Figure 4. Analytical Ultracentrifugation.

a) Sedimentation equilibrium ultracentrifugation of HIV-1 gp41^27–194: Panels are absorbance (bottom panel) and residuals (upper panel). Opened circles show 280nm absorbance gradients in the centrifuge cell. The solid line indicates the calculated fit for the monomer-trimer association. Residuals show the difference in the fitted and experimental values as a function of radial position. The data shown refers to a ¹⁵N¹³C²H labelled protein with a calculated monomeric mass of 22,000 (sample used directly for NMR analyses). The concentration range of the gradient (open circles) at equilibrium is ~ 1.0 µM – 20 µM. b) The monomer – trimer potential of gp41^27–194 is indicated as mole fraction of monomer and trimer plotted as function of total protein concentration. Profiles were constructed using the K^a value indicated in (A). The insert is at a tenfold lower protein concentration and the indicated protein molarity corresponds to ~ 50% monomer and 50% trimer composition. The molecular weight of the non-labelled gp41^27–194 construct is 19.65 kDa (1mg/ml = 50.9µM) and the NMR measurements described in the study were performed at ~ 20mg/ml (1mM).