Conformational
transitions involving aggregated proteins or peptides
are of paramount biomedical and biotechnological importance. Here,
we report an unusual freeze-induced structural reorganization within
a β-sheet-rich ionic coaggregate of poly(l-lysine),
PLL, and poly(l-glutamic acid), PLGA. Freezing aqueous suspensions
of the PLL–PLGA β-aggregate in the presence of low concentrations
of salt (NaBr) induces an instantaneous β-sheet-to-disorder
transition, as probed by infrared spectroscopy in the amide I′
band region. The conformational rearrangement of polypeptide chains
appears to be fully synchronized with the global liquid-to-ice phase
transition. In contrast to the known freeze-induced transitions, the
process described here is fully reversible: the subsequent thawing
results in an instantaneous disorder-to-β-sheet “refolding”.
However, in the absence of traces of soluble salts, the β-sheet
framework of the PLL–PLGA aggregate remains resistant to freezing
as no transition is observed. We note that the occurrence of the transition
depends on the type of salt present in the sample. Our results highlight
a hidden dimension of the structural dynamics within β-sheet-rich
aggregates. Possible scenarios of freeze-induced salt-bridge rupture
and removal of water from nanocanals are discussed.
Many of the potential applications of albumin-stabilized gold nanoclusters (AuNC) arise from the sensitivity of their luminescence to the presence of various ions and albumin-degrading proteases. However, the underlying photophysics...
Many of the potential applications of albumin-stabilized gold nanoclusters (AuNC) arise from the sensitivity of their luminescence to the presence of various ions and albumin-degrading proteases. However, the underlying photophysics and the mechanisms responsible for protease-induced quenching of AuNC luminescence are not fully understood. Here, we study proteinase K-induced digestion of bovine serum albumin (BSA)-AuNC conjugate under aerobic and anaerobic conditions. To this end, we adapt a Co(II)-catalyzed sulfite-based protocol enabling effective in situ deoxidization without deactivation of the enzyme. In the absence of proteinase K, the anaerobic conditions facilitate luminescence of BSA-AuNC reflected by a moderate increase in the red luminescence intensity. However, in the presence of proteinase K, we have observed a steeper decrease of emission intensity irrespective of whether the digestion was carried out under aerobic or anaerobic conditions. In both cases, the diminishing fluorescence occurred in phase with shifting of the emission maximum to longer wavelengths. These results contradict the previous hypothesis that protease-induced quenching of BSA-AuNC luminescence is a consequence of enhanced diffusion of oxygen to bare AuNC. Instead, aggregation of unprotected AuNCs and separation of nanoclusters from albumin’s side chains involved in energy transfers and luminescence-promoting electron donors may underlie the observed sensitivity of BSA-AuNC to protease treatment. Our findings are discussed in the context of mechanisms of formation and photophysics of BSA-AuNC conjugates.
Self-aggregation of individual polypeptide chains into amyloid fibrils is driven by interactions between amyloidogenic segments of whole proteins. The interplay between aggregation-prone and aggregation-resistant fragments within a single polypeptide chain is not well understood. Here, we examine fibrillization behavior of two designed chimeric peptides, ACC1-13E8 and ACC1-13E8(L/D), in which the highly amyloidogenic fragment of insulin’s A-chain (ACC1-13) is extended by an octaglutamate segment composed of all-L (E8), or alternating L/D residues (E8(L/D)). As separate entities, ACC1-13 readily forms fibrils with the infrared features of parallel β-sheet structure while acidified E8 forms so-called β2-aggregates consisting of antiparallel β-sheets and manifesting distinctly in the amide I band infrared region. This contrasts with profoundly aggregation-resistant behavior of E8(L/D) peptide although the alternating L/D motif has been hypothesized as compatible with aggregated α-sheets. ACC1-13E8 and ACC1-13E8(L/D) peptides are equally prone to fibrillization when the electrostatic repulsion between dissolved monomers is prevented either by lowering pH, or in the presence of Ca2+ ions. In the aggregated states, both ACC1-13E8 and ACC1-13E8(L/D) reveal the infrared characteristics of ordered parallel β-sheet structure with no spectral features attributable to β2-aggregates (ACC1-13E8) or α-sheets (ACC1-13E8(L/D)). Hence, the preferred structural pattern of ACC1-13 segment not only overrides the tendency of E8 to form the antiparallel β2-structure but also enforces formation of β-sheet structure within the E8(L/D) segment which on its own is entirely refractory to aggregation. We demonstrate how an alternating L/D sequence can be effectively forced to become a part of highly ordered amyloid structure scaffolded by an all-L amyloidogenic segment. Our study shows how the overall amyloidogenic characteristics of a larger hybrid sequence may be impacted and controlled by the properties of its most aggregation-prone part.
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