1,289 research outputs found
Mean-Field-Theory for Polymers in Mixed Solvents Thermodynamic and Structural Properties
Theoretical aspects of polymers in mixed solvents are considered using the
Edwards Hamiltonian formalism. Thermodynamic and structural properties are
investigated and some predictions are made when the mixed solvent approaches
criticality. Both the single and the many chain problems are examined. When the
pure mixed solvent is near criticality, addition of a small amount of polymers
shifts the criticality towards either enhanced compatibility or induced phase
separation depending upon the value of the parameter describing the interaction
asymmetry of the solvents with respect to the polymer. The polymer-solvent
effective interaction parameter increases strongly when the solvent mixture
approaches criticality. Accordingly, the apparent excluded volume parameter
decreases and may vanish or even become negative. Consequently, the polymer
undergoes a phase transition from a swollen state to an unperturbed state or
even take a collapsed configuration. The effective potential acting on a test
chain in strong solutions is calculated and the concept of Edwards screening
discussed. Structural properties of ternary mixtures of polymers in mixed
solvents are investigated within the Edwards Hamiltonian model. It is shown
that the effective potential on a test chain in strong solutions could be
written as an infinite series expansion of terms describing interactions via
one chain, two chains etc. This summation can be performed following a similar
scheme as in the Ornstein-Zernike series expansion.Comment: accepted in Macromol. Theory Simu
Plasmodium falciparum glyoxalase II: Theorell-Chance product inhibition patterns, rate-limiting substrate binding via Arg(257)/Lys(260), and unmasking of acid-base catalysis
Glyoxalase II (GloII) is a ubiquitous thioester hydrolase catalyzing the last step of the glutathione-dependent conversion of 2-oxoaldehydes to 2-hydroxycarboxylic acids. Here, we present a detailed structure-function analysis of cGloII from the malaria parasite Plasmodium falciparum. The activity of the enzyme was salt-sensitive and pH-log k(cat) and pH-log k(cat)/K-m profiles revealed acid-base catalysis. An acidic pK(a)(app) value of approximately 6 probably reflects hydroxide formation at the metal center. The glutathione-binding site was analyzed by site-directed mutagenesis. Substitution of residue Arg(154) caused a 2.5-fold increase of K-m(app), whereas replacements of Arg(257) or Lys(260) were far more detrimental. Although the glutathione-binding site and the catalytic center are separated, six of six single mutations at the substrate-binding site decreased the k(cat)(app) value. Furthermore, product inhibition studies support a Theorell-Chance Bi Bi mechanism with glutathione as the second product. We conclude that the substrate is predominantly bound via ionic interactions with the conserved residues Arg(257) and Lys(260), and that correct substrate binding is a pH-and salt-dependent rate-limiting step for catalysis. The presented mechanistic model is presumably also valid for GloII from many other organisms. Our study could be valuable for drug development strategies and enhances the understanding of the chemistry of binuclear metallohydrolases
Laparoscopic Cholecystectomy: Some Advantages or Just an Artifice of New Technology?
Twelve selected patients undergoing cholecystectomy were operated in a prospective randomised study
by laparoscopy (CO2 laser) or by classic surgery. Our preliminary results suggest that laparoscopic
cholecystectomy is of clinical benefit as compared to classic cholecystectomy since it reduces the surgical
trauma, limiting weight loss and shortening the hospital stay
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