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(I II) 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 an d studied as effective ditopic transport carriers of hydrophobic amino acid s and neurotransmitters through CHCl3 bulk liquid membranes (BLMs). A detai led analysis of the BLM transport of tryptophan and serotonin with these co mplexes, along with various derivatives, shows the ditopic nature of the me chanism of transport and reveals the importance of the role of both the cro wn ether function and the metal center. Kinetic data on the BLM transport o f tryptophan follow Michaelis-Menten behavior with K-m= 25 mM and V-m = 6.7 5 x 10(-3)h(-1). Variable temperature studies on the rate of transport of t ryptophan land serotonin) were conducted. From Arrhenius plots, the activat ion 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 calcu lated 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 wi th serotonin are reported.