Measurement of Acetylcholine Levels by pH Changes The concentration of acetylcholine (a neurotransmitter) in a sample can be determined from the pH changes that accompany its hydrolysis. When the sample is incubated with the enzyme acetylcholinesterase, acetylcholine is quantitatively converted into choline and acetic acid, which dissociates to yield acetate and a hydrogen ion: 0 CH HO CH3 CH-C-0-CH-CH,-N-CH. HO-CH,-CH,-N-CH, + CH-C0 + H || Acetate CH Choline CH3 Acetylcholine In a typical analysis, 15 mL of an aqueous solution containing an unknown amount of acetylcholine had a pH of 7.65. When incubated with acetylcholinesterase, the pH of the solution decreased to 6.87. As- suming that there was no buffer in the assay mixture, determine the number of moles of acetylcholine in the 15 mL sample.