Lanthanide chelates as bilayer alignment tools in NMR studies of membrane-associated peptides

Theequimolar complex, consisting of the lipid-like, amphiphilic chelating agent 1,11-bis[distearylamino]-diethylenetriamine pentaacetic acid (DTPA-18) and Tm(3+), is shown by deuterium ((2)H) NMR to be useful in aligning bicelle-like model membranes, consisting of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC). As shown previously (1996, R. S. Prosser et al., J. Am. Chem. Soc. 118, 269-270), in the absence of chelate, the lanthanide ions bind loosely with the lipid phosphate groups and confer the membrane with a sufficient positive magnetic anisotropy to result in parallel alignment (i.e., average bilayer normal along the field). Apparently, DTPA-18 sequesters the lanthanide ions and inserts into the phospholipid bilayer in such a manner that bilayer morphology is preserved over a wide temperature range (35-70 degrees C). The inherent paramagnetic shifts and line broadening effects are illustrated by (2)H NMR spectra of the membrane binding peptide, Leu-enkephalin (Lenk-d(2), Tyr-(Gly-d(2))-Gly-Phe-Leu-OH), in the presence of varying concentrations of Tm(3+), and upon addition of DTPA-18. Two conclusions could be drawn from this study: (1) The addition of Tm(3+) to the bicelle system is consistent with a conformational change in the surface associated peptide, and this effect is shown to be reversed by addition of the chelate, and (2) The paramagnetic shifts are shown to be significantly reduced by addition of chelate.