STEADY-STATE NONMONOTONIC CONCENTRATION PROFILES IN THE UNSTIRRED LAYERS OF BILAYER-LIPID MEMBRANES

Catalytic reactions in the unstirred layers near bilayer lipid membran es can induce nonmonotonic concentration profiles near the membrane su rface. In the case of transmembrane diffusion of a substrate immediate ly followed by its conversion due to the presence of an aqueous solubl e enzyme the size of the unstirred layer defined in terms of the conce ntration gradient at the membrane surface does not correspond to the w idth of the aqueous layer adjacent to the membrane where the concentra tion differs from the hulk phase concentration. Deducing of flux value s or convection parameters from the concentration gradient at zero dis tance from the membrane gives misleading results. An empirical equatio n for the estimation of the size of the concentration boundary layer i s proposed. It was derived from pH profiles registered with the help o f a microelectrode near a planar bilayer lipid membrane surrounded by a buffer solution containing at one side of the membrane acetaldehyde and sodium acetate and at the other side alcohol dehydrogenase. Since this parameter equals to the thickness of the unstirred layer in the c ase of exponential concentration profiles it may be applied to estimat e both mass transfer restrictions and kinetic of diffusion limited rea ctions occurring in the immediate membrane vicinity regardless the com plexity of the system under investigation.