Temperature coefficients of affinity and entropies of adsorption from enantiomeric pairs of compounds acting at muscarinic receptors in the guinea-pig ileum.

1 The affinities of a series of enantiomeric pairs of esters of phenylcyclohexylglycollic acid, of a pair of esters of alpha-methyltropic acid and of hyoscyamine methiodide have been measured for the muscarinic receptors of the guinea-pig ileum at 30 degrees and 37 degrees C and estimates have been made of their free energies, enthalpies and entropies of adsorption. 2 With (-)-S-hyoscyamine methiodide the enthalpy of adsorption is negative whereas with the (+)-R-enantiomer it is positive. 3 With the esters of phenylcyclohexylglycollic acid the size of the onium group appears to be as important as the stereochemical configuration in determining entropy. Large onium groups appear to be associated with an increase in entropy though this can only be measured approximately because of the narrow range of temperature which can be used.