A DSC study of the effects of sugars on thermal properties of rice starch gels before and after aging

Thermal characterization of gelatinized binary rice starch-water and ternary starch-sugar-water gels before and after aging was carried out using differential scanning calorimetry. The glass transition temperature of the maximally freeze-concentrated solution (T_g) in both fresh and aged gels was observed to decrease progressively with increasing sugar concentration. Aging of the gels generally shiftedT_g to higher temperatures, but had little or no effect on the ice melting peak temperature (T _m). The presence of various sugars could either accelerate or retard starch (amylopectin) recrystallization, depending on the type and concentration of sugar, as well as on starch/water ratio. A hypothesis based on the dual antiplasticizing-plasticizing effects of sugars was postulated to explain the observed effects. Of the sugars studied, xylose and fructose appeared to display exceptional retardative and accelerative effects, respectively, on retrogradation.This study was funded by a Sixth Malaysia Plan R&D grant under the Intensification of Research Priority Areas (IRPA) Program of the Ministry of Science, Technology and Environment, Malaysia. We thank Dr. Harry Levine and Dr. Louise Slade for giving us the opportunity to contribute a paper to this special issue of JTA.     

Synthesis of cryptophycins via an N-acyl-beta-lactam macrolactonization

An efficient and concise approach to the synthesis of the macrolide core of the cryptophycins has been developed. A novel macrolactonization utilizing a reactive acyl-beta-lactam intermediate incorporates the beta-amino acid moiety within the 16-membered macrolide core. This modular approach, involving a cyanide-initiated acyl-beta-lactam ring opening followed by cyclization, was successfully applied to the total synthesis of cryptophycin-24. The strategy was also used in an efficient synthesis of the 6,6-dimethyl-substituted dechlorocryptophycin-52. In this case, the cyanide-initiated ring opening of the bis-substituted 2-azetidinone followed by macrolactonization was achieved through a catalytic process.     

A novel three-component reaction for the diastereoselective synthesis of 2H-pyrimido[2,1-a]isoquinolines via 1,4-dipolar cycloaddition

[reaction: see text] The 1,4-dipole derived from isoquinoline and DMAD has been shown to react readily with N-tosylimines resulting in the diastereoselective synthesis of 2H-pyrimido[2,1-a]isoquinoline derivatives.     

Cumulative yields of the short-lived ruthenium isotopes in the spontaneous fission of252Cf

⁶⁷Ga is produced by the⁶⁶Zn(d,n) reaction at 9 MeV bombarding energy. Deuterons of about 30 A obtained from the T-11 Tandem accelerator are used to bombard targets of natural Zinc prepared by electroplating onto thick copper disks. Radiochemical separation of gallium is performed by using a cation exchange column. The optimum conditions for an efficient separation are given in detail.     

Gas Hydrate Formation in Reversed Micelles

We describe a technique to modify protein solubility and optimize enzyme activity in reversed micellar solutions. The technique is based on the ability of hydrates of natural gas to form in the micro-aqueous phase. Clathrate hydrates are crystalline inclusions of water and gas, and their formation in bulk water has traditionally been studied with relevance to natural gas recovery. We have found that hydrates can form in the environment of the microaqueous pools of reversed micelles, and that their extent of formation can be well controlled through the thermodynamic variables of temperature and pressure. Additionally, formation of hydrates affects the size and aggregation number of the micelles, and thus influences the solubility and conformation of encapsulated proteins. We demonstrate how the concept can be used in two applications: (i) protein extraction into reversed micelles and subsequent recovery, and (ii) optimization of enzyme activity in reversed micelles.     

Thermogravimetric analysis of PVC/ELNR blends

The thermal behaviour of a series of solution-cast blends of polyvinyl chloride/epoxidised liquid natural rubber (ELNR) of different mole percentage of epoxidation has been examined using thermogravimetric analysis. Thermal degradation is found to occur through a two-step route in which the first step corresponds to the dehydrochlorination of PVC to form a polyene and the second step is attributed to the decomposition of the ELNR and the polyene. Introduction of 20 and 50 mol% of epoxy group into the liquid NR is found to enhance the thermal stability of PVC. Probable mechanisms of degradation have been suggested on the basis of the kinetic analysis of the degradation studies. It is found that the mechanism is influenced by the epoxy content of the blend system. Activation energy for the degradation and the entropy change have also been reported.     

Di(2-ethyl hexyl) butyramide and di(2-ethyl hexyl)isobutyramide as extractants for uranium(VI) and plutonium(IV)

The extraction of uranium(VI) and plutonium(IV) from nitric acid into n-dodecane was studied using two isomeric branched alkyl amides, di(2-ethyl hexyl) butyramide (DEHBA) and di(2-ethyl hexyl) isobutyramide (DEHIBA). The extraction ratios of Pu(IV) at relatively high acidities were higher than the corresponding values for U(VI) in the case of DEHBA. However, with DEHIBA the values for Pu(IV) were negligibly small. Pu(IV) was found to be extracted as trisolvate by DEHBA and as disolvate by DEHIBA. U(VI) was extracted by both the amides. From the study of the extraction reactions at different temperatures, it was shown that all the reactions in the present investigation were enthalpy favoured and entropy disfavoured. Separation of Pu(IV) from bulk of U(VI) was feasible. However, the purity of the separated plutonium was not satisfactory in batch extraction studies.     

Length dependent folding kinetics of phenylacetylene oligomers: structural characterization of a kinetic trap

Using simulation to study the folding kinetics of 20-mer poly-phenylacetylene (pPA) oligomers, we find a long time scale trapped kinetic phase in the cumulative folding time distribution. This is demonstrated using molecular dynamics to simulate an ensemble of over 100 folding trajectories. The simulation data are fit to a four-state kinetic model which includes the typical folded and unfolded states, along with an intermediate state, and most surprisingly, a kinetically trapped state. Topologically diverse conformations reminiscent of alpha helices, beta turns, and sheets in proteins are observed, along with unique structures in the form of knots. The nonhelical conformations are implicated, on the basis of structural correlations to kinetic parameters, to contribute to the trapped kinetic behavior. The strong solvophobic forces which mediate the folding process and produce a stable helical folded state also serve to overstabilize the nonhelical conformations, ultimately trapping them. From our simulations, the folding time is predicted to be on the order of 2.5-12.5 mus in the presence of the trapped kinetic phase. The folding mechanism for these 20-mer chains is compared with the previously reported folding mechanism for the pPA 12-mer chains. A linear scaling relationship between the chain length and the mean first passage time is predicted in the absence of the trapped kinetic phase. We discuss the major implications of this discovery in the design of self-assembling nanostructures.     

Structures of Addition Products of Acetylenedicarboxylic Acid Esters with Various Dinucleophiles. An application of C,H-spin-coupling constants

Heterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by ¹³C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C,H-spin coupling is again demonstrated in the present series.     

Adhesion and debonding of soft elastic films: crack patterns, metastable pathways, and forces

We study the phenomenon of debonding in a thin soft elastic film sandwiched between two rigid plates as one of the plates is brought into intimate contact and then pulled away from contact proximity by application of a normal force. Nonlinear simulations based on minimization of total energy (composed of stabilizing elastic strain energy and destabilizing adhesive interaction energy) are employed to address the problems of contact hysteresis, cavitation, crack morphology, variation of contact area, snap-off distance, pull-off force, work done, and energy loss. Below a critical distance (d(c)) upon approach, simulations show the formation of columnar structures and nonrandom, regularly arranged nanocavities at the soft interface at a length scale of approximately 3h (h being the thickness of the film). The persistence of such instability upon withdrawal (distance >d(c)) indicates a contact hysteresis, which is caused by an energy barrier that separates the metastable states of the patterned configuration and the global minimum state of the flat film. The energy and the pull-off force are found to be nonequilibrium and nonunique properties depending on the initial contact, defects, noise, etc. Three broad pathways of debonding leading to adhesive failure of the interface, depending on the stiffness of the film, step size of withdrawal, and the imposed noise, are identified: a catastrophic column collapse mode, a peeling mode involving a continuous decrease in the contact area, and a column splitting mode. The first two modes are caused by a very high stress concentration near the cavity edges. These metastable patterned configurations engender pull-off forces that are orders of magnitude smaller than that required to separate two flat surfaces from contact.