simulation discover dual and mutually exclusive mechanisms of recovering charge interaction by zwitter-ionic osmolytes. However, the relative ability and the underlying mechanism of revival of electrostatic interactions are found to be strongly dependent of chemical nature of osmolyte. Specifically, glycine was found to competitively desorb the salt-ions from the surface via its direct interaction with the surface. On the other hand, TMAO and betaine counter-act salt stress by retaining adsorbed cations but partially neutralising their charge-density via ion-mediated interaction. We believe that the access to dual and mutually alternative modes of osmolytic actions, as elucidated here, would provide the cell the required adaptability in combating salt-stress

Osmolytes as Cryoprotectants Under Salt Stress

Cryoprotecting agent (CPAs)-guided preservation is essential for effective protection of cells from cryo-injuries. But current cryoprotecting technologies practised to cryopreserve cells for biomedical applications are met with extreme challenges due to associated toxicity of CPAs. Because of these limitations of present CPAs, the quest for nontoxic alternative for useful application in cell-based biomedicines is attracting growing interest. Towards this end, here we investigate naturally occurring osmolytes\u27 scope as biocompatible cryoprotectant under cold-stress condition in high saline medium. Via a combination of simulation and experiment on charged silica nano-structures, we render first-hand evidence that a pair of archetypal osmolytes glycine and betaine would act as cryoprotectant by restoring indigenous inter-surface electrostatic interaction, which had been a priori screened due to cold-effect under salt stress. While these osmolytes\u27 individual modes of action are sensitive to the subtle chemical variation, a uniform augmentation in the extent of osmolytic activity is observed with increase in temperature to counter the proportionately enhanced salt screening. The trend as noted in inorganic nano-structures is found to be recurrent and robustly transferrable in a charged protein interface. In hindsight, our observation justifies the sufficiency of reduced requirement of osmolytes in cells during critical cold condition and encourages their direct usage and biomimicry for cryopreservation

Mechanistic Insight into High Yield Electrochemical Nitrogen Reduction to Ammonia using Lithium Ions

Development of methods for economically feasible greener ammonia (NH3) production is gaining tremendous scientific attention. NH3 has its importance in fertilizer industry and it is envisaged as a safer liquid hydrogen carrier for futuristic energy resources. Here, an aqueous electrolysis based NH3 production in ambient conditions is reported, which yields high faradaic efficiency (~12%) NH3 via nitrogen reduction reaction (NRR) at lower over potentials (~ -0.6V vs. RHE or -1.1V vs. Ag/AgCl). Polycrystalline copper (Cu) and gold (Au) are used as electrodes for electrochemical NRR, where the electrolyte which yields high amount of NH3 (~41 µmol/L) is 5M LiClO4 in water with Cu as working electrode. A detailed study conducted here establishes the role of Li+ in stabilizing nitrogen near to the working electrode - augmenting the NRR in comparison to its competitor - hydrogen evolution reaction, and a mechanistic insight in to the phenomenon is provided. 15N2 assisted labeling experiments are also conducted to confirm the formation of ammonia via NRR. This study opens up the possibilities of developing economically feasible electrodes for electrochemical NRR at lower energies with only transient modifications of electrodes during the electrolysis, unlike the studies reported on complex electrodes or electrolytes designed for NRR in aqueous medium to suppress the hydrogen generation. </p

Role of Water Structure in Alkaline Water Electrolysis

A universal activity descriptor for catalytic alkaline hydrogen evolution reaction (HER) was unavailable, though metal-hydrogen binding energy can be considered as a good such descriptor in acidic medium. Herein, with the help of experimental and first principles density functional theory (DFT) based studies, we have shown that structural changes in the water coordination in electrolytes having high alkalinity can be a possible reason for the reduced catalytic activity of platinum (Pt) in high pH. Studies with polycrystalline Pt electrodes indicate that electrocatalytic HER activity reduces in terms of high overpotential required, high Tafel slope, and high charge transfer resistances in concentrated aqueous alkaline electrolytes (say 6M KOH) in comparison to that in low alkaline electrolytes (say 0.1M KOH), irrespective of the counter cations (Na+, K+ or Rb+) present. The changes in the water structure of bulk electrolytes with concentration are established using Raman, infrared, and 1H NMR based spectroscopic analyses. The changes in the interfacial water structure are also studied using in situ Raman scattering experiments where the changes in the coordination of water from tetrahedral to trihedral to free water are observed as the potential goes more cathodic towards HER. DFT based studies show enhanced water dissociation energy required for tetrahedrally coordinated water followed by trihedral, and then free water having the least dissociation energy for the Volmer process. But the water structure seems to be unaffected in anodic potentials. Hence the study paves new ways in studying the HER process in terms of the water structure near the electrode-electrolyte interface

A Tetranuclear Cobalt (II) Phosphate Possessing a D4R Core: An Efficient Water Oxidation Catalyst

The reaction of Co(OAc)2·4H2O with the sterically hindered phosphate ester, LH2, afforded the tetranuclear complex, [CoII(L)(CH3CN)]4∙5CH3CN (1) [LH2 = 2,6‐(diphenylmethyl)‐4‐isopropyl‐phenyl phosphate]. The molecular structure of 1 reveals that it is a tetranuclear assembly where the Co(II) centers are present in the alternate corners of a cube. The four Co(II) centers are held together by four di-anionic [L]2- ligands. The fourth coordination site on Co(II) is taken by an acetonitrile ligand. Changing the Co(II) precursor from Co(OAc)2·4H2O to Co(NO3)2.6H2O afforded the mononuclear complex [CoII(LH)2(CH3CN)2(MeOH)2](MeOH)2 (2). In 2, the Co(II) is surrounded by two monoanionic [LH]‒ ligands, and a pair of methanol and acetonitrile solvents in a six-coordinate arrangement. 1 has been found to be an efficient catalyst for the electrochemical water oxidation under high basic conditions while the mononuclear analogue, 2, does not respond towards electrochemical water oxidation. The tetranuclear catalyst has excellent electrochemcial stability and longevity, as established by the chronoamperometry and >1000 cycles durability test in high alkaline conditions. Excellent current densities of 1 and 10 mAcm‒2 were achieved with the overpotential of 354 and 452 mV respectively. The turnover frequency of this catalyst was calculated as 5.23 s−1 with excellent faradaic efficiency of 97%, indicating the selective oxygen evolution (OER) process happening with the aid of this catalyst. A mechanistic insight in to the higher activity of complex 1 towards OER compared to complex 2 is also provided with the help of density functional theory based calculations.</p

Ultra-Low Loaded Platinum Bonded Hexagonal Boron Nitride as Stable Electrocatalyst for Hydrogen Generation

Chemical stability of hexagonal boron nitride (hBN) ultra-thin layers in harsh electrolytes and the availability of nitrogen site to stabilize metals like Pt are used here to develop a high intrinsic activity hydrogen evolution reaction (HER) catalyst having low loaded Pt (5 weight% or < 1 atomic%). A catalyst having non-zero oxidation state for Pt (with a Pt-N bonding) is shown to be HER active even with low catalyst loadings (0.114 mgcm-2). Electronic modification of the shear exfoliated hBN sheets is achieved by Au nanoparticle-based surface decoration (hBN_Au), and further anchoring with Pt develops a catalyst (hBN_Au_Pt) with high turnover frequency for HER (~15), which is ~1.8 times higher than the benchmarked Pt/C HER catalyst. The hBN_Au_Pt is shown to be a highly durable catalyst even after the accelerated durability test for 10000 cycles and temperature annealing of 100 oC. Density functional theory-based calculations gave insights in to the electronic modifications of hBN with Au and the catalytic activity of the hBN_Au_Pt system, in line with the experimental studies, indicating the demonstration of a new class of catalyst system devoid of issues such as carbon corrosion and Pt leaching

Mechanistic Insight into Enhanced Hydrogen Evolution Reaction Activity of Ultrathin Hexagonal Boron Nitride-Modified Pt Electrodes

Enhancing the intrinsic activity of a benchmarked electrocatalyst such as platinum (Pt) is highly intriguing from fundamental as well as applied perspectives. In this work, hydrogen evolution reaction (HER) activity of Pt electrodes, benchmarked HER catalysts, modified with ultrathin sheets of hexagonal boron nitride (h-BN) is studied in acidic medium (Pt/h-BN), and augmented HER performance, in terms of the overpotential at a 10 mA cm^–2 current density (10 mV lower than that of Pt nanoparticles) and a lower Tafel slope (29 ± 1 mV/decade), of the Pt/h-BN system is demonstrated. The effects of h-BN surface modification of bulk Pt as well as Pt nanoparticles are studied, and the origin of such an enhanced HER activity is probed using density functional theory-based calculations. The HER charge transfer resistance of h-BN-modified Pt is found to be drastically reduced, and this enhances the charge transfer kinetics of the Pt/h-BN system because of the synergistic interaction between h-BN and Pt. An enormous reduction in the hydrogen adsorption energy on h-BN monolayers is also found when they are placed over the Pt electrode [−2.51 eV (h-BN) to −0.25 eV (h-BN over Pt)]. Corrosion preventive atomic layers such as h-BN-protected Pt electrodes that perform better than Pt electrodes do open possibilities of benchmarked catalysts by simple modification of a surface via atomic layers