Explaining the Atypical Reaction Profiles of Heme Enzymes with a Novel Mechanistic Hypothesis and Kinetic Treatment

Many heme enzymes show remarkable versatility and atypical kinetics. The fungal extracellular enzyme chloroperoxidase (CPO) characterizes a variety of one and two electron redox reactions in the presence of hydroperoxides. A structural counterpart, found in mammalian microsomal cytochrome P450 (CYP), uses molecular oxygen plus NADPH for the oxidative metabolism (predominantly hydroxylation) of substrate in conjunction with a redox partner enzyme, cytochrome P450 reductase. In this study, we employ the two above-mentioned heme-thiolate proteins to probe the reaction kinetics and mechanism of heme enzymes. Hitherto, a substrate inhibition model based upon non-productive binding of substrate (two-site model) was used to account for the inhibition of reaction at higher substrate concentrations for the CYP reaction systems. Herein, the observation of substrate inhibition is shown for both peroxide and final substrate in CPO catalyzed peroxidations. Further, analogy is drawn in the “steady state kinetics” of CPO and CYP reaction systems. New experimental observations and analyses indicate that a scheme of competing reactions (involving primary product with enzyme or other reaction components/intermediates) is relevant in such complex reaction mixtures. The presence of non-selective reactive intermediate(s) affords alternate reaction routes at various substrate/product concentrations, thereby leading to a lowered detectable concentration of “the product of interest” in the reaction milieu. Occam's razor favors the new hypothesis. With the new hypothesis as foundation, a new biphasic treatment to analyze the kinetics is put forth. We also introduce a key concept of “substrate concentration at maximum observed rate”. The new treatment affords a more acceptable fit for observable experimental kinetic data of heme redox enzymes

Electrochemical performance behavior of combustion-synthesized LiNi0.5Mn0.5O2 lithium-intercalation cathodes

LiNiO2, partially substituted with manganese in the form of a LiNi0.5Mn0.5O2 compound, has been synthesized by a gelatin assisted combustion method [GAC] method. Highly crystalline LiNi0.5Mn0.5O2 powders with R3m symmetry have been obtained at an optimum temperature of 850 ◦C, as confirmed by PXRD studies. The presence of cathodic and anodic CV peaks exhibited by the LiNi0.5Mn0.5O2 cathode at 4.4 and 4.3V revealed the existence of Ni and Mn in their 2+ and 4+ oxidation states, respectively. The synthesized LiNi0.5Mn0.5O2 cathode has been subjected to systematic electrochemical performance evaluation, via capacity tapping at different cut-off voltage limits (3.0–4.2, 3.0–4.4 and 3.0–4.6 V) and the possible extraction of deliverable capacity under different current drains (0.1C, 0.5C, 0.75C and 1C rates). The LiNi0.5Mn0.5O2 cathode exhibited a maximum discharge capacity of 174 mAh g−1 at the 0.1C rate between 3.0 and 4.6V. However, a slightly decreased capacity of 138 mAh g−1 has been obtained in the 3.0–4.4V range, when discharged at the 1C rate. On the other hand, extended cycling at the 0.1C rate encountered an acceptable capacity fade in the 3.0–4.4V range (<10%) for up to 50 cycles

High Voltage and High Capacity Characteristics of LiNi1/3Co1/3Mn1/3O2 Cathode for Lithium Battery Applications

Possibility of synthesizing LiNi1/3Co1/3Mn1/3O2 cathode via., soft chemistry based Gelatin Assisted Combustion [GAC] approach has been examined through the present study. GAC method with a calcination temperature as high as 750°C for a period of 24h. was found to be essential to prepare LiNi1/3Co1/3Mn1/3O2 powders with good hexagonal ordering and better cycling performance. The intensity ratio of (003) and (104) bragg peaks is greater than unity, which is an indication for the absence of cation mixing. The observed CV peaks confirm the presence of Ni, Co and Mn ions in their +3 oxidation state. A maximum discharge capacity of ~180mAh/g has been exhibited by the synthesized LiNi1/3Co1/3Mn1/3O2 cathode, when charged up to 4.6V. Hence, it is demonstrated that the LiNi1/3Co1/3Mn1/3O2 cathode synthesized through the present study could be exploited both as a high voltage and high capacity cathode material for use in rechargeable lithium battery applications

A preliminary investigation into the new class of lithium intercalating LiNiSiO4 cathode material

A unique attempt to exploit silicate chemistry for a possible enhancement of the electrochemical properties of a lithium ion system via exploration of the novel category lithium intercalating LiNiSiO4 cathode has been made through the present study. A novel citric acid assisted modified sol–gel method (CAM sol–gel) has been adopted to synthesize the title compound with a formation temperature positioned well below 500 ◦C, as derived from thermal studies. A powder x-ray diffraction (PXRD) pattern evidenced the absence of undesirable peaks and confirmed the formation of a hexagonal lattice structure with enhanced crystallinity and phase purity, and the presence of uniformly distributed particles of ∼200 nm size with well defined grain boundaries is obvious from the scanning electron microscopy (SEM) image of LiNiSiO4 material. Further, magic angle spinning (MAS) 7Li nuclear magnetic resonance (NMR) results from LiNiSiO4 confirmed the presence of a layered type of crystal arrangement. A cyclic voltammetry (CV) study performed on a LiNiSiO4 cathode revealed an excellent reversibility without any change in the peak position upon extended cycling, thus substantiating the structural stability upon progressive cycling

Synthesis and Characterization of LiMXFe1-XPO4 (M = Cu, Sn; X = 0.02) Cathodes - A study on the Effect of Cation Substitution in LiFePO4 Material

An attempt has been made for the possible augmentation and exploration of partially substituted LiFePO4 material as a positive electrode for lithium battery applications. In this regard, cationic substitution of Cu and Sn (2%) to the native LiFePO4/C electro active material has been carried out via. ball milling, with a view to understand the effect of respective transition and non-transition metals upon LiFePO4 individually. Uniformly distributed particles (SEM) of LiMXFe1-XPO4/C (M= Cu, Sn) with phase pure nature (XRD) and finer crystallite size (<1mm) were obtained. Further, it is interesting to note that irrespective of the nature of the dopant metal, the simple route of ball milled LiMXFe1- XPO4/C [M= Cu, Sn] cathodes endowed with improved conductivity and stable reversible capacity values (chare-discharge). In other words, the LiCu0.02Fe0.98PO4/C cathode delivered a reversible capacity of ~105 mAh/g with an excellent capacity retention characteristic. On the other hand LiSn0.02Fe0.98PO4/C cathodes exhibited an average specific capacity of ~100mAh/g with progressively enhanced efficiency values. Results of Fourier Transform Infra Red (FTIR) spectroscopy and Cyclic Voltammetric studies of LiMXFe1-XPO4/C (M= Cu, Sn) composites are also appended and correlated suitably

CAM sol–gel synthesized LiMPO4 (M=Co, Ni) cathodes for rechargeable lithium batteries

The emerging category cathode candidates such as LiCoPO4 and LiNiPO4 were synthesized at 800 �C using Citric acid assisted modified sol–gel (CAM sol–gel) method and examined for possible lithium intercalation behavior. Compound formation temperature is confirmed from thermogravimetry and differential thermal analysis (TG/DTA). Powder X-ray diffraction (PXRD) pattern evidenced the absence of undesirable peaks and confirmed the formation of phase pure LiMPO4 (M=Co, Ni) compounds with an orthorhombic structure and finer crystallite size. Presence of nanosized particles as observed from TEM image of LiCoPO4 and the presence of preferred local cation environment as understood from FT–IR studies are the added advantages of CAM sol–gel synthesis. Further, Cyclic voltametry (CV) and Impedance spectroscopy (EIS) studies performed on the synthesized LiCoPO4 and LiNiPO4 cathodes revealed excellent reversibility and structural stability of CAM sol–gel synthesized cathodes, especially upon storage as well as during cycling

LiMgy1Cry2Mn2-y1-y2O4 (0.0 < 0.30; Y2=0.30-Y1) as a cathode active material for lithium batteries

LiMn2O4 is an attractive 4 V positive material in lithium rechargeable batteries owing to its favourbale electrochemical characteristics besides its economic and environmental advantages. However, problems of limited cyclability, especiallyat elevated temperatures, have limited the uitlity and commercialization of this cathdoe material. Stabilization of the LiMn2O4 spinel structure has been sought to be ralzied by doping the spinel with suitable cations. In this paper, the results of an exploratory research on the cpacity and cyclability of LiMn2o4 cathodes simultaneously doped with Cr3+ and Mg2+ arereported

LiMgy1Cry2Mn2-y1-y2O4 (0.0 < 0.30; Y2=0.30-Y1) as a cathode active material for lithium batteries

LiMn2O4 is an attractive 4 V positive material in lithium rechargeable batteries owing to its favourbale electrochemical characteristics besides its economic and environmental advantages. However, problems of limited cyclability, especiallyat elevated temperatures, have limited the uitlity and commercialization of this cathdoe material. Stabilization of the LiMn2O4 spinel structure has been sought to be ralzied by doping the spinel with suitable cations. In this paper, the results of an exploratory research on the cpacity and cyclability of LiMn2o4 cathodes simultaneously doped with Cr3+ and Mg2+ arereported

Electrochemical behaviour of LiMyMn2–yO4 (M = Cu, Cr; 0 <= y <= 0.4)

Spinel lithium manganese oxide, LiMn2O4, is beset with problems of capacity fade upon repeated cycling. The loss in capacity upon cycling is attributable to Jahn–Teller distortion and manganese dissolution in the electrolyte in the charged state. One way to circumvent this capacity fade is to introduce other 3d transition metal ions in the LiMn2O4 lattice. In this paper, we report on the effect of partial substitution of manganese in the LiMn2O4 phase with copper (II) and chromium (III) ions. It has been shown that the higher octahedral stabilization energy of trivalent chromium imparts greater structural stability to chromium-doped LiMn2O4 spinels. Both copper and chromium reduce the capacity of the spinel in the 4 V region. In terms of its good reversible capacity and ability to sustain cycling with minimal capacity fade, LiCr0×1Mn1×9O4 may be considered as a potential cathode material for lithium rechargeable cells

Iron doped lithium cobalt oxides as lithium intercalating cathode materials

Layered transition metal oxides of the formula LiMO 2 have good lithium insertion properties for which reason LiCoO 2 and LiNiO2 have been exploited in practical lithium rocking chair batteries. Another member of the LiMO2 series, LiFeO2, should be an attractive cathode material considering the cheapness and environment-friendliness of iron compounds. Its rock-salt structure, however, does not allow significant amounts of lithium to be reversibly intercalated in its structure. Synthesis of layered LiFeO2 and study of its lithium intercalating properties have been of limited success. Therefore, an attempt has been made here to study LiCol_~FeyO 2 solid solutions (0 < y -< 0.4) as prospective cathode materials. XRD, FTIR, Atomic absorption spectroscopy, Particle size and Surface area analysis were carried out in this regard towards the physical characterization of the entire series of LiCo~_yFeyO 2 compounds. The electrochemical discharge capacity of these materials is explained as a function of the iron content