The SV experiment was carried out with a Beckman-Coulter XL-I analytical ultracentrifuge, following standard protocols as described by Brown et al. (2008a, b) . A monoclonal immunoglobulin G (IgG) preparation in phosphate-buffered saline (PBS) buffer was inserted in 12mm Epon centerpieces, temperature equilibrated at 18°C, and then accelerated to 45,000 rpm and scanned with absorbance optics at 280 nm. Data analysis was performed with SEDFIT 11.8 using c(s) models as described by Schuck et al. (2002) , the two-dimensional size-and-shape model c(s, f r ) as described by Brown and Schuck (2006) , and applying Bayesian prior knowledge as described in detail by Brown et al. (2007) . The computer used for these analyses was a Dell Precision T5400 workstation, with dual 32-bit quadcore 3.16-MHz processors and Windows operating system. 1