Formation of ionic channels in black lipid membranes by succinic derivatives of Gramicidin A

Abstract

Different succinyl derivatives of Gramicidin A were synthesized and their activity was investigated with different methods on lipid bilayer membranes. The succinyl derivatives of Gramicidin A can be classified as three different types, the O-succinyl derivative, the N-succinyl derivative and the N-O-succinyl derivative of Gramicidin A. An O-pyromellityl-N-succinyl gramicidin was synthesized which can be attributed to the latter class. It was found that O-succinyl gramicidin behaves like the unmodified Gramicidin A despite a charge effect on single-channel conductance, arising from the negative charge of the succinic residue, at the mouth of the channel. The activity of N-succinyl and N-O-succinyl gramicidin and of O-pyromellityl-N-succinyl-gramicidin depends strongly on the pH of the electrolyte solution. It is demonstrated that at low pH (≦5) the N-succinyl derivatives show high activity, whereas at high pH (≧7) the activity is sharply reduced or disappears totally. From these experiments it can be concluded that, for the formation of a dimeric gramicidin channel, the hydrogen of the formyl group can be replaced by a protonated carboxylic group of a succinic residue. Further results, obtained by measurement of the single-channel conductance and of the reaction rate constants for the channel formation, are discussed in terms of the structural basis of the single stranded model for the gramicidin channel. On this basis the double stranded helix can be, excluded and an interesting head-to-head single stranded β(π_L,D) helical channel is described which contains carboxyl groups at the head-to-head junction.