As we were interested in the changes of the lipid bilayer structure upon peptide insertion we had to exclude influences on the ordering and packing of lipids exerted by other parameters. These are, on the one hand, the constitution of the lipid species in terms of chain length and saturation (Seelig and Seelig 1977) , chain branching (Perly et al. 1985) , and headgroup structure (Cullis et al. 1986) , and on the other hand, the lipid phase state (Lafleur et al. 1990b; Perly et al. 1985; Sternin et al. 1988 ) and the cholesterol content (Dufourc et al. 1980; Oldfield et al. 1978; Stockton and Smith 1976) . The use of only one lipid component with a fixed cholesterol content eliminates the mentioned impacts. The uniformity of physical parameters, namely the temperature (Davis et al. 1980 ) and the relative humidity, was assured by the controlled sample environment.