Lipophilic metal-salicylideneimine-crown ether hybrids - Ditopic carriers in the facilitated transport of amphiphilic molecules across bulk liquid membranes

The crown ether-functionalized Mn-III salicylaldimine complexes manganese(III) 4,5-bis(3,5-di-tert-butylsalicylideneimine)benzo-18-crown-6-X [X = Cl-, I-, CH3COO-, (CH3)(3)CCOO-, B(Ph)(4)(-)] have recently been synthesized and studied as effective ditopic transport carriers of hydrophobic amino acids and neurotransmitters through CHCl3 bulk liquid membranes (BLMs). A detailed analysis of the BLM transport of tryptophan and serotonin with these complexes, along with various derivatives, shows the ditopic nature of the mechanism of transport and reveals the importance of the role of both the crown ether function and the metal center. Kinetic data on the BLM transport of tryptophan follow Michaelis-Menten behavior with K-m= 25 mM and V-m = 6.75 x 10(-3)h(-1). Variable temperature studies on the rate of transport of tryptophan land serotonin) were conducted. From Arrhenius plots, the activation energies, E-ar and pre-exponential factors, A, were determined to be 20 +/- 1 (17 +/- 5) kJ mol(-1) and 8200 (947) M(-1)h(-1), respectively. From these parameters, values for the thermodynamic activation energies, DeltaG(double dagger), DeltaH(double dagger), and DeltaS(double dagger) were calculated to be 26 +/- 4 (32 +/- 1) kJ mol(-1), 17 +/- 1 (14 +/- 5) kJ mol(-1), and -30 +/- 9 (-60 +/- 14) J K-1 mol(-1), respectively. Similar studies with serotonin are reported.