Charge transfer complexes of indole:catechol type II

Charge transfer complexes between tryptamine:epinephrine, tryptamine:isoproterenol, tryptamine:catechol, and indole:catechol were demonstrated spectrophotometrically in aqueous solutions containing 0.1 M HCl. It was possible to make precise measurements and to calculate thermodynamic parameters and molar absorptivities. In general, the equilibrium constant was found to be quite low and ranged from 4.00 +/- 0.04 to 0.30 +/- 0.02 M-1. These interactions are exothermic and have relatively large standard enthalpy and entropy changes (delta Ho values ranged from -7.4 +/- 0.3 to -5.2 +/- 0.4 kcal mol-1; delta So ranged from -25 +/- 5.0 to -16 +/- 1.3 cal K-1 mol-1). The similarity in delta Ho and delta So values for all the combinations indicates a similar interacting system. This result has led us to propose the involvement of the charge transfer effect, induced in the indole:catechol system, in the mechanism occurring in nerve conductions.