Amyloid fibrils, which are often associated with certain degenerative disorders, reveal a number of intriguing spectral properties. However, the relationship between the structure of fibrils and their optical traits remains poorly understood. Poly(L-glutamic) acid is a model polypeptide shown recently to form amyloid-like fibrils with an atypical infrared amide I' band at 1595 cm(-1), which has been attributed to the presence of bifurcated hydrogen bonds coupling C═O and N-D groups of the main chains to glutamate side chains. Here we show that this unusual amide I' band is observed only for fibrils grown from pure enantiomers of the polypeptide, whereas fibrils precipitating from equimolar mixtures of poly(L-glutamic) and poly(D-glutamic) acids have amide I' bands at 1684 and 1612 cm(-1), which are indicative of a typical intermolecular antiparallel β-sheet. Pure enantiomers of polyglutamic acid form spirally twisted superstructures whose handedness is correlated to the amino acid chirality, while fibrils prepared from the racemate do not form scanning electron microscopy (SEM)-detectable mesoscopically ordered structures. Vibrational circular dichroism (VCD) spectra of β-aggregates prepared from mixtures of all L- or D-polyglutamic acid in varying ratios indicate that the enhancement of VCD intensity correlates with the presence of the twisted superstructures. Our results demonstrate that both IR absorption and enhanced VCD are sensitive to subtle packing defects taking place within the compact structure of amyloid fibrils.

Alzheimer's disease (AD) is characterized by pathological aggregation of β-amyloid peptides and MAP-Tau protein. β-Amyloid (Aβ) is a peptide responsible for extracellular Alzheimer's plaque formation. Intracellular MAP-Tau aggregates appear as a result of hyperphosphorylation of this cytoskeletal protein. Small, oligomeric forms of Aβ are intermediate products that appear before the amyloid plaques are formed. These forms are believed to be most neurotoxic. Dendrimers are highly branched polymers, which may find an application in regulation of amyloid fibril formation. Several biophysical and biochemical methods, like circular dichroism (CD), fluorescence intensity of thioflavin T and thioflavin S, transmission electron microscopy, spectrofluorimetry (measuring quenching of intrinsic peptide fluorescence) and MTT-cytotoxicity assay, were applied to characterize interactions of cationic phosphorus-containing dendrimers of generation 3 and generation 4 (CPDG3, CPDG4) with the fragment of amyloid peptide (Aβ(1-28)) and MAP-Tau protein. We have demonstrated that CPDs are able to affect β-amyloid and MAP-Tau aggregation processes. A neuro-2a cell line (N2a) was used to test cytotoxicity of formed fibrils and intermediate products during the Aβ(1-28) aggregation. It has been shown that CPDs might have a beneficial effect by reducing the system toxicity. Presented results suggest that phosphorus dendrimers may be used in the future as agents regulating the fibrilization processes in Alzheimer's disease.

The irreversibility and autocatalytic character of amyloidogenesis and the polymorphism of amyloid fibrils underlie the phenomenon of self-propagating strains, wherein the mother seed, rather than the seeding environment, determines the properties of daughter fibrils. Here we study the formation of amyloid fibrils from bovine insulin and the recombinant Lys(B31)-Arg(B32) human insulin analog. The two polypeptides are similar enough to cross-seed but, upon spontaneous aggregation, form amyloid fibrils with distinct spectral features in the infrared amide I' band region. When bovine insulin is cross-seeded with the analog amyloid (and vice versa), the shape, absorption maximum, and even fine fingerprint features of the amide I' band are passed from the mother to daughter fibrils with a high degree of fidelity. Although the differences in primary structure between bovine insulin and the Lys(B31)-Arg(B32) analog of human insulin lie outside of the polypeptide's critical amyloidogenic regions, they affect the secondary structure of fibrils, possibly the formation of intermolecular salt bridges, and the susceptibility to dissection and denaturation with dimethyl sulfoxide (DMSO). All these phenotypic features of mother fibrils are imprinted in daughter amyloid upon cross-seeding. Analysis of noncooperative DMSO-induced denaturation of daughter fibrils suggests that the self-propagating polymorphism underlying the emergence of new amyloid strains is encoded on the level of secondary structure. Our findings have been discussed in the context of polymorphism of fibrils, amyloid strains, and possible implications for mechanisms of amyloidogenesis.