Teresa Griffin Fairbanks and Professor David D. Busath, Physics and Astronomy
Gramicidin A is a protein that forms cation selective ion channels in lipid bilayers (model cell membranes.) Although it is not found in natural systems, Gramicidin A is a good ion channel to study because its structure is known. It is generally thought that the passage of cations through the gramicidin channel does not cause the channel to move. Data for most cations seems to support this hypothesis. Unlike other cations, formamidinium, CH(NH2)2, causes a dramatic increase in current noise over baseline noise. This high noise level in the formamidinium channel suggests that some kind of channel movement may be taking place or some kind of blocking due to binding at the center of the channel such as that caused by guanidinium. A better understanding of this movement is found through analyzing how the noise changes with respect to applied voltage and with respect to formamidinium solution concentration.
Research was focused on the formamidinium-induced noise by using high frequency ffts (fast Fourier transforms) to analyze the current noise. Lorentzian fits were done on the average difference spectra (the channel power spectrum minus the baseline power spectrum) to find the characteristic frequency of the noise. These fits were done on a matrix of data made up of three different concentrations of formamidinium solution: 0. 1 molar, 0.3 molar, 1 molar, each at four different applied voltages: 50 mV, 100 mV, 150 mV, 200 mV.
Figure 1 shows the l molar difference spectra at the four voltage levels. Lorentzian fits for these graphs suggest that the characteristic frequency of the formamidinium-induced noise increases as the voltage increases. Subsequent analysis with higher resolution ffts and bigger ffts also seems to support this observation. Work is currently being done to determine the concentration dependence of the characteristic frequency.
The presence of a measurable characteristic frequency suggests that the formamidinium ions are inducing movement in the gramicidin channel. Because replacement of the tryptophans (residues at the ends of the gramicidin channel) in the channel eliminates the formamidinium-induced noise, we hypothesize that the passage of formamidinium ions causes the tryptophans to move, partially or entirely impeding the passage of formamidinium ions. Further analysis work which is now in process should enable us to better understand the cause of formamidinium-induced noise in gramicidin A channels.
References
- P. Lauger, Shot noise in ion channels, Biochimica et Biophysica Acta, 413 (1975): 1-1 0.
- Sang-Ah Seoh and David Busath, (Gramicidin Tryptophans Mediate Formamidinium-Induced Channel Stabilization, Biophysical Journal, 68 (June 1995):2272.
- Greg Hemsley and David Busath, Small iminium ions block gramicidin channels in lipid bilayers, Biophysical Journal, 59 (April 1991): 901-07.
- Teresa Fairbanks, Honors thesis, Brigham Young University, August 1996.