Amino Acids Change Solute Affinity for Lipid Bilayers.

Abstract

Time resolved fluorescence and differential scanning calorimetry were used to examine how two amino acids, L-Phenylalanine (L-PA) and N-Acetyl-DL-tryptophan (NAT), affect the temperature-dependent membrane affinity of two structurally similar coumarin solutes for DPPC vesicles. The 7-aminocoumarin solutes, Coumarin 151 (C151) and Coumarin 152 (C152), differ in their substitution at amine position - C151 is a primary amine and C152 is a tertiary amine - and both solutes show different tendencies to associate with lipid bilayers consistent with differences in their respective log P values. Adding L-PA to the DPPC vesicle solution did not change C151 s propensity to remain freely solvated in aqueous solution, but C152 showed a greater tendency to partition into the hydrophobic bilayer interior at temperatures below DPPC s gel-liquid crystalline transition temperature (Tgel-lc). This finding is consistent with L-PA s ability to enhance membrane permeability by disrupting chain-chain interactions. Adding NAT to DPPC vesicle-containing solutions changed C151 and C152 affinity for the DPPC membranes in unexpected ways. DSC data show that NAT interacts strongly with the lipid bilayer, lowering Tgel-lc by up to 2°C at concentrations of 10 mM. These effects disappear when either C151 or C152 is added to solution at concentrations below 10 μM, and Tgel-lc returns to a value consistent with unperturbed DPPC bilayers. Together with DSC results, fluorescence data imply that NAT promotes coumarin adsorption to the vesicle bilayer surface. NAT s effects diminish above Tgel-lc and imply that unlike L-PA, NAT does not penetrate into the bilayer but instead remains adsorbed to the bilayer s exterior. Taken in their entirety, these discoveries suggest that amino acids - and by inference, polypeptides and proteins - change solute affinity for lipid bilayers with specific effects that depend on individualized amino acid/lipid bilayer interactions.