The outer membrane of Gram-negative bacteria contains aqueous channels, porins, which aid the diffusion of small hydrophilic molecules across it. Escherichia coli, as enteric bacteria, are able to survive a hostile environment of proteases, surfactants, and drastic changes of osmotic pressure. Rhodobacter capsulatus is not an enteric bacterium and as such has not evolved to resist the same challenges. Porins, which have molecular weight of approximately 35 kDa, form trimeric channels with a solute exclusion limit of about 600 Da. Most of them open and close in a controlled manner as a function of p.d. This function is little understood at present. The functional properties of single trimers of the major porin of Rhodobacter capsulatus 37b4 have been investigated in planar artificial bilayers. On application of a suitable p.d. the observed trimer closes in approximately three equal steps. The behaviour is completely symmetrical as regards closure in response to p.d.'s of opposite polarity and is strongly cation selective.
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