Alpha-helix to beta-sheet transition in long-chain poly-L-lysine: Formation of alpha-helical fibrils by poly-L-lysine

The temperature-induced alpha-helix to beta-sheet transition in long-chain poly-L-lysine (PLL), accompanied by the gauche-to-trans isomerization of CH2 groups in the hydrocarbon side chains of Lys amino acid residues, and formation of beta-sheet as well as alpha-helix fibrillar aggregates of PLL have been studied using Fourier-transform infrared (FT-IR) and vibrational circular dichroism (VCD) spectroscopy, and transmission electron microscopy (TEM). In a low-temperature alkaline water solution or in a methanol-rich water mixture, the secondary structure of PLL is represented by alpha-helical conformations with unordered and gauche-rich hydrocarbon side chains. Under these conditions, PLL molecules aggregate into alpha-helical fibrils. PLLS dominated by extended antiparallel beta-sheet structures with highly ordered trans-rich hydrocarbon side chains are formed in a high-temperature range at alkaline pD and aggregate into fibrillar, protofibrillar, and spherical forms. Presented data support the idea that fibrillar aggregation is a varied phenomenon possible in repetitive structural elements with not only a beta-sheet-rich conformation, but also an alpha-helical-rich conformation. (C) 2017 Elsevier B.V. and Societe Francaise de Biochimie et Biologie Moleculaire (SFBBM). All rights reserved.