Insulin amyloid superstructures as templates for surface enhanced Raman scattering

Vortex-induced amyloid superstructures of insulin and its component A and B chains

Insulin is an amyloid-forming polypeptide built of two disulfide-linked chains (A and B), both themselves amyloidogenic. An interesting property of insulin is that agitation strongly influences the course of its aggregation, resulting in characteristic chiral superstructures of amyloid fibrils. Here, we investigate the self-assembly of these superstructures by comparing the quiescent and vortex-assisted aggregation of insulin and its individual A and B chains in the presence or absence of reducing agent tris(2-carboxyethyl)phosphine (TCEP). Our study shows that only the B chain in the presence of TCEP is converted into aggregates with morphology (according to atomic force microscopy) and optical activity (manifested as an extrinsic Cotton effect induced in bound thioflavin T) characteristic of amyloid superstructures that are normally formed by insulin in the absence of TCEP. In contrast to more rigid B-peptide fibrils, elongated aggregates of the A peptide become amorphous upon agitation. Moreover, the aggregation of equimolar mixture of both peptides does not produce highly ordered entities. Our results suggest that the dynamics of the B chain are the driving force for the assembly of superstructures, with the A chain being complicit as long as its own dynamics are controlled by the firm attachment to the B chain provided by the intact covalent structure of insulin.

Conformational memory effect reverses chirality of vortex-induced insulin amyloid superstructures

Formation of amyloid fibrils is often associated with intriguing far-from-equilibrium phenomena such as conformational memory effects or flow-driven self-assembly. Insulin is a model amyloidogenic polypeptide forming distinct structural variants of fibrils, which self-propagate through seeding. According to infrared absorption, fibrils from bovine insulin ([BI]) and Lys(B31)-Arg(B32) human insulin analogue ([KR]) cross-seed each other and imprint distinct structural features in daughter fibrils. In the absence of preformed [KR] amyloid seeds, bovine insulin agitated at 60 °C converts into chiral amyloid superstructures exhibiting negative extrinsic Cotton effect in bound thioflavin T. However, when agitated bovine insulin is simultaneously cross-seeded with [KR] amyloid, daughter fibrils reveal a positive extrinsic Cotton effect. Our study indicates that dramatic changes in global properties of amyloid superstructures may emerge from subtle conformational-level variations in single fibrils (e.g., alignment and twist of β-strands) that are encoded by memory effects.

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

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 β(2)-type aggregates with morphological and tinctorial properties typical for amyloid fibrils. In contrast to regular antiparallel β-sheet (β(1)), the amide I' vibrational band of β(2)-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 (β(1)) 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 β(1)-type aggregates are metastable, and they slowly convert to fibrils with the infrared characteristics of β(2)-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.

In situ formation of one-dimensional assemblies of gold nanoparticles in confined media

The spontaneous in situ formation of one-dimensional (1D) assemblies of gold nanoparticles (NP) in oleylamine/bis(2-ethylhexyl) sulfosuccinate sodium salt/water/octane (OAm/AOT/w/o) microemulsions by exploiting both the aurophilic bonding between OAm and gold salt, and the interactions between OAm and AOT surfactant is presented. Control on the structure of the resulting assemblies is achieved by changing in the solvent quality, the [Au]/[AOT] molar ratio and the presence of different cosolutes. A possible mechanism of the formation of the 1D parallel Au NP arrays is proposed.

Investigation of encapsulation of insulin biotemplate within C-methylresorcin[4]arenes

Solution structure of insulin templated C-methyl resorcin[4]arene nanocapsules has been investigated using neutron scattering. The insulin biotemplate has enhanced the limits of encapsulation and enabled formation of a larger spherical molecular host.