AN IMPROVED APPROXIMATE SOLUTION OF THE LAMM EQUATION FOR THE SIMULTANEOUS ESTIMATION OF SEDIMENTATION AND DIFFUSION-COEFFICIENTS FROM SEDIMENTATION-VELOCITY EXPERIMENTS

Sedimentation and diffusion coefficients are important parameters to d escribe size and shape of macromolecules in solution. The data can be obtained from sedimentation velocity experiments by a nonlinear fittin g procedure using approximate solutions for the Lamm equation. Here, w e present a modification of such a model function that was originally proposed by Fujita [H. Fujita, Mathematical Theory of Sedimentation An alysis, Wiley, New York, 1962]. The extended model function is well su itable to study low molecular mass compounds. The improvement of this solution given here is based on using an adjustable value for the expl icit integration variable, z, the reduced radius. This modification le ads to more accurate sedimentation and diffusion coefficients compared to using a constant value of 0.5 as used by Fujita. The advantage of our modification was demonstrated by the analysis of noise-free curves calculated using the finite element method, as well as experimental c urves obtained for the peptides angiotensin I and II. The relatively l ow sedimentation and diffusion coefficients found for both substances indicate that the peptides exist as extended chains of about 3.65 nm ( angiotensin I) or 3.04 nm length (angiotensin II) in solution. The lac k of higher-order structure of the peptides that was derived also from CD spectra might facilitate receptor binding, and could be one reason for the fast proteolytic digestion of the free peptides. (C) 1998 Els evier Science B.V.