A method for directly fitting the time derivative of sedimentation velocity data and an alternative algorithm for calculating sedimentation coefficient distribution functions

The time-derivative method for deriving the sedimentation coefficient distr ibution, g(s*), from sedimentation velocity data that was developed by Wait er Stafford has many advantages and is now widely used. By fitting Gaussian functions to the g(s*) distribution both sedimentation and diffusion coeff icients (and therefore molecular masses) for individual species can be obta ined. However, some of the approximations used in these procedures limit th e accuracy of the results. An alternative approach is proposed in which the dc/dt data are fitted rather than g(s*). This new approach gives improved accuracy, extends the range to sedimentation coefficients below 1 S, and en hances resolution of multiple species. For both approaches the peaks from i ndividual species are broadened when the data cover too wide a time span, a nd this effect is explored and quantified. An alternative algorithm far cal culating (g) over cap(s*) from the dc/dt curves is presented and discussed. Rather than first averaging the dc/dt data for individual scan pairs and t hen calculating (g) over cap(s*) from that average, the (g) over cap(s*) di stributions are calculated for every scan pair and then subsequently averag ed. This alternative procedure yields smaller error bars for g(s*) and some what greater accuracy for fitted hydrodynamic properties when the time span becomes large. (C) 2000 Academic Press.