Martins A. F., Oliveira A. C., Morfin J.-F., Laurents D. V., Tóth E. and Geraldes C. F.
Journal of biological inorganic chemistry (2015) 21 (1) 83-99 - doi : 10.1007/s00775-015-1316-9
We have conjugated the tetraazacyclododecane-tetraacetate (DOTA) chelator to Pittsburgh compound B (PiB) forming negatively charged lanthanide complexes, Ln(L4), with targeting capabilities towards aggregated amyloid peptides. The amphiphilic Gd(L4) chelate undergoes micellar aggregation in aqueous solution, with a critical micellar concentration of 0.68 mM, lower than those for the neutral complexes of similar structure. A variable temperature 17O NMR and NMRD study allowed the assessment of the water exchange rate, k ex 298 = 9.7 x 106 s-1, about the double of GdDOTA, and for the description of the rotational dynamics for both the monomeric and the micellar forms of Gd(L4). With respect to the analogous neutral complexes, the negative charge induces a significant rigidity of the micelles formed, which is reflected by slower and more restricted local motion of the Gd3+ centers as evidenced by higher relaxivities at 20-60 MHz. Surface Plasmon Resonance results indicate that the charge does not affect significantly the binding strength to Abeta1-40 [K d = 194 +/- 11 muM for La(L4)], but it does enhance the affinity constant to human serum albumin [K a = 6530 +/- 68 M-1 for Gd(L4)], as compared to neutral counterparts. Protein-based NMR points to interaction of Gd(L4) with Abeta1-40 in the monomer state as well, in contrast to neutral complexes interacting only with the aggregated form. Circular dichroism spectroscopy monitored time- and temperature-dependent changes of the Abeta1-40 secondary structure, indicating that Gd(L4) stabilizes the random coil relative to the alpha-helix and beta-sheet. TEM images confirm that the Gd(L4) complex reduces the formation of aggregated fibrils.