Determination of amino acids in apple extracts by high performance liquid chromatography

Summary A rapid and sensitive method for the simultaneous determination of primary amino acids in apple is described. After sample preparation, amino acids were derivatized with o-phthaldialdehyde/2-mercaptoethanol and separated on a reversed phase column with a gradient of phosphate buffer-tetrahydrofuran-methanol as the mobile phase. Detection was carried out with a fluorescence detector at excitation and emission wavelengths of 340 nm and 425 nm respectively. Recovery studies showed good results for all substances (91–109%) (with coefficients of variation ranging, from 0.1 to 9.0%). This method was applied to the monitoring of amino acids during the ripening of apples.     

Multiple Stable Conformations Account for Reversible Concentration‐Dependent Oligomerization and Autoinhibition of a Metamorphic Metallopeptidase

Molecular plasticity controls enzymatic activity: the native fold of a protein in a given environment is normally unique and at a global free‐energy minimum. Some proteins, however, spontaneously undergo substantial fold switching to reversibly transit between defined conformers, the “metamorphic” proteins. Here, we present a minimal metamorphic, selective, and specific caseinolytic metallopeptidase, selecase, which reversibly transits between several different states of defined three‐dimensional structure, which are associated with loss of enzymatic activity due to autoinhibition. The latter is triggered by sequestering the competent conformation in incompetent but structured dimers, tetramers, and octamers. This system, which is compatible with a discrete multifunnel energy landscape, affords a switch that provides a reversible mechanism of control of catalytic activity unique in nature.     

Volume approximation of smooth convex bodies by three-polytopes of restricted number of edges

For a given convex body K in \Bbb R³{\Bbb R}^3 with C ² boundary, let P ^c _n be the circumscribed polytope of minimal volume with at most n edges, and let P ⁱ _n be the inscribed polytope of maximal volume with at most n edges. Besides presenting an asymptotic formula for the volume difference as n tends to infinity in both cases, we prove that the typical faces of P ^c _n and P ⁱ _n are asymptotically regular triangles and squares, respectively, in a suitable sense.     

Sequential extraction--spectrofluorometric determination of lead and cadmium using cryptands.

A spectrofluorimetric method for the determination of ultratrace amounts of lead and cadmium is described based on the sequential extraction of the ternary ion-association complexes formed between the cation, a cryptand as the ligand and eosin as a counter ion. A linear working range from the detection limit (0.5 ng ml(-1) to 250 ng ml(-1) of lead and to 1 50 ng ml(-1) of cadmium was obtained. The relative standard deviation was 2-4%. The proposed method has been applied successfully to the determination of lead and cadmium in zinc metals and soft drinks.     

Dido's problem in the plane for domains with fixed diameter

We find the connected compact domains in the closed half-plane, with fixed area and diameter, which minimize the relative perimeter.This work has been partially supported by the DGICYT (Spain) Grant no. PB 91-0324.     

Liquid-liquid extraction: A graphical method for equilibrium stage calculations for quaternary systems

Ruiz Beviá, F., Prats Rico, D. and Marcilla Gomis, A.F., 1984. Liquid-liquid extraction: a graphical method for equilibrium stage calculations for quaternary systems. Fluid Phase Equilibria, 15: 257-265.A very simple graphical method for stage calculations has been developed for quaternary systems. The calculation method is based on the equilibrium representation method proposed by Ruiz and Prats (1983a), and requires only two graphs for solution of the material balances. The method is based completely on experimental data and does not require any simplifying hypotheses.     

The influence of temperature on the liquid-liquid-solid equilibrium of the ternary system water-potassium chloride-2-propanol

The lowest temperature at which water---KCl---2-propanol mixtures cannot be split has been determined as 25.3°C. Seven diagrams of liquid-liquid-solid equilibria for the ternary system water---potassium chloride-2-propanol around this temperature have been measured and the transition between their two different shapes has been studied.     

Sequential extraction-spectrofluorimetric determination of ultratraces of zinc using cryptand 2.2.1 and eosin

A highly sensitive and selective spectrofluorimetric method has been developed for the determination of ultratraces of zinc, based on solvent extraction with 1,2-dichloroethane of the ion-pair formed between eosinate anion and the positively charged cryptate of zinc with cryptand ethers. The detection limit for zinc is 1.5 ng/ml, and the linear working range up to 200 ng/ml. The relative standard deviation is 1.8% at the 100 ng/ml level. The proposed method has been tested for determination of zinc in coal and fly ash.