Conformations of synthetic β peptides in solid state and in aqueous solution: relation to toxicity in PC12 cells

Abstract

The secondary structures of peptides β25-35 (the active toxic fragment) and β35-25 (reverse sequence and non-toxic fragment), as well as of the amidated β(25-35)-NH 2 peptide were investigated in aqueous solution and in the solid state by means of Fourier-transformed infrared spectroscopy and circular dichroism spectroscopy. The conformations of the β25-35 and β35-25 in solid state were identical and contained mostly β-sheet structures. In the solid state the amidated β(25-35)-NH 2 peptide also contained mostly β-sheet structures. Freshly prepared aqueous solutions of the β25-35 (0.5–3.8 mM) contained a mixture of β-sheet and random coil structures. Within 30–60 min incubation at 37°C in water or in phosphate-buffered saline solution (PBS), β25-35 was almost fully converted to a β-sheet structure. Decreasing the temperature from 37°C to 20°C decreased the rate of conversion from random coil to β-sheet structures, 1–2 h being required for complete conversion. In contrast β35-25 in water or in PBS buffer had mostly a random coil structure and remained so for 6 days. The amidated β(25-35)-NH 2 peptide in water (2.7 mM) was also mostly random coil. However, when this peptide (2–2.7 mM) was dissolved in PBS (pH 7.4) or in 140 mM NaCl, a gel was formed and its conformation was mostly β-sheet. Decreasing the concentration of β(25-35)-NHZ peptide in 140 mM NaCl aqueous solution from 2 mM to 1 mM or below favored the conversion from β-sheet structures to random coil structures. The β25-35 was toxic to PC12 cells while β35-25 was not. The amidated peptide β(25-35)-NH 2 was at least 500-fold less toxic than β25-35. Structural differences between these β peptides in aqueous solutions may explain the difference in their respective toxicities.