Interactions of proteins in aqueous electrolyte solutions from fluorescence anisotropy and circular-dichroism measurements

Understanding aqueous protein-protein interactions is crucial for the devel opment of a molecular-thermodynamic model for salt-induced protein precipit ation. In addition, protein interactions are important in many disease stat es, including cataract formation and a-amyloid diseases. Fluorescence aniso tropy provides a means to measure intermolecular interactions. In this work , monomer-dimer equilibrium of the peptide T4 LYS(11-36) was studied by flu orescence anisotropy over the pH range 4-7 and the NaCl concentration range 0.0-1.0 M, in a 25 mM sodium phosphate buffer. This 26 amino-acid peptide is derived from the P-sheet region of the T4 lysozyme molecule and has the potential to form amyloid fibrils. The association constant for dimerizatio n increases with rising pH and ionic strength. The potential of mean force for peptide-peptide interactions was calculated from these association cons tants. Circular-dichroism measurements show that the peptide becomes more s tructured as the pH rises, possibly contributing to increased association. (C) 2000 Elsevier Science B.V. All rights reserved.