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G Kegeles

Publications and source records attributed to G Kegeles.

At least 19 recordsLinked to original sources
PubMed1992

The critical micelle condition revisited.

Several simple alternatives have been examined as a possible basis for micellar size distributions which are internally consistent with the experimentally required concept of a threshold concentration, the critical micelle condition. Among these are the two-state system, monomer in equilibrium with a single high polymer, indefinite self-association, and continuous self-association with an arbitrary upper limit beyond which all further association is absolutely prohibited. Of these, only the last is a possible choice, although lacking experimental support. This report considers in deeper detail a thermodynamic model for micelle distributions, the so-called shell model of the present author, which is in basic agreement with an earlier statistic-mechanical study [Hoeve CA & Benson GC (1957), Colloid Polym Sci, 252, 56] in predicting the possibility of broad distributions of micellar species. A self-consistent distribution model which predicts a critical micelle condition must also predict a location with respect to degree of polymerization having a minimum concentration. Thus, the molar distribution function for the shell model satisfies this requirement, whereas the equivalent concentration distribution function fails to do so, as would the statistical models for multiple ligand binding. Moreover, the position of this minimum is predicted to change with concentration.

Kinetics↗
PubMed1984

A micelle model for the sedimentation behavior of bovine beta-casein.

The monomer-single polymer model of G.A. Gilbert (Disc. Faraday Soc. 20 (1955) 68) for moving boundary sedimentation has been used by Payens and colleagues to explain the observed results for bovine caseins, and by Harrington and colleagues to explain the observed results for myosin fibrils. Electron microscope pictures of Buchheim and Schmidt have subsequently revealed micellar beta-casein in the form of slightly elongated or spherical particles having a bimodal size distribution, but with a broad range of particle sizes, at concentrations not too far above the critical micelle concentration. The equilibrium properties of a broadly distributed micellar system can be fitted by the shell model developed by one of us, and in the present article, the shell model is extended to predict the moving boundary sedimentation behavior of such a system. The observed sedimentation patterns, as well as the critical concentration predictions of the monomer-single polymer Gilbert sedimentation model, are satisfactorily described with the present model, based on a continuous distribution of intermediates between monomers and the largest possible spherical micelles. For one example considered, the predicted frequency distribution of molecular weight is in qualitative agreement with the frequency distribution of particle volume found by Buchheim and Schmidt.

Animals↗
PubMed1984

Pressure-jump kinetics of bovine beta-casein micellization.

Two samples of highly purified bovine beta-casein supplied to us by Dr. T.A.J. Payens of the National Institute for Dairy Research, Ede, The Netherlands, were studied over a range of concentration from just below the critical micelle concentration (CMC) to 0.46%, in 0.2 ionic strength phosphate buffer (pH 7.0), at 20 and 25 degrees C. The relaxation process studied by pressure jump using a 90 degree scattered light detector was also confirmed by the temperature-jump method. In the pressure-jump experiments, the process could be separated into two general time domains, with an approximate ratio of 10-25:1, a behavior reminiscent of that found for synthetic micellar systems. The faster relaxation process was still exhibited below the CMC, however. The concentration dependence of the faster relaxation time agreed very satisfactorily with predictions from the micelle model described in the companion paper.

Animals↗
PubMed1984

Perturbation of ribosome subunit interaction by glutaraldehyde fixation.

Ultracentrifugal analysis of ribosomal purity is complicated by the rapid reequilibration of ribosomes with their subunits, and this is further enhanced by the effects of hydrostatic pressure. Fixation of the ribosome system prior to ultracentrifugal analysis supposedly freezes the reequilibration, and thus tends to obviate these difficulties. However, no redistribution of the ribosome-subunit population must be allowed to occur during fixation. Thus, it is necessary that fixation be extremely rapid compared to the ribosome-subunit reequilibration, in order to avoid errors in analysis. It was the purpose of this investigation to make a direct experimental comparison of the rates of these two processes, fixation and ribosome-subunit reequilibration, using the stopped-flow technique with a light-scattering detector, under a variety of buffer environment readjustments. The following findings resulted from this study: (1) Fixation in Tris buffers could not be followed by light scattering because of interaction of glutaraldehyde with Tris, leading to continuous production of high molecular weight contaminants. (2) Polymerization of glutaraldehyde leads to increased ultraviolet absorption, which must not be confused with scattering changes. (3) Undialyzed ribosome solutions prepared by dissolving stock suspensions stored at high levels of magnesium and univalent electrolyte into a known standard buffer produced solutions having free Mg2+ levels lower than those of original buffer, complicating kinetic observations and threatening ribosome stability. (4) Addition of malonic acid as a buffer for Mg2+ largely eliminated this problem. (5) The kinetics of glutaraldehyde were measured quantitatively by studying the perturbation of ribosome association and dissociation kinetics produced during shifts of Mg2+ levels. (6) At fixative levels not producing coagulation, fixation kinetics can be competitive with those of association-dissociation. (7) Mass action effects like those of dilution can be caused by fixation, and can give rise to excess subunits, which may be mistaken for loose couples in original ribosome preparations.

Aldehydes↗
PubMed1984

Models for the analytical ultracentrifuge behavior of Helix pomatia alpha-hemocyanin. I. Experimental testing of previous models.

The 'microheterogeneity model' (R.J. Siezen and R. van Driel, Biochim. Biophys. Acta 295 (1973) 131) and the 'incompetent whole molecule model' (G. Kegeles, Arch. Biochem. Biophys. 180 (1977) 530) for the dissociation of Helix pomatia alpha-hemocyanin whole molecules to half molecules were tested experimentally, using ultracentrifugation and stopped-flow dilution analysis. Results of differential sedimentation experiments followed by stopped flow analysis of separated fractions of 60 S and 100 S molecules were not entirely as predicted by the incompetent model, the agreement depending on the pH and ionic strength of analysis. A considerable amount of stopped-flow dilution response could be attributed to material sedimenting between 60 and 100 S. This material appears to be the main equilibrating fraction, and its amount is considerably larger than that predicted by the microheterogeneity model. Increased hydrostatic pressure was found to enhance this fraction, whereas fixation or low ionic strength reduced or eliminated this fraction. Nonequilibrium components of 30, 50 and 80 S were detected and partially purified by differential sedimentation.

Journal Article↗
PubMed1979

Relaxation kinetics of E. coli ribosomes: evidence for the reaction of 30S . IF3 complex with 50S ribosomal subunits.

Addition of initiation factor IF3 to solutions of E. coli ribosomes dramatically alters their behavior in pressure-jump relaxation kinetic experiments in which 90 degrees light-scattering is used to monitor the macromolecular reaction. The effect of IF3 on relaxation processes attributed to "tight" couples is strongly dependent on the Mg2+ concentration. At 2.5 mM Mg2+, addition of 1 molar equivalent of IF3 decreases the relaxation amplitude by a factor of 3 relative to ribosome solutions without IF3. However, at 5.0 mM Mg2+, addition of 1 molar equivalent of IF3 produces a marked increase in the relaxation amplitude, by a factor of 2-8 fold relative to ribosomes in the absence of IF3. IF3 has no effect on the relaxation process attributed to "loose" couples at 10 mM Mg2+. While we are unable to propose a precise mechanism for IF3 action with the data on hand, our results require that the 30S . IF3 complex either reacts with the 50S subunit, forming a 70S . IF3 intermediate, or acts as a pool of reactive 30S subunit. Further kinetic evidence is required to distinguish between these possible pathways.

Escherichia coli↗
PubMed1976

Volume of reaction by the Archibald ultracentrifuge method (lobster hemocyanin).

Samples of lobster hemocyanin (Homarus americanus) under conditions of reversible reaction between whole (25 S) and half (17 S) molecules have been subjected to accurately known nitrogen pressures in analytical ultracentrifuge cells. A modified pressurization chamber of the type developed by Schumaker and colleagues has been constructed for this purpose. The molecular weight was then determined at the top (liquid-gas) meniscus, by means of the Archibald method. The logarithmic dependence upon pressure of the derived equilibrium constant then gave directly the volume of reaction. Experiments were performed in veronal-citrate buffers at pH 8, where the molar volume of formation of whole (dodecameric) molecules from half molecules appears to be negative, and at pH 8.46 in veronal-citrate buffer in the presence of 0.003 molar free calcium ion, where the molar volume of formation was estimated to be + 390 cm3/mole. In glycine-sodium hydroxide buffer at pH 9.6 containing 0.0047 molar free calcium, the molar volume of formation of whole molecules was estimated to be +120 +/- 70 cm3, corresponding to an estimated difference in partial specific volume between whole molecules and half molecules of only 1.3 (10)-4cm3/gram. The correctness of the sign of this value in glycine buffer has been verified by pressure-jump light-scattering experiments.

Animals↗
PubMed1975

Mechanism of the hexamer-dodecamer reaction of lobster hemocyanin.

The overall forward and reverse rate constants for the hexamer-dodecamer reaction of lobster hemocyanin have been determined in 0.1 ionic strength glycine buffers at pH 9.6, at free calcium ion levels from 0.0031 to 0.0053 molar, at 25 degrees C. Concentration-jump relaxation experiments in a stopped-flow apparatus were monitored by light scattered at 90 degrees. The reaction is pseudobimolecular, and the overall forward rate constant bears virtually all of the calcium ion concentration-dependence, while the overall reverse rate constant is truly unimolecular. Four calcium ions appear to participate in the reaction between two hexameric molecules, and appear to become an integral part of the structure of the dodecameric molecule under these conditions.

Animals↗
PubMed1975

Relaxation kinetics of E coli ribosomes: evidence for the reaction of 30S . IF3 complex with 50S ribosomal subunits.

Addition of initiation factor IF3 to solutions of E.coli ribosomes dramatically alters their behavior in pressure-jump relaxation kinetic experiments in which 90 degrees light-scattering is used to monitor the macromolecular reaction. The effect of IF3 on relaxation processes attributed to "tight" couples is strongly dependent on the Mg2+ concentration. At 2.5 mM Mg2+, addition of 1 molar equivalent of IF3 decreases the relaxation amplitude by a factor of 3 relative to ribosome solutions without IF3. However, at 5.0 mM Mg2+, addition of 1 molar equivalent of IF3 produces a marked increase in the relaxation amplitude, by a factor of 2-8 fold relative to ribosomes in the absence of IF3. IF3 has no effect on the relaxation process attributed to "loose" couples at 10 mM Mg2+. While we are unable to propose a precise mechanism for IF3 action with the data on hand, our results require that the 30S . IF3 complex either reacts with the 50S subunit, forming a 70S . IF3 intermediate, or acts as a pool of reactive 30S subunit. Further kinetic evidence is required to distinguish between these possible pathways.

Journal Article↗