10189
J. Phys. Chem. 1994, 98, 10189-10199
Solute Interactions with Immobilized Artificial Membranes Shaowei Ong, Xiaoxing Qiu, and Charles Pidgeon' Department of Medicinal Chemistry, School of Pharmacy, Purdue University, West Lafayette, Indiana 47907 Received: March 2, 1994; In Final Form: June 2, 1994@
Solute binding to immobilized aritifical membrane (IAM) chromatography surfaces was studied. The IAM and e*erIAM.PGC10/C3.The solutes studied were tryptophan (Trp) and surfaces studied were e*erIAM.PCC'o/C3 glycocholic acid (GA). Equilibrium binding constants ( K ) and the free energy of adsorption (AG) were obtained from fitting experimental equilibrium binding data to Langmuir adsorption isotherms. Trp adsorbs to both e*erIAM.PCC10/C3 and e*erIAM.PGC'0/C3surfaces at both acidic (pH 3.4) and neutral pH (pH 7.0). Surface-associated Trp did not change the 31PNMR line shape of e*erIAM.PGC10'C3at either pH 3.4 or 7.0; however, the 31PNMR line shape of e*erIAM.PCc10/c3 was broadened from surface-associated Trp. The line broadening was directly proportional to the amount of surface-associated Trp. Spectral simulations of the 31PNMR line shape indicated that Trp,bound to the interfacial region of the e*erIAM.PCc10/c3surface, slowed the wobbling motion of the phospholipid head group. The correlation time (ZL) for the lipid head group wobbling motion increased linearly with the amount of surface-associated Trp. GA strongly adsorbed to (K 7.06 x lo4) and e*erIAM.PCC10/C3(K 3.44 x lo4) at acidic conditions (pH both e*erIAM.PGC10/C3 3.4). However, in contrast to Trp, surface-associated GA did not change the 31P NMR line shape of etherIAM,PCC10/C3 but broadened the 31PNMR line shape of e*erIAM.PGC10/C3. The extent of line broadening and ZL of immobilized PG increased exponentially with the amount of surface-associated GA. In addition, 31PNMR line shape analysis and T2 measurements indicated that for Trp the adsorption rate (on) and desorption rate (off) of the e*erIAM.PCC'o/C3 surface was faster than the exchange rates measurable by the NMR time surface were slower scale (i.e., odoff rates > lo3 s-l). For GA, the odoff rates from the e*erIAM.PGC'o/C3 than the NMR time scale (i.e., odoff rates
