Stereochemical studies on cyclic peptides. Part XI. Conformation of cyclic pentapeptides having intramolecular 3 leads to 1 hydrogen bonds.

Conformational analysis of cyclic pentapeptides having two intra-ring 3 leads to 1 hydrogen bonds has been carried out. It is found that the structure can easily be formed with trans planar peptide units without causing significant angular strain at the alpha-carbon atoms. Four different types of conformations designated Types I--IV are possible for the backbone structure. Details of these four types of conformations and also the accommodating possibility of these types for allglycyl and all-alanyl residues are presented. Three of the four types have relatively low energies for glycyl residues whereas the other one has a slightly higher energy. When alanyl residues are introduced at the five alpha-carbon atoms, the types that are energetically favourable depend upon the sequence of isomers. Energy calculations have also been carried out for the combinations of glycyl, L- and D-alanyl residues. The theoretical results are compared with available experimental observations both from solution and solid state studies.

Studies on Unsteady Pressure Fields in the Region of Separating and Reattaching Flows

Experimental studies on the measurement of pressure fields in the region of separating and reattaching flows behind several two-dimensional fore-bodies and one axisymmetric body are reported. In particular, extensive measurements of mean pressure, surface pressure fluctuation, and pressure fluctuation within the flow were made for a series of two-dimensional fore-body shapes consisting of triangular nose with varying included angle. The measurements from different bodies are compared and one of the important findings is that the maximum values of rms pressure fluctuation levels in the shear layer approaching reattachment are almost equal to the maximum value of the surface fluctuation levels.

Laboratory studies on ball wear in the grinding of a hematite-magnetite ore

Marked-ball grinding tests were carried out under different grinding conditions and environments. Three types of balls were used, namely, cast hyper steel, high chrome cast iron and EN-31 (forged), which cover a wide range of chemical composition, microstructure and media hardness. The effect of pulp density on ball wear and grinding efficiency was also studied. Relative pulp viscosities at different percent solids for the ore slurry were also determined. As the Kudremukh ore contained about 0.2% pyrite, the effect of addition of pyrite on ball wear was studied separately. Results of marked-ball grinding tests indicated that ball wear increased with time and showed a sharp increase for wet grinding over dry grinding. Ball wear under wet grinding conditions was also influenced by the gaseous atmosphere in the mill. At 70% solids, the best results in terms of reduced ball wear coupled with satisfactory grinding efficiency were obtained. The influence of oxygen on the corrosive wear of grinding balls was increasingly felt only if sulphide minerals such as pyrite were also present in the ore. The various ball materials could be arranged in the following order with respect to their overall wear resistance: high chrome cast iron > EN-31 (forged) > cast hyper steel.Possible ball wear mechanisms involved in the grinding of Kudremukh ore are discussed.

Conformational studies on beta-bend containing a cis peptide unit

Conformational studies have been carried out on the X-cis-Pro tripeptide system (a system of three linked peptide units, in the trans-cis-trans configuration) using energy minimization techniques. For X, residues Gly, L-Ala, D-Ala and L-Pro have been used. The energy minima have been classified into different groups based upon the conformational similarity. There are 15, 20, 18 and 6 minima that are possible for the four cases respectively and these fall into 11 different groups. A study of these minima shows that, (i) some minima contain hydrogen bonds - either 4-->1 or 1-->2 type, (ii) the low energy minima qualify themselves as bend conformations, (iii) cis' and trans' conformations are possible for the prolyl residue as also the C(gamma)-endo and C(gamma)-exo puckerings, and (iv) for Pro-cis-Pro, cis' at the first prolyl residue is ruled out, due to the high energy. The available crystal structure data on proteins and peptides, containing cis-Pro segment have been examined with a view to find the minima that occur in solid state. The data from protein show that they fall under two groups. The conformation at X in X-cis-Pro is near extended when it is a non-glycyl residue. In both peptides and proteins there exists a preference for trans' conformation at prolyl residue over cis' when X is a non-glycyl residue. The minima obtained can be useful in modelling studies.

Differential scanning calorimetry cure studies on the effect of addition of epoxy diluents of tetrafunctional epoxy resins

The effect of the addition of p-aminophenol and aniline-based epoxy diluents on the curing behavior of highly viscous tetraglycidyl diamino diphenyl methane resin with diamino diphenyl sulfone hardener have been investigated kinetically by differential scanning calorimetry. Dynamic scans were carried out over a temperature range 30–300°C for different resin formulations. Isothermal scans at four differnt temperatures have also been carried out for the evaluation of kinetic parameters. Heat flow measurements at different heating rates have indicated the evidence of autocatalytic behavior of curing reaction following a simple nth-order kinetics.

Molecular Modeling Studies on Amphotericin B and its Complex with Phospholipid,

Molecular dynamics simulation studies on polyene antifungal antibiotic amphotericin B, its head-to-tail dimeric structure and lipid - amphotericin B complex demonstrate interesting features of the flexibilities within the molecule and define the optimal interactions for the formation of a stable dimeric structure and complex with phospholipid.

Two forms of Pf1 Inovirus: X-ray diffraction studies on a structural phase transition and a calculated libration normal mode of the asymmetric unit

Inovirus is a helical array of alpha-helical protein asymmetric units surrounding a DNA core. X-ray fibre diffraction studies show that the Pf1 species of Inovirus can undergo a reversible temperature-induced transition between two similar structural forms having slightly different virion helix parameters. Molecular models of the two forms show no evidence for altered interactions between the protein and either the solvent or the viral DNA; but there are significant differences in the shape and orientation of the protein asymmetric unit, related to the changes in the virion parameters. Normal modes involving libration of whole asymmetric units are in a frequency range with appreciable entropy of libration, and the structural transition may be related to changes in libration.

CD and NMR studies on the aggregation of amphotericin-B in solution

We report in this paper the aggregation properties of amphotericin-B (amp-B) in solution using CD and 1H-NMR techniques. Our results indicate that the preferred structure of amp-B in dimethylsulfoxide is a monomer at low concentrations (10−4M and below) and a stable dimer at higher concentrations (range 5 · 103 M to 10−2M). In a DMSO/ethanol mixture (1:1 (v/v)), the antibiotic is monomeric, irrespective of the concentration within the range studied. We propose a head-to-tail model based on NMR data. An understanding of the head-to-tail dimer, is, we believe important, particularly in view of the recent report wherein it is proposed that the drug inserts into bilayers as head-to-tail oligomers.

Computer modeling studies on the binding of 2',5'-linked dinucleoside phosphates to ribonuclease T1-influence of subsite interactions on the substrate specificity

The modes of binding of Gp(2',5')A, Gp(2',5')C, Gp(2',5')G and Gp(2',5')U to RNase T1 have been determined by computer modelling studies. All these dinucleoside phosphates assume extended conformations in the active site leading to better interactions with the enzyme. The 5'-terminal guanine of all these ligands is placed in the primary base binding site of the enzyme in an orientation similar to that of 2'-GMP in the RNase T1-2'-GMP complex. The 2'-terminal purines are placed close to the hydrophobic pocket formed by the residues Gly71, Ser72, Pro73 and Gly74 which occur in a loop region. However, the orientation of the 2'-terminal pyrimidines is different from that of 2'-terminal purines. This perhaps explains the higher binding affinity of the 2',5'-linked guanine dinucleoside phosphates with 2'-terminal purines than those with 2'-terminal pyrimidines. A comparison of the binding of the guanine dinucleoside phosphates with 2',5'- and 3',5'-linkages suggests significant differences in the ribose pucker and hydrogen bonding interactions between the catalytic residues and the bound nucleoside phosphate implying that 2',5'-linked dinucleoside phosphates may not be the ideal ligands to probe the role of the catalytic amino acid residues. A change in the amino acid sequence in the surface loop region formed by the residues Gly71 to Gly74 drastically affects the conformation of the base binding subsite, and this may account for the inactivity of the enzyme with altered sequence i.e., with Pro, Gly and Ser at positions 71 to 73 respectively. These results thus suggest that in addition to recognition and catalytic sites, interactions at the loop regions which constitute the subsite for base binding are also crucial in determining the substrate specificity.

Crystallization studies on amorphous AI-Y-Ni and AI-Y-Cu alloys

AI83Y10Ni7, AI80Y10Ni10 and AI80Y10Cu10 alloys were studied by the rapid solidification processing route. The glass-forming ability was found to decrease in the order of alloys mentioned above. Differential scanning calorimetry (DSC) of these amorphous alloys showed that the amorphous phase in AI-Y-Ni alloys has a higher thermal stability when compared to that in AI-Y-Cu alloys. A four-stage crystallization sequence could be identified for the AI-Y-Ni amorphous alloys. Even though the AI80Y10Cu10 alloy showed four exothermic peaks in the DSC study, a definite crystallization sequence could not be arrived at due to the coexistence of many crystalline phases along with the amorphous phase in the melt-spun condition.

Electrical-Resistivity Studies on GE-SE Glasses at High-Pressures and Low-Temperatures

High pressure electrical resistivity measurements were carried out on GexSe100-x (0 less-than-or-equal-to x less-than-or-equal-to 40) glasses at ambient and low temperatures using the Bridgman anvil system. All the melt quenched glasses show a discontinuous glassy semiconductor to crystalline metal transition at high pressures. The high pressure phases of Ge-Se samples do not correspond to any of the equilibrium phases of the system. Additionally, the variation of transition pressure (P(T)), ambient resistivity (rho0) and the activation energy (DELTAE(t)) with composition, exhibit a change in behaviour at x = 20 and 33. The unusual variations observed in these glasses are discussed in the light of chemical and percolation thresholds occurring in the glassy system.

Two forms of PF1 INOVIRUS: X-ray diffraction studies on a structural phase transition and a calculated libration normal mode of the assymetric unit

Inovirus is a helical array of agr-helical protein asymmetric units surrounding a DNA core. X-ray fibre diffraction studies show that the Pf1 species of Inovirus can undergo a reversible temperature-induced transition between two similar structural forms having slightly different virion helix parameters. Molecular models of the two forms show no evidence for altered interactions between the protein and either the solvent or the viral DNA; but there are significant differences in the shape and orientation of the protein asymmetric unit, related to the changes in the virion parameters. Normal modes involving libration of whole asymmetric units are in a frequency range with appreciable entropy of libration, and the structural transition may be related to changes in libration.

Studies on a stove for powdery biomass

The sawdust stove, classically known for several decades, is considered here in a scientific study. The poor ignition characteristics and smoky start up are related to improper geometric dimensions. Based on a parametric study, the startup procedure and the dimensions of the stove were modified to achieve a smooth start up. Also, the range of acceptable fuels was enlarged to include tiny unprocessed dry twigs, weeds and wood sticks o the extent of about 50%, with the rest being sawdust-like material. The efficiency of the stove was measured to be 30–40%, depending on the relative size and shape of the vessel and the power level of the stove. A simple procedure for designing this class of stove for various power levels, as well as burning times, is presented. A new concept of multiport design is also discussed.