Electrostatic potential of the acetylcholine binding sites in the nicotinic receptor probed by reactions of binding-site cysteines with charged methanethiosulfonates.

All of the potent agonists and competitive antagonists of the acetylcholine receptors are positively charged, onium compounds. Among the interactions involved in the binding of these compounds, electrostatic forces undoubtedly make an important contribution. There is evidence that the acetylcholine binding site contains both acidic and aromatic amino acids. The acidic side chains could provide long-range charge-charge interactions with acetylcholine, while the aromatic side chains could provide short-range cation-pi-electron and hydrophobic interactions. To probe the long-range electrostatic interactions in the binding site, the rate constants for the reactions of sulfhydryl-specific reagents with cysteines in the binding site have been determined as a function of ionic strength. The reagents are the positively charged methanethiosulfonate ethylammonium and methanethiosulfonate ethyltrimethylammonium, the negatively charged methanethiosulfonate ethylsulfonate, and the neutral methyl methanethiosulfonate. In addition, the rate constants of the reactions of these methanethiosulfonates with positively charged, negatively charged, and uncharged simple thiol compounds have been similarly determined. An analysis of these rate constants in terms of absolute rate theory and Debye-Hückel theory is consistent with the acetylcholine binding site containing two to three negative charges and an electrostatic potential at zero ionic strength of about -80 mV relative to bulk solution.