An FT-IR Study on Packing Defects in Mixed β-Aggregates of Poly(l-glutamic acid) and Poly(d-glutamic acid): A High-Pressure Rescue from a Kinetic Trap

Abstract

Under favorable conditions of pH and temperature, poly­(l-glutamic acid) (PLGA) adopts different types of secondary and quaternary structures, which include spiral assemblies of amyloid-like fibrils. Heating of acidified solutions of PLGA (or PDGA) triggers formation of β₂-type aggregates with morphological and tinctorial properties typical for amyloid fibrils. In contrast to regular antiparallel β-sheet (β₁), the amide I′ vibrational band of β₂-fibrils is unusually red-shifted below 1600 cm^–1, which has been attributed to bifurcated hydrogen bonds coupling CO and N–D groups of the main chains to glutamic acid side chains. However, unlike for pure PLGA, the amide I′ band of aggregates precipitating from racemic mixtures of PLGA and PDGA (β₁) is dominated by components at 1613 and 1685 cm^–1typically associated with intermolecular antiparallel β-sheets. The coaggregation of PLGA and PDGA chains is slower and biphasic and leads to less-structured assemblies of fibrils, which is reflected in scanning electron microscopy images, sedimentation properties, and fluorescence intensity after staining with thioflavin T. The β₁-type aggregates are metastable, and they slowly convert to fibrils with the infrared characteristics of β₂-type fibrils. The process is dramatically accelerated under high pressure. This implies the presence of void volumes within structural defects in racemic aggregates, preventing the precise alignment of main and side chains necessary to zip up ladders of bifurcated hydrogen bonds. As thermodynamic costs associated with maintaining void volumes within the racemic aggregate increase under high pressure, a hyperbaric treatment of misaligned chains leads to rectifying the packing defects and formation of the more compact form of fibrils