Chain-length dependence of (alpha)-helix to (beta)-sheet transition in polylysine: Model of protein aggregation studied by temperature-tuned FTIR spectroscopy

The chain-length dependence of the (alpha)-helix to (beta)-sheet transition in poly(L-lysine) is studied by temperaturetuned FTIR spectroscopy. This study shows that heterogeneous samples of poly(L-lysine), comprising polypeptide chains with various lengths, undergo the (alpha)-(beta) transition at an intermediate temperature compared to homogenous ingredients. This holds true as long as each individual fraction of the polypeptide is capable of adopting an antiparallel (beta)-sheet structure. The tendency is that the longer chain is, the lower the (alpha)-(beta) transition temperature is, which has been linked to the presence of distorted or solvated helices with turns or (beta) sheets in elongating chains of poly(L-lysine). As such helical structures are apparently conducive to the (alpha)-(beta) transition, this draws a comparison to the hypothesis of metastable protein conformational states being a common stage in amyloid-formation pathways. The antiparallel architecture of the (beta) sheet is likely to reflect the pretransition interhelical interactions in poly(L-lysine). Namely, the chains are arranged in an antiparallel manner because of energetically favored antiparallel preassembly of dipolar (alpha) helices.