Figure 6.

Oligomerization of DsRed as shown by analytical ultracentrifugation. (A) Equilibrium radial absorbance profile (circles) measured at 20,000 rpm, overlaid by theoretical curve fits for species ranging from monomer to pentamer. The curves were globally fitted to data at 10,000, 14,000, and 20,000 rpm, although only the latter data are presented here. (B) Residual errors from the theoretical fits, illustrating that a tetramer (gray solid line) fits the experimental data better than monomer, dimer, trimer, or pentamer. (C) Residual errors on an expanded scale comparing simple tetramer (gray solid line, same values as in B) with a model allowing the tetramer to dimerize to an octamer (black solid line) with a K_d of 39 μM, i.e., [octamer] = [tetramer]²/(39 μM). The variance for the tetramer + octamer model was 1.18 × 10⁻⁵, somewhat better than that for the tetramer alone, 1.95 × 10⁻⁵.