The Origin of Regioselectivity in a-Cleavage Reactions of Cyclopropenethiones: Potential Role of Pseudo-Jahn-Teller Effect in Substituted Cyclopropenyl Systems

Arylalkylcyclopropenethiones undergo highly regioselective photochemical a-cleavage via thioketene carbene intermediates, giving rise to products derived from the less stabilized carbene. UHF MIND0/3 calculations provide an insight into this unexpected regioselectivity. The nx* triplet of cyclopropenethione is calculated to have a highly unsymmetrical geometry with an elongated C-C bond, a delocalized thiaaUyl fragment, and a pyramidal radicaloid carbon (which eventually becomes the carbene center). From this molecular electronic structure, aryl group stabilization is expected to be more effective at the thiaallyl group rather than at the pyramidal radical center. Thus, the stability of the substituted triplet thione rather than that of the thioketene carbene determines the preferred regiochemistry of cleavage. The unusual structure of the cyclopropenethione triplet is suggested to be related to one of the Jahn-Teller distorted forms of the cyclopropenyl radical. An alternative symmetrical structure is adopted by the corresponding triplet of cyclopropenone, partly accounting for its differing photobehavior. A similar structural dichotomy is demonstrated for the corresponding radical anions as well.

EPR study on the role of Mn in enhancing PTC of BaTiO3

Low concentration of Mn (< 0.05 atom%) added to lanthanide-doped ceramics for enhancing the PTC effect did not show any EPR signal due to Mn in the tetragonal phase. Above Tc (400 K) it showed the six-line signal arising from Mn2+. This is explained on the basis of Mn existing as Mn3+ ion with short relaxation time at room temperature. Oxidation state changes to Mn2+ above Tc; thus Mn3+ acts as an electron trap. This augments the function of activated defect centres (VBa /ag VBa) in diminishing the charge carrier concentration across the phase transformation.

Crystallographic and modelling studies on Mycobacterium tuberculosis RuvA: Additional role of RuvB-binding domain and inter species variability

RuvA, along with RuvB, is involved in branch migration of heteroduplex DNA in homologous recombination. The structures of three new crystal forms of RuvA from Mycobacterium tuberculosis (MtRuvA) have been determined. The RuvB-binding domain is cleaved off in one of them. Detailed models of the complexes of octameric RuvA from different species with the Holliday junction have also been constructed. A thorough examination of the structures presented here and those reported earlier brings to light the hitherto unappreciated role of the RuvB-binding domain in determining inter-domain orientation and oligomerization. These structures also permit an exploration of the interspecies variability of structural features such as oligomerization and the conformation of the loop that carries the acidic pin, in terms of amino acid substitutions. These models emphasize the additional role of the RuvB-binding domain in Holliday junction binding. This role along with its role in oligomerization could have important biological implications.

The role of transition metal oxides in the decomposition and explosion of arylammonium perchlorates

The influence of MnO2, CuO, and NiO on the thermal decomposition and explosivity of arylammonium perchlorates has been studied by differential thermal analysis (DTA) and explosive sensitivity measurements. The metal oxides considerably sensitize both decomposition and explosion and the sensitizing effect is in the order NiO < CuO < MnO2. The accelerated decomposition or explosion seems to occur via the formation of an intermediate, metal perchlorate arylamine complex. The experimental evidence for the mechanism put forward has been included.

Role of free volume in strain softening of as-cast and annealed bulk metallic glass

Plasticity in amorphous alloys is associated with strain softening, induced by the creation of additional free volume during deformation. In this paper, the role of free volume, which was a priori in the material, on work softening was investigated. For this, an as-cast Zr-based bulk metallic glass (BMG) was systematically annealed below its glass transition temperature, so as to reduce the free volume content. The bonded-interface indentation technique is used to generate extensively deformed and well defined plastic zones. Nanoindentation was utilized to estimate the hardness of the deformed as well as undeformed regions. The results show that the structural relaxation annealing enhances the hardness and that both the subsurface shear band number density and the plastic zone size decrease with annealing time. The serrations in the nanoindentation load-displacement curves become smoother with structural relaxation. Regardless of the annealing condition, the nanohardness of the deformed regions is similar to 12-15% lower, implying that the prior free volume only changes the yield stress (or hardness) but not the relative flow stress (or the extent of strain softening). Statistical distributions of the nanohardness obtained from deformed and undeformed regions have no overlap, suggesting that shear band number density has no influence on the plastic characteristics of the deformed region.

Role of alloys in the thermal decomposition and combustion of ammonium perchlorate

The participation of aluminum in the decomposition reaction of ammonium perchlorate (AP) is enhanced if magnesium is added—either as a mixture of Al and Mg powders or as an alloy of Mg in Al. The differential thermal analyses of the compositions show a sensitization in the temperatures of decomposition, as well as increase in the heat of reaction. The AP-Mg and Ap-(Mg---Li) alloy pellets also show increased reactivity. The burning rates of AP-(Al-10% Mg) alloy pellets increase with increase in the alloy content, while calorimetric values peak at 40% alloy content. The combustion product gases of AP-40% (Al-10% Mg) alloy contain large quantities of hydrogen.

Role of H2O2 in phagocytosis and parasitism

Abstract is not available.

Role of Mn impurity in the deterioration of ZnS:Cu, Br electroluminescent phosphors

ZnS:Cu, Br powder EL phosphors showed 6-line EPR signal at 25°C whose intensity increases with Cu content and on annealing in Zn-vapour. The signal arises from native Mn impurity. The starting material does not show any EPR signal since Mn2+ acts as an affinity potential well for a hole in ZnS, forming Mn3+ - a chemically uncommon situation in sulfides. In doped ZnS, holes are trapped at Cu such that Mn2+ persists. Deterioration of EL brightness is accompanied by the decrease in EPR signal intensity due to field assisted hole transference to Mn2+. Intentional addition of Mn in ZnS:Cu, Br decreases the brightness and shortens life time. Stable phosphors require ZnS with Mn content less than 1014 cm−3.

Structural effects of a dimer interface mutation on catalytic activity of triosephosphate isomerase.The role of conserved residues and complementary mutations

The active site of triosephosphate isomerase (TIM, EC: 5.3.1.1), a dimeric enzyme, lies very close to the subunit interface. Attempts to engineer monomeric enzymes have yielded well-folded proteins with dramatically reduced activity. The role of dimer interface residues in the stability and activity of the Plasmodium falciparum enzyme, PfTIM, has been probed by analysis of mutational effects at residue 74. The PfTIM triple mutant W11F/W168F/Y74W (Y74W*) has been shown to dissociate at low protein concentrations, and exhibits considerably reduced stability in the presence of denaturants, urea and guanidinium chloride. The Y74W* mutant exhibits concentration-dependent activity, with an approximately 22-fold enhancement of kcat over a concentration range of 2.5–40 μm, suggesting that dimerization is obligatory for enzyme activity. The Y74W* mutant shows an approximately 20-fold reduction in activity compared to the control enzyme (PfTIM WT*, W11F/W168F). Careful inspection of the available crystal structures of the enzyme, together with 412 unique protein sequences, revealed the importance of conserved residues in the vicinity of the active site that serve to position the functional K12 residue. The network of key interactions spans the interacting subunits. The Y74W* mutation can perturb orientations of the active site residues, due to steric clashes with proximal aromatic residues in PfTIM. The available crystal structures of the enzyme from Giardia lamblia, which contains a Trp residue at the structurally equivalent position, establishes the need for complementary mutations and maintenance of weak interactions in order to accommodate the bulky side chain and preserve active site integrity.

Study of Friction and Transfer Layer Formation in Copper-Steel Tribo-System: Role of Surface Texture and Roughness Parameters

In the present investigation, experiments were conducted on a tribological couple-copper pin against steel plate-using an inclined pin-on-plate sliding tester to understand the role of surface texture and roughness parameters of the plate on the coefficient friction and transfer layer formation. Two surface characteristics of the steel plates-roughness and texture-were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture of the plate. The plowing component of friction was highest for the surface texture that promotes plane strain conditions while it was lowest for the texture that favors plane stress conditions at the interface. Dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing and hence the plane strain/stress type deformations taking place at the asperity level.

Probing into the role of conserved N-glycosylation sites in the Tyrosinase glycoprotein family

N-linked glycosylation has a profound effect on the proper folding, oligomerization and stability of glycoproteins. These glycans impart many properties to proteins that may be important for their proper functioning, besides having a tendency to exert a chaperone-like effect on them. Certain glycosylation sites in a protein however, are more important than other sites for their function and stability. It has been observed that some N-glycosylation sites are conserved over families of glycoproteins over evolution, one such being the tyrosinase related protein family. The role of these conserved N-glycosylation sites in their trafficking, sorting, stability and activity has been examined here. By scrutinizing the different glycosylation sites on this family of glycoproteins it was inferred that different sites in the same family of polypeptides can perform distinct functions and conserved sites across the paralogues may perform diverse functions.

Sequence dependence and role of 5'-phosphate in the B to Z transition

Spectroscopic studies on pd(CG)3 and pd(GC)3 have been carried out to elucidate the sequence dependence and effect of free 5'-phosphate on the B to Z transition. Unlike d(CG)3, pd(CG)3 fails to undergo salt-induced B to Z transition at ambient temperature. Model building studies have been carried out to determine the inhibitory role of the 5'-phosphate group, but have been unsuccessful.

Role of prolactin(prl) in regulation of the nocturnal surge of testosterone (t) in the bonnet monkey (macaca radiata)

Earlier studies from this lebordory have shown thet adult male bonnet monkeys exhibit nychthemrel rhythmicity la the secretion of serum 'T' the levele reehlng peek by 22OOhr. Of the gonedotropine cnelyeed only serum PRL showed a concommitent increme with T(Biol.of Reprod. 24,814, 1981). In the present study mMinietretion of l rgobromocryptin (EBC) either by i.v.route(2mg)or by naeel l pr~(100~)reeulted in blockade of nocturnal increase of both PRL end T(Controle T-18.6ng/ml: PRL 130=29ng/ml: EBC treated T-2.2&1.2ng/ml; PRL n.d.to 15nng/ml). Adminietretion of N oPRL could not reverse the effect of EBC. Although, increaeed serum PRL induced by injection of Chlorprommine did not result in increase in serum 'T' during the dey time, the nocturnel 'T' surge could not be obeeerved. EBC treeted monkeys, however, showed normal testosterone response to exogenous hCG. These IeSUlte a0 SwgeStive of high levels of PRL me&in6 reeponeiveneee of testes to tonic levels of serum IX. (Aided by grant8 from ICMR, Kew Delhi, WHO, Geneva eld FPF, India).

Novel role of the nitrite transporter NirC in Salmonella pathogenesis: SPI2-dependent suppression of inducible nitric oxide synthase in activated macrophages

Activation of macrophages by interferon gamma (IFN- ) and the subsequent production of nitric oxide (NO) are critical for the host defence against Salmonella enterica serovar Typhimurium infection. We report here the inhibition of IFN- -induced NO production in RAW264.7 macrophages infected with wild-type Salmonella. This phenomenon was shown to be dependent on the nirC gene, which encodes a potential nitrite transporter. We observed a higher NO output from IFN- -treated macrophages infected with a nirC mutant of Salmonella. The nirC mutant also showed significantly decreased intracellular proliferation in a NO-dependent manner in activated RAW264.7 macrophages and in liver, spleen and secondary lymph nodes of mice, which was restored by complementing the gene in trans. Under acidified nitrite stress, a twofold more pronounced NO-mediated repression of SPI2 was observed in the nirC knockout strain compared to the wild-type. This enhanced SPI2 repression in the nirC knockout led to a higher level of STAT-1 phosphorylation and inducible nitric oxide synthase (iNOS) expression than seen with the wild-type strain. In iNOS knockout mice, the organ load of the nirC knockout strain was similar to that of the wild-type strain, indicating that the mutant is exclusively sensitive to the host nitrosative stress. Taken together, these results reveal that intracellular Salmonella evade killing in activated macrophages by downregulating IFN- -induced NO production, and they highlight the critical role of nirC as a virulence gene.

Role of conformational dynamics in kinetics of an enzymatic cycle in a nonequilibrium steady state

Enzyme is a dynamic entity with diverse time scales, ranging from picoseconds to seconds or even longer. Here we develop a rate theory for enzyme catalysis that includes conformational dynamics as cycling on a two-dimensional (2D) reaction free energy surface involving an intrinsic reaction coordinate (X) and an enzyme conformational coordinate (Q). The validity of Michaelis-Menten (MM) equation, i.e., substrate concentration dependence of enzymatic velocity, is examined under a nonequilibrium steady state. Under certain conditions, the classic MM equation holds but with generalized microscopic interpretations of kinetic parameters. However, under other conditions, our rate theory predicts either positive (sigmoidal-like) or negative (biphasic-like) kinetic cooperativity due to the modified effective 2D reaction pathway on X-Q surface, which can explain non-MM dependence previously observed on many monomeric enzymes that involve slow or hysteretic conformational transitions. Furthermore, we find that a slow conformational relaxation during product release could retain the enzyme in a favorable configuration, such that enzymatic turnover is dynamically accelerated at high substrate concentrations. The effect of such conformation retainment in a nonequilibrium steady state is evaluated.