Effect of surface charge on the interfacial orientation and conformation of FB1 in model membranes

The toxicity of FB1 is usually explained through the enzymatic disruption of lipidic metabolism. However, it may lie in the thermodynamics of the membrane and its cooperative phase behavior rather than in the activity of individual proteins. Here, we investigate the effects of FB1 at the molecular and mesoscopic levels in FB1-phospholipid mixed Langmuir films. Mean molecular area vs FB1 molar fractions (x(FB1)) and phase diagram analysis allowed us to define miscibility conditions and phase states at different x(FB1). Surface potential measurements, evaluated as a function of the molecular packing and x(FB1), revealed the FB1-induced change in the collective dipolar reorientation leaded to neutralization of charged films. Size, shape, and distribution of 2D-domain analysis from epifluorescence data suggested the increase in the mixing entropy and film relaxation rate. Finally, PM-IRRAS revealed the orientation of FB1 with the amine end (zwitterionic and negatively charged monolayers) or the tricarballylic acid end (positively charged monolayers) pointing to the air. The globular-extended conformational equilibrium of FB1 is dynamically defined by the membrane charge becoming a toxicity enhancing factor. The specificity of the toxin-protein interaction might also be perturbed by the FB1-induced remodeling of the membrane topography by affecting the raft-like platforms where membrane enzymes are considered to be located.