Experiments on electron transfer at the membrane-solution interface

Abstract

The objective of the present work is to establish that electron transfer occurs at a biosurface in contact with an ionic solution. For this purpose, electrical contact between evaporated films of metals and metal oxides and the membrane (dipalmitoylphosphatidylcholine alone or containing gramicidin) was made by use of a Langmuir-Blodgett trough technique. Three or five layers of lipid were thus formed on the metal substrate (gold, platinum, or tin dioxide); electronic connection was made to the latter but contact to the solution was via the membrane. A solution containing redox ions (p-benzoquinone-hydroquinone system) was used to make an interface with the membrane.

Experiments were particularly designed to guard against the possibility that electrons would reach the quinone via unintended contact of the solution with the underlying metal via pinholes in the membrane, and in diffusion through the membrane. Thus, experiments were made with the biolipid alone, and then with the biolipid-gramicide mixture.

Behavior of these two systems differed radically in that the biolipid alone showed time dependence in its behavior and after a few minutes behaved as though it presented only a permeable barrier between the ionic solution and the underlying metal (so that electron transfer occurred to the ions in solution from the metal or metal oxide). However, the biolipid-gramicidin behaved radically differently in that electrical currents measured in its presence did not change with time. Underpotential deposition on the biolipid-gramicidin differed from that on the biolipid alone; the latter showed anodic stripping peaks equal to those of the metal substrate, but on the former, the peaks were decisively shifted.

Tafel lines for Q+2H⁺+2e→QH₂ in the presence of the biolipid membrane at least a few minutes old were essentially the same as on the Au, Pt, and SnO₂, respectively, underlying the membrane (hence it is permeable to ions from solution). However, measured Tafel lines for the same reaction and electrode system, except for the fact that gramicidin was mixed with the biolipid, were independent of the age of the membrane and differed from those obtained on the metallic underlay. Hence, electron transfer to the quinone occurs at the gramicidin-solution interface.