Anion transport properties of amine and amide-sidechained peptides are affected by charge and phospholipid composition

Abstract

Four synthetic anion transporters (SATs) having the general formula (n-C₁₈H₃₇)₂N-COCH₂OCH₂CO-(Gly)₃Pro-Lys(ε-N-R)-(Gly)₂-O-n-C₇H₁₅ were prepared and studied. The group R was Cbz, H (TFA salt), t-Boc, and dansyl in peptides 1, 2, 3, and 4 respectively. The glutamine analog (GGGPQAG sequence) was also included. A dansyl-substituted fluorescent SAT was used to probe peptide insertion; the dansyl sidechain resides in an environment near the bilayer's midpolar regime. When the lysine sidechain was free or protected amine, little effect was noted on final Cl⁻ transport rate in DOPC : DOPA (7 : 3) liposomes. This stands in contrast to the significant retardation of transport previously observed when a negative glutamate residue was present in the peptide sequence. It was also found that Cl⁻ release from liposomes depended on the phospholipid composition of the vesicles. Chloride transport diminished significantly for the free lysine containing SAT, 2, when the lipid was altered from DOPC : DOPA to pure DOPC. Amide-sidechained SATs 1 and 5 showed a relatively small decrease in Cl⁻ transport. The effect of lipid composition on Cl⁻ transport was explained by differences in electrostatic interaction between amino acid sidechain and lipid headgroup, which was modeled by computation.