Nuclear magnetic resonance diffusion and relaxation time study of acetylcholine

Abstract

The dynamic rotational and translational processes associated with the acetylcholine ion are of interest in view of its role as a neurochemical transmitter; the diffusion process is of particular interest as this is involved in the movement of the ion across the synaptic cleft.

Proton n.m.r. measurements of diffusion and relaxation together with high resolution studies have been carried out on solutions of acetylcholine chloride in D₂O. Diffusion data cover the range 20–80°C while the relaxation data cover the range –60 to +90°C. The diffusion coefficient of acetylcholine at 20°C is 12.2 × 10^–10 m² s^–1 and the activation energy of the diffusion process 23 kJ mol^–1. The relaxation data give activation energies of 47 kJ mol^–1 and 13 kJ mol^–1 below and above room temperature respectively. The 47 kJ mol^–1 activation energy is thought to be due to rotation about the partial double bond between the ester oxygen and carboxyl carbon of the acetylcholine molecule. The 13 kJ mol^–1 activation energy is likely to be due to rotation of the acetylcholine molecule as a whole.