The effect of S-layer protein adsorption and crystallization on the collective motion of a planar lipid bilayer studied by dynamic light scattering

A dedicated dynamic light scattering (DLS) setup was employed to study the undulations of freely suspended planar lipid bilayers, the so-called black lipid membranes (BLM), over a previously inaccessible spread of frequencies (relaxation times ranging from 10(-2) to 10(-6) s) and wavevectors (250 cm (-1) < q < 38,000 cm(-1)). For a BLM consisting of 1,2-dielaidoyl-sn-3-glyc ero-phosphocholine (DEPC) doped with two different proportions of the catio nic lipid analog dioctadecyl-dimethylammonium bromide (DODAB) we observed a n increase of the lateral tension of the membrane with the DODAB concentrat ion. The experimentally determined dispersion behavior of the transverse sh ear mode was in excellent agreement with the theoretical predictions of a f irst-order hydrodynamic theory. The symmetric adsorption of the crystalline bacterial cell surface layer (S-layer) proteins from Bacillus coagulans E3 8-66 to a weakly cationic BLM (1.5 mot % DODAB) causes a drastic reduction of the membrane tension well beyond the previous DODAB-induced tension incr ease. The likely reason for this behavior is an increase of molecular order along the lipid chains by the protein and/or partial protein penetration i nto the lipid headgroup region. S-layer protein adsorption to a highly cati onic BLM (14 mol % DODAB) shows after 7 h incubation time an even stronger decrease of the membrane tension by a factor of five, but additionally a si gnificant increase of the (previously negligible) surface viscosity, again in excellent agreement with the hydrodynamic theory. Further incubation (24 h) shows a drastic increase of the membrane bending energy by three orders of magnitude as a result of a large-scale, two-dimensional recrystallizati on of the S-layer proteins at both sides of the BLM. The results demonstrat e the potential of the method for the assessment of the different stages of protein adsorption and recrystallization at a membrane surface by measurem ents of the collective membrane modes and their analysis in terms of a hydr odynamic theory.