Thermal stability of antithrombin Wibble (dashed line) and a normal control (continuous line) measured by changes in circular dichroic ellipticity at 222 nm. Addition of the high-affinity heparin pentasaccharide remarkably stabilizes both antithrombins (curves on right), with the even greater stabilization and sharper melting transition of antithrombin Wibble, probably reflecting its increased heparin affinity. The stable horizontal dashed line is latent antithrombin Wibble from peak II in Fig 2.