Ca(2+)- and Al(3+)-induced conformational transitions of amyloid fragment H-Ile-Ile-Gly-Leu-Met-NH2.

The effect of Ca2+ and Al3+ binding on the conformation of the neurotoxic amyloid fragment H-Ile-Ile-Gly-Leu-Met-NH2 [betaA(31-35)NH2] was studied in trifluoroethanol solutions and in the presence of liposomes. Comparative circular dichroism and Fourier-transform infrared spectroscopic studies revealed that the peptide forms a specific 1:1 complex with Ca2+ which coordinates the polar amide carbonyl groups of the peptide backbone. The results suggest the importance of a folded structure in the complexation of Ca2+. On the contrary, the increasing Al3+ concentration causes a gradual shift of the conformational equilibrium toward beta-sheet structure reflecting no specific binding site for Al3+. In the presence of liposomes the peptide adopts a conformation similar to that of the Ca(2+)-peptide complex. The relevance of the stabilization of peptide conformation by Ca2+ and liposome binding to the bioactive conformation of betaA(31-35)NH2 is also discussed.