Studies On Thermal-Decomposition Of Double-Base Propellants

The scanning thermogram of a block sample of a double-base propellant shows a shoulder around 200°C which is not observed in a powder sample of the sample propellant. The heat of decomposition was also found to be different In the two cases. Product analysis and activation energy calculations show that nitroglycerine un dergoes decomposition in the block sample, whereas it vaporizes in the powder sample.

Oxygen transfer in circular surface aeration tanks

Surface aeration systems employed in activated sludge plants are the most energy-intensive units of the plants and typically account for a higher percentage of the treatment facility's total energy use. The geometry of the aeration tank imparts a major effect on the system efficiency. It is said that at optimal geometric onditions, systems exhibits the maximum efficiency. Thus the quantification of the optimal geometric conditions in surface aeration tanks is needed. Optimal geometric conditions are also needed to scale up the laboratory result to the field installation. In the present work, experimental studies have been carried out on baffled and unbaffled circular surface aeration tanks to ascertain the optimal geometric conditions. It is found that no optimal geometric conditions exist for the liquid/water depth in circular surface aeration tanks; however, for design purposes, a standard value has been assumed. Based on the optimal geometric conditions, a scale-up equation has been developed for the baffled circular surface aeration tanks.

Curing Reactions in Elastomers. 11. Carboxy- Terminated Polybutadiene

Attempts have been made to understand the curing reactions in carboxy-terminated polybutadiene (CTPB), which happens to be the most practical binder in advanced solid composite propellants. The curing of CTPB has been studied for different ratios of curing agents (MAPO and Epoxide) by gel content, molecular weight, crosslink density, and penetration temperature measurements, and the optimum composition of curators for effective curing of CTPB has been determined. Activation energy calculations on the curing of CTPB with 9.5% epoxide and 0.5% MAPO in the temperature range 75100°C gave 14.1 kcal/mol for which a diffusion-controlled or acid-catalyzed epoxide ring opening mechanism has been suggested for the curing process in CTPB.

Self-Organization of n-Alkane Chains in Water: Length Dependent Crossover from Helix and Toroid to Molten Globule

We demonstrate a chain length dependent crossover in the structural properties of linear hydrocarbon (n-alkane) chains using detailed atomistic simulations in explicit water. We identify a number of exotic structures of the polymer chain through energy minimization of representative snapshots collected from molecular dynamics trajectory. While the collapsed state is ring-like (circular) for small chains (CnH2n+2; n <= 20) and spherical for very long ones (n = 100), we find the emergence of ordered helical structures at intermediate lengths (n similar to 40). We find different types of disordered helices and toroid-like structures at n = 60. We also report a sharp transition in the stability of the collapsed state as a function of the chain length through relevant free energy calculations. While the collapsed state is only marginally metastable for C20H42, a clear bistable free energy surface emerges only when the chain is about 30 monomers long. For n = 30, the polymer exhibits an intermittent oscillation between the collapsed and the coil structures, characteristic of two stable states separated by a small barrier.

Topochemical dimerization of non-parallel double bonds: 7-methoxycoumarin

X-ray crystal structure analysis of 7-methoxycoumarin reveals that the reactive double bonds are rotated by about 65° with respect to each other, the centre-to-centre distance between the double bonds being 3.83 Å. In spite of this unfavourable arrangement, photodimerization occurs in the crystalline state yielding the syn-head-tail dimer as the only product. Lattice energy calculations on ground-state molecules in crystals throw light on the mechanism of the reaction.

Complex carbohydrates: 1. Conformational studies on some oligosaccharides related to N-glycosyl proteins which interact with concanavalin A

Empirical potential energy calculations have been carried out to determine the preferred conformations of some oligosaccharides having the trimannosidic core structure (Man3GlcNAc2) and which interact with concanavalin A. In the minimum energy conformations for the trimannosidic core the mannose residue on the Man α(1–6) arm comes close to one of the N-acetylglucosamine residues of the core. The addition of N-acetylglucosamine residues to the terminal mannose residues does not alter the preferred conformation of the trimannosidic core although it alters the relative preference of some of the higher energy conformations. The minimum energy conformation broadly agrees with available X-ray data. The presence of a bisecting N-acetylglucosamine residue on the middle mannose does not push the trimannosidic core to any new conformation but it does alter the relative preference for a particular conformation.

Theoretical studies on Beta-lactam antibiotics VI: Conformational analysis and structure-activity relationships of penicillin sulfoxides and cephalosporins

Conformational energy calculations were carried out on penicillin α-and Β-sulfoxides and δ2- and δ3-cephalosporins, in order to identify the structural features governing their biological activity. Results on penicillin Β-sulfoxide indicated that in its favoured conformation, the orientation of the aminoacyl group was different from the one required for biological activity. Penicillin α sulfoxide, like penicillin sulfide, favoured two conformations of nearly equal energies, but separated by a much higher energy barrier. The reduced activity of the sulfoxides despite the nonplanarity of their lactam peptide indicated that the orientations of the aminoacyl and carboxyl groups might also govern biological activity. δ3-cephalosporins favoured two conformations of nearly equal energies, whereas δ2-cephalosporins favoured only one conformation. The lactam peptide was moderately nonplanÄr in the former, but nearly planar in the latter. The differences in the.preferred orientations of the carboxyl group between penicillins and cephalosporins were correlated with the resistance of cephalosporins to penicillinases.

Is 3'-nucleotide rigid?

Conformational analysis of nucleic acids and polynucleotides is far more complex than that of proteins and polypeptides, due to five single bond rotations in addition to the sugar puckerings in the monomer. Sundaralingam1 proposed the concept of the 'rigid' nucleotides from analysis of crystal structure data, with the flexibility allowed only about the phosphodiester bonds. However, the crystal structure of deoxyguanosine-5'−phosphate2,3 indicates at gt conformation about the C-4'−C-5' bond against gg in a conformationally rigid nudeotide1. Jack et al. 4 considered the flexibility of nucleotides in tRNA about the C-4'−C-5' bond, thereby introducing the concept of 'non-rigid' ribonucleotides. Conformational flexibility of the f uranose ring in DNA and RNA and their energetics using classical and quantum chemical methods have been reported5−8. We have examined the flexibility of 3'-nucleotides. alpha, the most important of the conformational parameters defining the 3'-end of a nucleotide unit9, has a value in the range 195°−270° in all the 3'-nucleotides, dinucleoside monophosphates and higher oligomers which have been surveyed. A survey of the proposed structures of polyribonudeotides10,11 also shows the values of a to be greater than 200°. However, the structures proposed for B-DNA by Arnott and Hukins12,13 and D-DNA by Arnott et al. 14 have values of alpha of 155° and 141° respectively, much lower than the lowest observed value. The structure for B-DNA has two strong, short contacts (C-2'...OP-1 = 2.64 Å and HC-2"...OP-1 = 1.79 Å) which lead to an energetically unfavourable conformation. Hence, it is of interest to investigate whether, by allowing flexibility to the sugar moiety in the nucleotide unit, it is possible to make the structure energetically favourable. Here, conformational energy calculations were carried out to determine the range of alpha which would give rise to energetically favoured conformations with different sugar puckerings. Our analysis has shown that the theoretically obtained range is nearly the same as the preferred range in crystals, indicating the flexibility of the 3'-nucleotides.

Theoretical studies on penicillins, penicillin sulphones and their 1-oxa-1-dethia and 1-carba-1-dethia analogues: binding specificities of transeptidases and penicillinases

Empirical potential energy calculations have been carried out to determine the preferred conformations of penicillins and penicillin sulphones and their 1-oxa-1-dethia and 1-carba-1-dethia analogues. With the exception of 1-oxa-1-dethia penicillins, all the other compounds favour C2 and the C3 puckered conformations of their five-membered rings. Replacement of C2 methyl groups by hydrogen atoms as in bisnorpenicillin V or oxidation of sulphur in position 1 as in sulphones, makes the C3 puckered form much less favourable. Addition of an amino-acyl group at the C6 atom, however, makes the C3 puckered form more favoured in penicillin G or V and in 1-carba-1-dethia penicillins. Through the replacement of the sulphur atom at position 1 by an oxygen atom or by a -CH2 group increases the non-planarity of the lactam peptide bond, it significantly affects the relative disposition of the C3 carboxyl group with respect to the β-lactam ring. These conformational differences have been correlated with the biological activities of these compounds. The present study suggests that the conformation of the bicyclic ring system may be more important for initial binding with the crosslinking enzyme(s) involved in the biosynthesis of bacterial cell-wall peptidoglycan and that the mode of binding is influenced by the nature of the side-group at the C6 atom. These studies predict, in agreement with experimental results, that the 1-oxa-1-dethia penicillin nulceus is an inhibitor of penicillianses. The study also suggests that the stereospecificities of the crosslinking enzyme(s) and penicillinases are very similar with regard to the nature of the side-group at the 6 atom and the confirmation of the bicyclic ring system. However, the confirmational requirement for the bicyclic ring system appears to be more specific in the former enzyme than in the latter.

Conformational studies of phynyl and isoxazolyl penicillins

Conformational energy calculations have been carried out on a few representative b-lactamase resistant and susceptible, phenyl and isoxazolyl penicillins. These studies, in agreement with those of earlier workers, show that the 6b-side chains of resistants penicillins are highly rigid as compared to those of susceptible penicillins. The present studies also suggest that the degree of resistant to b-lactamases depends not only on the rigidity of the side chain but also on the nature and orientation of the substituent, beyond the amide carbonyl group in the side chain. The overall shapes of these penicillins correlate well with their antibacterial properties.

A semi-empirical theory of molten salts

A semi-empirical model is presented for describing the interionic interactions in molten salts using the experimentally available structure data. An extension of Bertaut's method of non-overlapping charges is used to estimate the electrostatic interaction energy in ionic melts. It is shown, in agreement with earlier computer simulation studies, that this energy increases when an ionic salt melts. The repulsion between ions is described using a compressible ion theory which uses structure-independent parameters. The van der Waals interactions and the thermal free energy are also included in the total energy, which is minimised with respect to isostructural volume variations to calculate the equilibrium density. Detailed results are presented for three molten systems, NaCl, CaCl2 and ZnCl2, and are shown to be in satisfactory agreement with experiments. With reliable structural data now being reported for several other molten salts, the present study gains relevance.

Conformation And C-N Rotational Barrier Of Dithiocarbamate Esters Nmr And Molecular-Orbital Studies

The variable temperature proton and ambient temperature carbon-13 NMR spectra of S-methyl dithiocarbamate esters have been recorded. The results of the theoretical energy calculations (CNDO/2 and EHT types) together with the experimental data have been interpreted in terms of the molecular conformations. The barrier heights for the rotation about the thioamide C—N bond are calculated using the CNDO/2 method and the results are discussed in terms of the computed charge densities and bond orders.

An explanation for the rare occurrence of cis peptide units in proteins and polypeptides

The rarity of occurrence of cis peptide units is only partially explained by the higher intrinsic energy of the cis over the trans form, which provides a probability of 0·01 for cis peptide units to occur. An additional factor is the conformational restriction imposed by the occurrence of a cis peptide unit in a chain of trans units. Taking a section of three peptide units having the sequences trans-trans-trans (ttt) and trans-cis-trans (tct), conformational energy calculations indicate that the latter can occur only to an extent of 0·1%, unless there occurs the sequence X-Pro, in which case it is of the order of 30%. This explains the extreme rarity of cis peptide units, in general; however, it follows that even with non-prolyl residues, cis peptide units are not forbidden, but can occur in some rare examples and should be looked for.

Conformational analysis of cyclo (l-cystine

Conformational analysis of cyclo(L-cystine) shows that the diketopiperazine ring has to exist only in the boat form. With this geometry, the molecule can adopt two distinct forms differing mainly in the chirality of the disulphide bridge. In both the P- and M-models, corresponding to dihedral angles of nearly + 90° and —90° respectively about the S-S bond, the molecule displays an approximate two-fold symmetry. According to our semi-empirical energy calculations, the minimum energy of the M-model is —9.2 kcal/mol, only 0.3 kcal/mol lower than that of the P-model. Because the difference between the two minima is so small, neither form is clearly superior to the other. However, the number of low energy conformations of the M-model in the allowed conformational space is significantly larger than that of the P-model by a ratio of 3 to 1, and therefore the former is likely to be thermodynamically favoured.

Conformation of polypeptide-chains containing both l-residues and D-residues .2. Double-helical structures of poly-ld-peptides

Polypeptides with alternating L- and D-amino acid residues can take up stereochemically satisfactory coaxial double-helical structures, both antiparallel and parallel, which are stabilized by systematic interchain NH O hydrogen bonds. Semiempirical energy calculations over allowed regions of conformational space have yielded the characteristics of these double-helices. There are four possible types of antiparallel double-helices - A3, A4, A5 and A6, with n, the number of LD peptide units per turn, around 2.8, 3.6, 4.5 and 5.5 respectively, while for the parallel double-helices there are two types, P3 and P4, having similar helical parameters as in A3 and A4. The hydrogen-bonding scheme restricts the pitch in all the models to the narrow range of 10.0 to 11.5 Å. All these helices have large central cores whose radii increase proportionately with n. In this respect, A3 and A4 are suitable models for the structure of gramicidin A. In terms of their relative energies, antiparallel double-helices are marginally more stable than those with parallel strands. Our results indicate that the energy differences amongst the members in the antiparallel family are not significant and thus provide an explanation for the polymorphism reported for poly(γ-benzyl-LD-glutamate).