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Numerical study of a binary Yukawa model in regimes characteristic of globular proteins in solutions

Achille Giacometti and Domenico Gazzillo

Giorgio Pastore

Tushar Kanti Das*

  • Istituto Nazionale per la Fisica della Materia and Dipartimento di Chimica Fisica, Università di Venezia, S. Marta DD 2137, I-30123 Venezia, Italy

  • Dipartimento di Fisica Teorica, Università di Trieste and INFM-DEMOCRITOS, National Simulation Center, Strada Costiera 11, Miramare, I-34100 Trieste, Italy

  • ICTP, Diploma Course, Strada Costiera 11, Miramare P.O Box 586, I-34100 Trieste, Italy

  • *Present address: Department of Physics, Shahjalal University of Science & Technology, Sylhet-3100, Bangladesh.
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Phys. Rev. E 71, 031108 – Published 21 March, 2005

DOI: https://doi.org/10.1103/PhysRevE.71.031108

Abstract

The main goal of this paper is to assess the limits of validity, in the regime of low concentration and strong Coulomb coupling (high molecular charges), of a simple perturbative approximation to the radial distribution functions (RDF’s), based upon a low-density expansion of the potential of mean force and proposed to describe protein-protein interactions in a recent small-angle-scattering (SAS) experimental study. A highly simplified Yukawa (screened Coulomb) model of monomers and dimers of a charged globular protein (β-lactoglobulin) in solution is considered. We test the accuracy of the RDF approximation, as a necessary complementary part of the previous experimental investigation, by comparison with the fluid structure predicted by approximate integral equations and exact Monte Carlo (MC) simulations. In the MC calculations, an Ewald construction for Yukawa potentials has been used to take into account the long-range part of the interactions in the weakly screened cases. Our results confirm that the perturbative first-order approximation is valid for this system even at strong Coulomb coupling, provided that the screening is not too weak (i.e., for Debye length smaller than monomer radius). A comparison of the MC results with integral equation calculations shows that both the hypernetted-chain (HNC) and Percus-Yevick closures have a satisfactory behavior under these regimes, with the HNC being superior throughout. The relevance of our findings for interpreting SAS results is also discussed.

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Phys. Rev. E 71, 031108– Published 21 March, 2005
  • Received 28 June 2004

© 2005 American Physical Society

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