Nonideal Associating Systems
Leslie A. Holladay, Alkis J. Sophianopoulos · Journal of Biological Chemistry · 1972
Abstract The study describes an algorithm and specifies conditions for obtaining data by sedimentation equilibrium to study nonideal associating systems. The algorithm can analyze systems of up to four macromolecular components, and it returns values of their concentrations plus a single nonideality coefficient B. By appropriate mathematical manipulations the algorithm is made to solve by least squares for linear coefficients, rather than attempting the more cumbersome iterative fitting of nonlinear regression. Synthetic data with realistic errors, both random and systematic, were used to test the reliability of the answers obtained and to define the optimum experimental conditions which can yield data capable of analysis. Equations are derived which permit the simultaneous solution of data from two ultracentrifuge cells and thus extend the concentration range studied. Extension of the concentration range is essential for all systems with more than two macromolecular components. Synthetic systems of widely varying association constants, number of species, and degree of nonideality were analyzed successfully. The dissociation constants varied from 1.0 x 10-3 m to 3.74 x 10-6 m; the second virial coefficient was varied from 0 to 3.0 x 10-5 mole dl g-2. Statistical tests for constructing confidence regions are used in determining the reliability of the parameters obtained.