Lorentzian noise in single gramicidin A channel formamidinium currents

Seoh & Busath (1995) showed that in the presence of formamidinium, single gramicidin A channels were lengthened, had uniformly noisy currents at low voltages and had superlinear current-voltage relationships, all three properties being absent in gramicidin M- channels in which the interfacial tryptophan residues in gramicidin A are all replaced by phenylalanine. We measured the single channel noise power spectra (PSDs) in small monoolein (GMO) bilayers with formamidinium chloride solutions to help identify the mechanism of noise process. PSDs were Lorentzian with characteristic frequencies of 0.1-1.0 kHz in 0.1 and 0.3 M formamidinium chloride solutions, and from. 1-6 kHz in 1 M solution. Si(0), where measurable, ranged from approximately 50-200 fA2/Hz. The time course of the noise process could not be detected in these experiments. The low fc suggests slow motions or rare states of the blocking 'gates' which, judging from the result with gramicidin M-, must be equal to or related to the Trp residues.