CONFORMATIONS OF SYNTHETIC BETA-PEPTIDES IN SOLID-STATE AND IN AQUEOUS-SOLUTION - RELATION TO TOXICITY IN PC12 CELLS

The secondary structures of peptides beta 25-35 (the active toxic frag ment) and beta 35-25 (reverse sequence and non-toxic fragment), as wel l as of the amidated beta(25-35)-NH2 peptide were investigated in aque ous solution and in the solid state by moans of Fourier-transformed in frared spectroscopy and circular dichroism spectroscopy, The conformat ions of the beta 25-35 and beta 35-25 in solid state were identical an d contained mostly beta-sheet structures, In the solid state the amida ted beta(25-35)-NH2 peptide also contained mostly beta-sheet structure s. Freshly prepared aqueous solutions of the beta 25-35 (0.5-3.8 mM) c ontained a mixture of beta-sheet and random coil structures, Within 30 -60 min incubation at 37 degrees C in water or in phosphate-buffered s aline solution (PBS). beta 25-35 was almost fully converted to a beta- sheet structure. Decreasing the temperature from 37 degrees C to 20 de grees C decreased the rate of conversion from random coil to beta-shee t structures, 1-2 h being required for complete conversion. In contras t beta 35-25 in water or in PBS buffer had mostly a random coil struct ure and remained so for 6 days. The amidated beta(25-35)-NH2 peptide i n water (2.7 mM) was also mostly random coil. However, when this pepti de (2-2.7 mM) was dissolved in PBS (pH 7.4) or in 140 mM NaCl, a gel w as formed and its conformation was mostly beta-sheet. Decreasing the c oncentration of beta(25-35)-NH2 peptide in 140 mM NaCl aqueous solutio n from 2 mM to 1 mM or below favored the conversion from beta-sheet st ructures to random coil structures. The beta 25-35 was toxic to PC12. cells while beta 35-25 was not, The amidated peptide beta(25-35)-NH2 w as at least 500-fold less toxic than beta 25-35, Structural difference s between these beta peptides in aqueous solutions may explain the dif ference in their respective toxicities.