Tropical Clays .1. Index Properties And Microstructural Aspects

Soils showing changes in plasticity characteristics upon driving form an important group in tropical soils. These changes are attributed to the grouping of particles into aggregates either due to mineralogy or presence of cementing agents and/or pore fluid characteristics. These changes are found to be permanent. In this paper, the effect of these changes leading to changes in index properties is discussed. The coefficient of permeability has been found to be comparable at liquid limit water content for different soils of varying liquid limit values. Permeability is an indirect reflection of microstructure and indicates the flow rate, which depends upon pore geometry. Other mechanical properties like compressibility and shear strength also depend upon pore geometry. These microstructural aspects of liquid limit as a reference state for the analysis of engineering behavior of tropical soils are examined in detail.

Synthesis, Reactivity, Structural Aspects, and Solution Dynamics of Cyclopalladated Compounds of N,N `,N `'-Tris(2-anisyl)guanidine

N,N',N `'-Tris(2-anisyl)guanidine, (ArNH)(2)C=NAr (Ar = 2-(MeO)C6H4), was cyclopallaclated with Pd(OC(O)R)(2) (R = Me, CF3) in toluene at 70 degrees C to afford palladacycles Pd{kappa(2)(C,N)-C6H3-(OMe)-3(NHC(NHAr)(=NAr))-2}(mu-OC(O)R)](2)(R = Me (1a) and CF3 (1b)) in 87% and 95% yield, respectively. Palladacycle 1a was subjected to a metathetical reaction with LiBr in aqueous ethanol at 78 degrees C to afford palladacycle Pd{kappa(2)(C,N)-C6H3(OMe)-3(NHC(NHAr)(=NAr))-2}(mu-Br)](2) (2) in 90% yield. Palladacycle 2 was subjected to a bridge-splitting reaction with Lewis bases in CH2Cl2 to afford the monomeric palladacycles Pd{kappa(2)(C,N)-C6H3(OMe)-3(NHC(NHAr)(=NAr))-2}Br(L)] (L = 2,6-Me2C5H3N (3a), 2,4-Me2C5H3N (3b), 3,5-Me2C5H3N (3c), XyNC (Xy = 2,6-Me2C6H3; 4a), (BuNC)-Bu-t (4b), and PPh3 (5)) in 87-95% yield. Palladacycle 2 upon reaction with 2 equiv of XyNC in CH2Cl2 afforded an unanticipated palladacycle, Pd{kappa(2)(C,N)-C(=NXy)(C6H3(OMe)-4)-2(N=C-(NH Ar)(2))-3} Br(CNXy)] (6) in 93% yield, and the driving force for the formation of 6 was ascribed to a ring contraction followed by amine-imine tautomerization. Palladacycles 1 a,b revealed a dimeric transoid in-in conformation with ``open book'' framework in the solid state. In solution, 1 a exhibited a fluxional behavior ascribed to the six-membered ``(C,N)Pd'' ring inversion and partly dissociates to the pincer type and kappa(2)-O,O'-OAc monomeric palladacycles by an anchimerically assisted acetate cleavage process as studied by variable-temperature H-1 NMR data. Palladacycles 3a,b revealed a unique trans configuration around the palladium with lutidine being placed trans to the Pd-C bond, whereas cis stereochemistry was observed between the Pd-C bond and the Lewis base in 4a (as determined by X-ray diffraction data) and 5 (as determined by P-31 and C-13 NMR data). The aforementioned stereochemical difference was explained by invoking relative hardness/softness of the donor atoms around the palladium center. In solution, palladacycles 3a-c exist as a mixture of two interconverting boat conformers via a planar intermediate without any bond breaking due to the six-membered ``(C,N)Pd'' ring inversion, whereas palladacycles 4a,b and 5 exist as a single isomer, as deduced from detailed H-1 NMR studies.

DNA Binding, Coprotease, and Strand Exchange Activities of Mycobacterial RecA Proteins: Implications for Functional Diversity among RecA Nucleoprotein Filaments

One of the fundamental questions concerning homologous recombination is how RecA or its homologues recognize several DNA sequences with high affinity and catalyze all the diverse biological activities. In this study, we show that the extent of single-stranded DNA binding and strand exchange (SE) promoted by mycobacterial RecA proteins with DNA substrates having various degrees of GC content was comparable with that observed for Escherichia coli RecA. However, the rate and extent of SE promoted by these recombinases showed a strong negative correlation with increasing amounts of sequence divergence embedded at random across the length of the donor strand. Conversely, a positive correlation was seen between SE efficiency and the degree of sequence divergence in the recipient duplex DNA. The extent of heteroduplex formation was not significantly affected when both the pairing partners contained various degrees of sequence divergence, although there was a moderate decrease in the case of mycobacterial RecA proteins with substrates containing larger amounts of sequence divergence. Whereas a high GC content had no discernible effect on E. coli RecA coprotease activity, a negative correlation was apparent between mycobacterial RecA proteins and GC content. We further show clear differences in the extent of SE promoted by E. coli and mycobacterial RecA proteins in the presence of a wide range of ATP:ADP ratios. Taken together, our findings disclose the existence of functional diversity among E. coli and mycobacterial RecA nucleoprotein filaments, and the milieu of sequence divergence (i.e., in the donor or recipient) exerts differential effects on heteroduplex formation, which has implications for the emergence of new genetic variants.

Glass Transition in the Density Functional Theory of Freezing

A mean-field description of the glass transition in the hard-sphere system is obtained by numerically locating "glassy" minima of a model free-energy functional. These minima, characterized by inhomogeneous but aperiodic density distributions, appear as the average density is increased above the value at which equilibrium crystallization takes place. Investigations of the density distribution and local bond-orientational order at these minima yield results similar to those obtained from simulations.

Diversity in Functional Organization of Class I and Class II Biotin Protein Ligase

The cell envelope of Mycobacterium tuberculosis (M. tuberculosis) is composed of a variety of lipids including mycolic acids, sulpholipids, lipoarabinomannans, etc., which impart rigidity crucial for its survival and pathogenesis. Acyl CoA carboxylase (ACC) provides malonyl-CoA and methylmalonyl-CoA, committed precursors for fatty acid and essential for mycolic acid synthesis respectively. Biotin Protein Ligase (BPL/BirA) activates apo-biotin carboxyl carrier protein (BCCP) by biotinylating it to an active holo-BCCP. A minimal peptide (Schatz), an efficient substrate for Escherichia coli BirA, failed to serve as substrate for M. tuberculosis Biotin Protein Ligase (MtBPL). MtBPL specifically biotinylates homologous BCCP domain, MtBCCP87, but not EcBCCP87. This is a unique feature of MtBPL as EcBirA lacks such a stringent substrate specificity. This feature is also reflected in the lack of self/promiscuous biotinylation by MtBPL. The N-terminus/HTH domain of EcBirA has the selfbiotinable lysine residue that is inhibited in the presence of Schatz peptide, a peptide designed to act as a universal acceptor for EcBirA. This suggests that when biotin is limiting, EcBirA preferentially catalyzes, biotinylation of BCCP over selfbiotinylation. R118G mutant of EcBirA showed enhanced self and promiscuous biotinylation but its homologue, R69A MtBPL did not exhibit these properties. The catalytic domain of MtBPL was characterized further by limited proteolysis. Holo-MtBPL is protected from proteolysis by biotinyl-59 AMP, an intermediate of MtBPL catalyzed reaction. In contrast, apo-MtBPL is completely digested by trypsin within 20 min of co-incubation. Substrate selectivity and inability to promote self biotinylation are exquisite features of MtBPL and are a consequence of the unique molecular mechanism of an enzyme adapted for the high turnover of fatty acid biosynthesis.

Bioinorganic and medicinal chemistry: aspects of gold(I)-protein complexes

Gold(I)-based drugs have been used successfully for the treatment of rheumatoid arthritis (RA) for several years. Although the exact mechanism of action of these gold(I) drugs for RA has not been clearly established, the interaction of these compounds with mammalian enzymes has been extensively studied. In this paper, we describe the interaction of therapeutic gold(I) compounds with mammalian proteins that contain cysteine (Cys) and selenocysteine (Sec) residues. Owing to the higher affinity of gold(I) towards sulfur and selenium, gold(I) drugs rapidly react with the activated cysteine or selenocysteine residues of the enzymes to form protein-gold(I)-thiolate or protein-gold(I)-selenolate complexes. The formation of stable gold(I)-thiolate/selenolate complexes generally lead to inhibition of the enzyme activity. The gold-thiolate/selenolate complexes undergo extensive ligand exchange reactions with other nucleophiles and such ligand exchange reactions alter the inhibitory effects of gold(I) complexes. Therefore, the effect of gold(I) compounds on the enzymatic activity of cysteine-or selenocysteine-containing proteins may play important roles in RA. The interaction of gold(I) compounds with different enzymes and the biochemical mechanism underlying the inhibition of enzymatic activities may have broad medicinal implications for the treatment of RA.

Electrooxidation of methanol in sulfuric-acid electrolyte on platized-carbon electrodes with several functional-group characteristics

The effect of acid/base functional-groups associated with platinized-carbon electrodes on their catalytic activity toward electro-oxidation of methanol in sulfuric acid electrolyte at 60-degrees-C is studied. Platinized-carbon electrodes with sm amounts of functional groups exhibit higher catalytic activity compared to those with large concentrations of acidic/basic surface functionalities. The overpotential for methanol oxidation is minimum on electrodes of platinized carbons with pHzpc values between 6 and 7. An x-ray photoelectron spectroscopic study of various platinized carbons suggests that the acid/base surface functional-groups produce ample amounts of surface Pt-oxides and a consequent decrease in activity toward methanol oxidation.

Ionic conductivity, mechanical strength and Li-ion battery performance of mono-functional and bi-functional (''Janus'') ``soggy sand'' electrolytes

Influence of dispersion of uniformly sized mono-functional and bi-functional (''Janus'') particles on ionic conductivity of novel ``soggy sand'' electrolytes and its implications on mechanical strength and lithium-ion battery performance are discussed here.

Intraseasonal Variation of the Summer Monsoon I:Observational Aspects

Within the summer monsoon, the circulation and rainfall over the Indian region exhibit large variations over the synoptic scale of 3-7 days and the supersynoptic scales of 10 days and longer. In this paper we discuss some facets of intraseasonal variation on the supersynoptic scale on the basis of existing observational studies and some new analysis. The major variation of the summer monsoon rainfall on this scale is the active-break cycle. The deep convection over the Indian region on a typical day in the active phase is organized over thousands of kilometers in the zonal direction and is associated with a tropical convergence zone (TCZ). The intraseasonal variations on the supersynoptic scale are also coherent on these scales and are related to the space-time variation of the large-scale TCZ. The latitudinal distribution of the occurrence of the TCZ is bimodal with the primary mode over the heated continent and a secondary mode over the ocean. The variation of the continental TCZ is generally out of phase with that of the oceanic TCZ. During the active spells, the TCZ persists over the continent in the monsoon zone. The revival from breaks occurs either by northward propagation of the TCZ over the equatorial Indian Ocean or by genesis of a disturbance in the monsoon zone (often as a result of westward propagations from W. Pacific). The mechanisms governing the fluctuation between active spells and breaks, the interphase transition and the complex interactions of the TCZ over the Indian subcontinent with the TCZ over the equatorial Indian Ocean and the W. Pacific, have yet to be completely understood.

Establishment of the functional importance of thiamin carrier protein in pregnant rats by using monoclonal antibodies

Monoclonal antibodies (mAbs) to chicken thiamin carrier protein (TCP) have been produced by hybridoma technology to identify the crucial epitopes involved in bioneutralization of the vitamin carrier. The monoclonality of these mAbs (A4C4, F3H6, H8H3, C8C1 and G7H10) was sought to be confirmed by sub-class isotyping; they all belong to IgG1, k type. The epitopes recognized by all the five mAbs are conserved in TCP from the chicken to the rat as assessed by liquid phase RIA and immunoprecipitation of I-125-labelled proteins from pregnant rat serum. Among these mAbs, passive immunization of pregnant rats with the mAb C8C1 only on three consecutive days (day 10, 11 and 12) resulted in embryonic resorption. These results demonstrate the importance of epitopic structure specified by the mAb C8C1 on TCP during pregnancy in rats.

Functional analysis of conserved motifs in EcoP15I DNA methyltransferase

EcoP15I DNA methyltransferase recognizes the sequence 5'-CAGCAG-3' and transfers a methyl group to N-6 of the second adenine residue in the recognition sequence. All N-6 adenine methyltransferases contain two highly conserved sequences, FxGxG (motif I), postulated to form part of the S-adenosyl-L-methionine binding site and (D/N/S)PP(Y/F) (motif IV) involved in catalysis. We have altered the second glycine residue in motif I to arginine and serine, and substituted tyrosine in motif IV with tryptophan in EcoP15I DNA methyltransferase, using site-directed mutagenesis. The mutant enzymes were overexpressed, purified and characterized by biochemical methods. The mutations in motif I completely abolished AdoMet binding but left target DNA recognition unaltered. Although the mutation in motif IV resulted in loss of enzyme activity, we observed enhanced crosslinking of S-adenosyl-L-methionine and DNA. This implies that DNA and AdoMet binding sites are close to motif IV. Taken together, these results reinforce the importance of motif I in AdoMet binding and motif IV in catalysis. Additionally, limited proteolysis and UV crosslinking experiments with EcoP15I DNA methyltransferase imply that DNA binds in a cleft formed by two domains in the protein. Methylation protection analysis provides evidence for the fact that EcoP15I DNA MTase makes contacts in the major groove of its substrate DNA. Interestingly, hypermethylation of the guanine residue next to the target adenine residue indicates that the protein probably flips out the target adenine residue. (C) 1996 Academic Press Limited