As the differences in the values for a and n when comparing the different peptide species cannot be solely attributed to a hydrophobic mismatch situation (see above), lateral peptide parameters such as the peptide cross-sectional area or the peptide/lipid contact surface are assumed to be the primary cause for the observed differences. In line with the lipid disordering scenario discussed above, larger peptide cross-sectional areas and peptide lipid contact surfaces would probably cause higher values for the lipid acyl chain distance a and smaller values for the lipid correlation length n. This is true if peptide/lipid interactions are neglected or a repulsive potential between lipids and peptides is assumed (Lagüe et al. 2001) . For the present sample sequence: homodimer (4) ? hairpin (5) ? recognition system (3/6), this would imply an increase in a and a decrease in n. The fact that the observed results point to the opposite may be taken as evidence for attractive peptide lipid interactions. Due to the sequence identity of the antiparallel TMD strands of all peptide species, and therefore, the constitutional similarity of the postulated attractive peptide/lipid interactions, only the peptide secondary structure, i.e., the lateral sizes of the peptides, and the size of the lipid/peptide interfacial plane should affect a and n. Therefore, a monotonous decrease in the lipid chain distance would be expected for samples 1-3, which is in line with the obtained results (Fig. 5b) . On the contrary, a monotonous increase in the chain correlation length was not observed. The value for n of the homodimeric species 4 (sample 2) appears higher than that for the hairpin 5 (sample 3). This might be explained by the fact that oligomer 4 is known to experience a monomer-dimer equilibrium (Küsel et al. 2007 ) including a structural change from a membrane-spanning b 5.6 helix to two reasonable broader and shorter b 6.3 single helices (Fig. 2) . This would actually change the peptide dimensions in the z direction as well as the molecular constitution of the helix surface and therefore cause different hydrophobic mismatch situations as well as changed lateral peptide/lipid interactions.