Showing papers by "Indian Institute of Science published in 1990"
TL;DR: There still is a great gap between human reading and machine reading capabilities, and a great amount of further effort is required to narrow-down this gap, if not bridge it.
508 citations
01 Dec 1990
TL;DR: It is shown that prevention and avoidance of FMS deadlocks can be implemented using Petri net models and a generic, Petri-net-based online controller for implementing deadlock avoidance in real-world FMSs is developed.
Abstract: Deadlocks constitute an important issue to be addressed in the design and operation of FMSs. It is shown that prevention and avoidance of FMS deadlocks can be implemented using Petri net models. For deadlock prevention, the reachability graph of a Petri net model of the given FMS is used, whereas for deadlock avoidance, a Petri-net-based online controller is proposed. The modeling of the General Electric FMS at Erie, PA, is discussed. For such real-world systems, deadlock prevention using the reachability graph is not feasible. A generic, Petri-net-based online controller for implementing deadlock avoidance in such real-world FMSs is developed. >
422 citations
TL;DR: The Asian elephant's foraging strategy in its natural habitat and in cultivation was studied in southern India during 1981-83 and crop raiding can be thought of as an extension of the elephant's optimal foraging Strategy.
Abstract: The Asian elephant's foraging strategy in its natural habitat and in cultivation was studied in southern India during 1981-83. Though elephants consumed at least 112 plant species in the study area, about 85% of their diet consisted of only 25 species from the order Malvales and the families Leguminosae, Palmae, Cyperaceae and Gramineae. Alteration between a predominantly browse diet during the dry season with a grass diet during the early wet season was related to the seasonally changing protein content of grasses. Crop raiding, which was sporadic during the dry season, gradually increased with more area being cultivated with the onset of rains. Raiding frequency reached a peak during October-December, with some villages being raided almost every night, when finger millet (Eleusine coracana) was cultivated by most farmers. The monthly frequency of raiding was related to the seasonal movement of elephant herds and to the size of the enclave. Of their total annual food requirement, adult bull elephants derived an estimated 9.3% and family herds 1.7% in quantity from cultivated land. Cultivated cereal and millet crops provided significantly more protein, calcium and sodium than the wild grasses. Ultimately, crop raiding can be thought of as an extension of the elephant's optimal foraging strategy.
286 citations
TL;DR: In this paper, the surface areas of the as-prepared metal aluminates using carbohydrazide fuel were higher compared with urea (1 to 20 $m^2g^{-1}$).
Abstract: Fine-particle metal aluminates,$MAl_20_4$ where $M = Mg$, $Ca$, $Sr$, $Ba$, $Mn$, $Co$, $Ni$, $Cu$ and $Zn$ as well as $3CaO . Al_2O_3$ $(C_3A)$, $CaAl_{12}O_{19}$ $(CA_6)$ and $MgCeAl_{11}O_{19}$ have been prepared by the combustion of mixtures of the respective metal nitrates (oxidizers) and urea or carbohydrazide (fuels) at 500 or $350^0c$ respectively, over a time of 5min. The solid combustion products were identified by their characteristic X-ray powder diffraction patterns. The fine-particle nature of these metal aluminates was investigated using SEM, TEM, particle size analysis and surface area measurements. The surface areas of the as-prepared metal aluminates using carbohydrazide fuel were higher (45 to 85 $m^2g^{-1}$) compared with urea (1 to 20 $m^2g^{-1}$).
255 citations
TL;DR: A statistical-mechanical theory wherein spatial fluctuations of the order parameter are incorporated is used to study the shapes and areas of the hysteresis loops as functions of the amplitude and frequency of the magnetic field.
Abstract: A systematic study of hysteresis in model continuum and lattice spin systems is undertaken by constructing a statistical-mechanical theory wherein spatial fluctuations of the order parameter are incorporated. The theory is used to study the shapes and areas of the hysteresis loops as functions of the amplitude (${\mathit{H}}_{0}$) and frequency (\ensuremath{\Omega}) of the magnetic field. The response of the spin systems to a pulsed magnetic field is also studied. The continuum model that we study is a three-dimensional (${\mathrm{\ensuremath{\Phi}}}^{2}$${)}^{2}$ model with O(N) symmetry in the large-N limit. The dynamics of this model are specified by a Langevin equation. We find that the area A of the hysteresis loop scales as A\ensuremath{\sim}${\mathit{H}}_{0}^{0.66}$${\mathrm{\ensuremath{\Omega}}}^{0.33}$ for low values of the amplitude and frequency of the magnetic field. The hysteretic response of a two-dimensional, nearest-neighbor, ferromagnetic Ising model is studied by a Monte Carlo simulation on 10\ifmmode\times\else\texttimes\fi{}10, 20\ifmmode\times\else\texttimes\fi{}20, and 50\ifmmode\times\else\texttimes\fi{}50 lattices. The framework that we develop is compared with other theories of hysteresis. The relevance of these results to hysteresis in real magnets is discussed.
237 citations
TL;DR: In this article, the effect of PbO on the borate network was studied by Raman spectroscopy with a view to understand the effect on boroxol rings in the form of pentaborate and diborate units.
Abstract: Glasses in the system $xPbO:(100- x)B_2O_3 (22\leq x\leq 85$ mol%) have been studied by Raman spectroscopy with a view to understand the effect of PbO on the borate network. The conversion of three-fold to four-fold coordinated boron takes place primanly by incorporation of $BO_4$ into boroxol rings in the form of pentaborate and diborate units. For high lead content, back conversmn of four to three coordinatmn of borons occurs and the network consists primardy of pyroborate, orthoborate and metaborate groups.
235 citations
TL;DR: In this paper, a two source-location method was proposed for measurement of the four-pole or the transfer matrix parameters for an acoustic element or a subsystem of elements by means of four-microphone technique and use of the transfer function approach.
227 citations
TL;DR: It is shown that an `assured fitness returns' model provides a selective pressure for the evolution of worker behaviour which is at least about as strong as that of haplodiploidy, but free from such requirements of the latter as high levels of worker-brood genetic relatedness and ability of workers to manipulate brood sex ratios.
Abstract: Delineation of the selective pressures responsible for the evolution of sterile worker castes found in social insect colonies remains a major unsolved problem in evolutionary biology. There has therefore been a great deal of interest in suggesting ways by which the inclusive fitness of sterile workers can potentially be larger than those of solitary nest-builders. Queller's (1989) head-start hypothesis (Proc. natn. Acad. Sci. U.S.A. 86, 3224) suggests that workers may gain relatively more inclusive fitness because they have access to young of various ages which can be quickly brought to the age of independence, whereas a solitary foundress has to survive for the entire duration of the development of her brood. I argue here that Queller's quantitative analysis is incorrect because it gives an unfair advantage to workers, either by giving full credit of rearing an offspring to a worker who only cared for it for a short while or, by assuming that a worker can do much more work per unit time than a solitary foundress. I show, however, that workers do indeed have an advantage over solitary foundresses because they have assured fitness returns, even if in small amounts, for short periods of work. This results from a different reckoning from that used by Queller and gives a more moderate advantage, arising essentially from saving the wasted effort that occurs when lone foundress nests fail. Using field and laboratory data on the primitively eusocial wasp Ropalidia marginata, and re-analysing data on the four species of polistine wasps used by Queller, I show that such an `assured fitness returns' model provides a selective pressure for the evolution of worker behaviour which is at least about as strong as that of haplodiploidy, but free from such requirements of the latter as high levels of worker-brood genetic relatedness and ability of workers to manipulate brood sex ratios.
167 citations
TL;DR: It is concluded that PPN is a better guidance law in a practical sense than TPN and its generalizations.
Abstract: Two generic classes of proportional navigation (PN) laws are compared in detail. One class consists of pursuer-velocity-referenced systems, which include pure proportional navigation (PPN) and its variants; the second category consists of line-of-sight- (LOS-) referenced systems such as true proportional navigation (TPN), generalized true proportional navigation (GTPN), and generalized guidance laws. The existing closed-form solutions are discussed in detail, and the classical linear and quasilinear analytical solutions are summarized. A critical comparison is then made with regard to the definition, implementation, and analytical aspects of the guidance laws, including the method, the nature of solution, and an appraisal of the behavior of the pursuer motion resulting from the laws. It is established that, in spite of some restricted advantages in the solvability of the equations of motion, the LOS-referenced PN schemes suffer from serious limitations in terms of implementation and trajectory behavior. Among the major drawbacks are forward velocity variation requirement, relatively large control effort requirement, restrictions on initial engagement conditions to ensure intercept, lack of robustness, and possibility of unbounded acceleration. It is concluded that PPN is a better guidance law in a practical sense than TPN and its generalizations. >
124 citations
TL;DR: In this paper, a decomposition theorem for SU (2) matrices in terms of factors which are symmetric fourth and eighth roots of the identity was established. But this decomposition was based on the assumption that all SU(2) polarization transformations can be realized by the minimum number of quarter-wave and half-wave plates.
121 citations
TL;DR: Computer-aided generation of parting surfaces and the determination of projected area, flatness and draw for a parting surface, identification of surfaces to which draft is provided, recognition of component segments causing undercuts, testing for dimensional stability, and location of flash, machined surfaces and feeders are presented.
Abstract: A scientific approach is presented and the related logic developed for design of parting surfaces of patterns, moulds and dies used in the manufacture of cast, forged, injection-moulded and die-cast components. This has enabled computer-aided generation of parting surfaces and the determination of projected area, flatness and draw for a parting surface, identification of surfaces to which draft is provided, recognition of component segments causing undercuts, testing for dimensional stability, and location of flash, machined surfaces and feeders. Influencing criteria for parting-surface design have been formulated and developed into algorithms implemented on a personal computer. This approach greatly aids the engineer in rational decision making, paving the way for a systemized code for parting-surface design.
01 Jun 1990-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, the relative stability of evaluated oxides in the order of increasing stability follows the sequence CeO2 − ZrO2, Gd2O3, didymium oxide, Sm2O 3, Nd2 O3, Y2 O 3.
Abstract: Attempts have been made to evaluate the thermal stability of rare earth oxide face coats against liquid titanium. Determination of microhardness profiles and concentration profiles of oxygen and metallic constituents of oxide in investment cast titanium rods has allowed grActation of relative stability of rare earth oxides. The relative stability of evaluated oxides in the order of increasing stability follows the sequence CeO2 — ZrO2 — Gd2O3 — didymium oxide — Sm2O3 —Nd2O3 — Y2O3. The grading does not follow the free energy data of the formation of these oxides. A better correlation with the experimental observations is obtained when the solubility of the metallic species in titanium is also taken into consideration.
TL;DR: Indole was metabolized by a new pathway via indoxyl (3-hydroxyindole), N-formylanthranilic acid, anthranilic Acid, 2,3-dihydroxybenzoic acid, and catechol, which was further degraded by ortho cleavage, and was demonstrated in a cell-free system.
Abstract: Indole and its derivatives form a class of toxic recalcitrant environmental pollutants. The growth of Aspergillus niger was inhibited by very low concentrations (0.005 to 0.02%) of indole, even when 125- to 500-fold excess glucose was present in the medium. When 0.02% indole was added, the fungus showed a lag phase for about 30 h and the uptake of glucose was inhibited. Indole was metabolized by a new pathway via indoxyl (3-hydroxyindole), N-formylanthranilic acid, anthranilic acid, 2,3-dihydroxybenzoic acid, and catechol, which was further degraded by ortho cleavage. The enzymes N-formylanthranilate deformylase, anthranilate hydroxylase, 2,3-dihydroxybenzoate decarboxylase, and catechol dioxygenase were induced by indole as early as after 5 h of growth, and their activities were demonstrated in a cell-free system.
TL;DR: In this paper, fine particle fluorescent aluminous oxide materials like Cr3+-doped α-Al2O3 (ruby), MgAl 2O4, LaAlO3, Y3Al5O12, LaMgAl11O19, CaAl12O19 and CeMgal 11O19 have been prepared by the combustion of the corresponding metal nitrate-aluminium nitrate/urea/carbohydrazide mixtures at 500°C in less than 5 min.
Abstract: Fine particle fluorescent aluminous oxide materials like Cr3+-doped α-Al2O3 (ruby), MgAl2O4, LaAlO3, Y3Al5O12 and Ce3+-doped Y3Al5O12, LaMgAl11O19, CaAl12O19 and CeMgAl11O19 have been prepared by the combustion of the corresponding metal nitrate-aluminium nitrate-urea/carbohydrazide mixtures at 500°C in less than 5 min. Formation of these Cr3+- and Ce3+-doped aluminous oxides has been confirmed by their characteristic XRD, colour, UV-visible and fluorescence spectra as well as decay time measurements. Ruby (Cr3+/α-Al2O3) powder showed characteristic excitation bands at 406 and 548 nm and emission band at 695 nm with the decay time of 3·6 ms.
TL;DR: A review of the literature on gangliosides and their roles in neuronal function, synaptic transmission, and the role of ganglioides in synaptic neurotransmission finds that the former is important for neuronal function and the latter for synaptic transmission.
11 Jun 1990
TL;DR: In this paper, a PWM rectifier/inverter system using IGBTs (insulated-gate bipolar transistors), capable of switching at 20 kHz, is reported.
Abstract: A PWM (pulse width modulation) rectifier/inverter system using IGBTs (insulated-gate bipolar transistors), capable of switching at 20 kHz, is reported. The base drive circuit for the IGBT, incorporating short circuit protection, is presented. The inverter uses an Undeland snubber together with a simple energy recovery circuit, which ensures reliable and efficient operation even for 20 kHz switching. The front end for the system is a regenerative single-phase full-bridge IGBT inverter along with an AC reactor. The steady-state design considerations are explained, and control techniques for unity power factor operation and fast current control of the front end converter, in rotating as well as stationary reference frames, are discussed and compared. Results from computer simulations and experimental results for a 1.5 kW prototype system using GE type 6E20 IGBTs are presented. >
TL;DR: In particular, this article showed that a subset consisting of half the full set of constraints is shown to have vanishing mutual brackets, and some other constraint brackets are also shown to be zero.
TL;DR: The present study reveals the exquisite specificity of A. integrifolia lectin for the T-antigen, and suggests that the primary subsite can accommodate only Gal or GalNAc or alpha-linkage with other sugars such as in A- and B-blood-group trisaccharides, Gal alpha 1-3Gal and Galalpha 1-4Gal.
Abstract: Thermodynamic analysis of carbohydrate binding by Artocarpus integrifolia (jackfruit) agglutinin (jacalin) shows that, among monosaccharides, Me alpha GalNAc (methyl-alpha-N-acetylgalactosamine) is the strongest binding ligand. Despite its strong affinity for Me alpha GalNAc and Me alpha Gal, the lectin binds very poorly when Gal and GalNAc are in alpha-linkage with other sugars such as in A- and B-blood-group trisaccharides, Gal alpha 1-3Gal and Gal alpha 1-4Gal. These binding properties are explained by considering the thermodynamic parameters in conjunction with the minimum energy conformations of these sugars. It binds to Gal beta 1-3GalNAc alpha Me with 2800-fold stronger affinity over Gal beta 1-3GalNAc beta Me. It does not bind to asialo-GM1 (monosialoganglioside) oligosaccharide. Moreover, it binds to Gal beta 1-3GalNAc alpha Ser, the authentic T (Thomsen-Friedenreich)-antigen, with about 2.5-fold greater affinity as compared with Gal beta 1-3GalNAc. Asialoglycophorin A was found to be about 169,333 times stronger an inhibitor than Gal beta 1-3GalNAc. The present study thus reveals the exquisite specificity of A. integrifolia lectin for the T-antigen. Appreciable binding of disaccharides Glc beta 1-3GalNAc and GlcNAc beta 1-3Gal and the very poor binding of beta-linked disaccharides, which instead of Gal and GalNAc contain other sugars at the reducing end, underscore the important contribution made by Gal and GalNAc at the reducing end for recognition by the lectin. The ligand-structure-dependent alterations of the c.d. spectrum in the tertiary structural region of the protein allows the placement of various sugar units in the combining region of the lectin. These studies suggest that the primary subsite (subsite A) can accommodate only Gal or GalNAc or alpha-linked Gal or GalNAc, whereas the secondary subsite (subsite B) can associate either with GalNAc beta Me or Gal beta Me. Considering these factors a likely arrangement for various disaccharides in the binding site of the lectin is proposed. Its exquisite specificity for the authentic T-antigen, Gal beta 1-3GalNAc alpha Ser, together with its virtual non-binding to A- and B-blood-group antigens, Gal beta 1-3GalNAc beta Me and asialo-GM1 should make A. integrifolia lectin a valuable probe for monitoring the expression of T-antigen on cell surfaces.
01 Nov 1990
TL;DR: In this article, the effects of the induced magnetic field and sources or sinks have been included in the analysis of flow and heat transfer over a stretching sheet with a magnetic field in an electrically conducting ambient fluid.
Abstract: The flow and heat transfer over a stretching sheet with a magnetic field in an electrically conducting ambient fluid have been studied. The effects of the induced magnetic field and sources or sinks have been included in the analysis. Both non-isothermal wall and constant heat flux conditions have been considered. The governing equations have been solved numerically using a shooting method. It is observed that for the prescribed wall temperature the skin friction, induced magnetic field at the wall and heat transfer are enhanced due to the magnetic field, but in general, they reduce as the reciprocal of the magnetic Prandtl number increases. For constant heat flux case, the temperature at the wall reduces as the magnetic field increases, but it increases with the reciprocal of the magnetic Prandtl number. The heat transfer is strongly affected by the Prandtl number, wall temperature and sink. Whenm 2.5 the unrealistic temperature distributions are encountered. The present analysis is more general than any previous investigation.
TL;DR: The diphosphinoamines react with group 6 metal carbonyl derivatives to yield the cis-chelate complexes [M(CO)4(L-PP′).
TL;DR: It was concluded that this species may be an aggregate of the native protein and that such an aggregation might be taking place on storage in the dry state at -20 degrees C, leading to loss in activity.
TL;DR: In this article, molecular adsorption of oxygen has been studied on (111, (110) and (100) surfaces of Ag, Cu, and Ni employing XPS and UPS.
TL;DR: In this paper, a novel combustion process using corresponding rare earth nitrate, aluminum nitrate and urea/carbohydrazide mixtures was described, and a saturated aqueous solution of the mixture when rapidly heated at 500°C foams and ignites to yield voluminous, crystalline single phase rare earth orthoaluminates.
TL;DR: In the value‐8 crystal the helices aggregate in a parallel mode, whereas the aggregation is antiparallel in the VALU‐7 crystal, which gives a static representation of a step in a possible helix unwinding or folding process.
Abstract: The crystal structures of two helical peptides Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe (VALU-7) and Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Aib-OMe (VALU-8) have been determined to a resolution of 1.0 and 0.9 A, respectively. Both the seven and eight residue peptides crystallize with two conformers per asymmetric unit. The VALUS conformers are completely helical and differ only at the C-terminus by a sign reversal of the \phi, \psi angles of the last residue. One of the VALU-7 conformers occurs as a normal \alpha-helix, whereas in the other, the N(7)=O(3) \alpha-type hydrogen bond is ruptured by the entry of a water molecule (W) into the helix, which in turn makes hydrogen bonds N(7)…W = 2.97 A and W…O(3) = 2.77 A. The other side of the water molecule is surrounded by a hydrophobic pocket. These two conformers give a static representation of a step in a possible helix unwinding or folding process. In the VALU-8 crystal the helices aggregate in a parallel mode, whereas the aggregation is anti-parallel in the VALU-7 crystal. The crystal parameters are VALU-7, $P2_1$, a = 10.203 (3) A, b = 19.744 (6) A, c = 22.561 (6) A, \beta = 96.76 deg, Z = 4, $C_{38}H_{69}N_7O_{10}$.0.5 $H_2O$, R = 6.65% for 3674 reflections observed >3\sigma(F); and VALU-8, $P2_1$ a = 10.593 (4) A, b = 27.57 (6) A, c = 17.745 (5) A, \beta = 95.76 (3) deg, Z = 4, $C_{42}H_{76}N_8O_{11}$.0.25 $CH_3OH$, R = 6.63% for 4701 reflections observed >3\sigma(F).
TL;DR: The vanadate has the expected layered structure and is ferroelectric with a Curie temperature 720 K as discussed by the authors, which is the vanadium analog of the first member of the Aurivillius family of oxides of the general formula.
Abstract: $Bi_2VO_{5.5} (Bi_4V_2O_{11}) $which is the vanadium analog of the first member of the Aurivillius family of oxides of the general formula $Bi_2A_{n-1}B_nO_{3n+3}$ has been prepared and characterized. The vanadate has the expected layered structure and is ferroelectric with a Curie temperature 720 K. While we have not been able to synthesize the vanadium analog of the$ n = 2$ member of the Aurivillius family, we have examined the structure and properties of a vanadate of the composition Bi2V3O9.
TL;DR: In this paper, the authors reported the preparation of fine-particle, large-surface-area ceria by the combustion process developed for preparation of $\alpha$-alumina, aluminates, chromites, and ferrites.
Abstract: Fine-particle ceria having narrow size distribution is of interest as polishing material for ophthalmological lenses, television tubes, plates and mirrors [1]. The preparation has been reported of such ceria by the hydrolysis of cerium isopropoxide [2] and cerium nitrate [3, 4] followed by thermal decomposition, as well as by the thermal decomposition of cerium oxalate hydrazinate [5] and cerium hydrazine carboxylate [6]. In this letter we report the preparation of fine-particle, large-surface-area ceria by the combustion process developed for the preparation of $\alpha$-alumina [7], aluminates [8], chromites [9] and ferrites [10]. The process involves rapid decomposition of a saturated aqueous solution containing ceric ammonium nitrate and urea $(CH_4N_20)$, carbohydrazide $(CH_6N_4O (CH))$, oxalyl dihydrazide $(C_2H_{16}N_4O_2 (ODH))$ or tetraformal trisazine $(C_4H_{16}N_6O_2 (TFTA))$ redox mixtures.
TL;DR: In this paper, it is shown that given a dynamical theory with second-class constraints, it is sometimes possible to construct another theory with first-class constraint, i.e., a gauge-invariant one, which is physically equivalent to the first theory.
TL;DR: The structure of the low humidity form of hen egg white lysozyme has been analyzed, using x-ray data collected at 88% relative humidity, in order to explore the variability in protein hydration caused by a change in the amount of water surrounding the protein molecule and the consequent conformational perturbations in the molecule.
TL;DR: In this article, a phenomenological analysis of the linear polarisation data of iron electrodes at various states-of-charge values is envisaged, in the light of depassivation and grain-refinement models, suggesting that the incorporation of sulphide enhances the bulk electrode conductivity.
TL;DR: The importance and usefulness of local doublet parameters in understanding sequence dependent effects has been described for A- and B-DNA oligonucleotide crystal structures and an interesting feature that emerges is that the local rise along the z-axis is highly correlated with the difference in the buckle values of the two basepairs in the doublet.
Abstract: The importance and usefulness of local doublet parameters in understanding sequence dependent effects has been described for A- and B-DNA oligonucleotide crystal structures. Each of the two sets of local parameters described by us in the NUPARM algorithm, namely the local doublet parameters, calculated with reference to the mean z-axis, and the local helical parameters, calculated with reference to the local helix axis, is sufficient to describe the oligonucleotide structures, with the local helical parameters giving a slightly magnified picture of the variations in the structures. The values of local doublet parameters calculated by NUPARM algorithm are similar to those calculated by NEWHELIX90 program, only if the oligonucleotide fragment is not too distorted. The mean values obtained using all the available data for B-DNA crystals are not significantly different from those obtained when a limited data set is used, consisting only of structures with a data resolution of better than 2.4 A and without any bound drug molecule. Thus the variation observed in the oligonucleotide crystals appears to be independent of the quality of their crystallinity. No strong correlation is seen between any pair of local doublet parameters but the local helical parameters are interrelated by geometric relationships. An interesting feature that emerges from this analysis is that the local rise along the z-axis is highly correlated with the difference in the buckle values of the two basepairs in the doublet, as suggested earlier for the dodecamer structures (Bansal and Bhattacharyya, in Structure & Methods: DNA & RNA, Vol. 3 (Eds., R.H. Sarma and M.H. Sarma), pp. 139-153 (1990)). In fact the local rise values become almost constant for both A- and B-forms, if a correction is applied for the buckling of the basepairs. In B-DNA the AA, AT, TA and GA basepair sequences generally have a smaller local rise (3.25 A) compared to the other sequences (3.4 A) and this seems to be an intrinsic feature of basepair stacking interaction and not related to any other local doublet parameter. The roll angles in B-DNA oligonucleotides have small values (less than +/- 8 degrees), while mean local twist varies from 24 degrees to 45 degrees. The CA/TG doublet sequences show two types of preferred geometries, one with positive roll, small positive slide and reduced twist and another with negative roll, large positive slide and increased twist.(ABSTRACT TRUNCATED AT 400 WORDS)