FIGURE 4.

Circular dichroism of HET-C2. A, far-UV CD. The spectra indicate high helical content in HET-C2 with large negative n-π* transitions at 222 nm as well as π-π* transitions split into two transitions because of exciton coupling, resulting in negative bands at ∼208 nm and positive bands at ∼192 nm. Data are presented in units of molar ellipticity per residue. B, near-UV CD. Slightly altered spectral signals from Tyr/Trp are obtained by incubation of apoHET-C2 with membranes lacking glycolipid but not with membranes containing glycolipid, consistent with small conformational changes triggered by membrane interaction. C, derivative plots for temperature dependence of molar ellipticity at 222 nm. The relatively low midpoints of the unfolding transitions are consistent with a lack of conformational stabilization by intramolecular disulfides, as expected for a GLTP fold. ApoHET-C2 conformation is slightly destabilized by membrane interaction. Spectra were recorded at pH 7.4 as detailed under “Experimental Procedures.”