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

The equimolar complex, consisting of the lipid-like, amphiphilic chelating agent 1,11-bis[distearylamino]-diethylenetriamine pentaacetic acid (DTPA-18 ) and Tm3+, is shown by deuterium (H-2) NMR to be useful in aligning bicell e-like model membranes, consisting of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC). As shown previously (1996, R. S. Presser et al., J. Am. Chem. Sec. 118, 269-270), in the absence of chelate , the lanthanide ions bind loosely with the lipid phosphate groups and conf er the membrane with a sufficient positive magnetic anisotropy to result in parallel alignment (i.e., average bilayer normal along the field). Apparen tly, DTPA-18 sequesters the lanthanide ions and inserts into the phospholip id bilayer in such a manner that bilayer morphology is preserved over a wid e temperature range (35-70 degrees C). The inherent paramagnetic shifts and line broadening effects are illustrated by H-2 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 Tm3+, and upon addition of D TPA-18. Two conclusions could be drawn from this study: (1) The addition of Tm3+ to the bicelle system is consistent with a conformational change in t he surface associated peptide, and this effect is shown to be reversed by a ddition of the chelate, and (2) The paramagnetic shifts are shown to be sig nificantly reduced by addition of chelate. (C) 1999 Academic Press.