Ag0 Nanoparticles Synthesized in R-phycoerythrin: Change in Bioconjugate Properties upon Ripening of Nanoparticles

BACKGROUND

Bioconjugates prepared from photoactive molecules and metal nanoparticles are suitable for the development of new optoelectronic devices and for theranostic applications in medicine.

METHODS

We produced a bioconjugate of R-phycoerythrin (R-PE), a photosynthetic pigment of red algae, with Ag0 nanoparticles (Ag0 NPs) synthesized in its tunnel cavities by mixing aqueous solutions of AgNO3 and R-PE without exogenous reductant. In freshly prepared bioconjugate (Ag0⋅R-PE), the diameter of Ag0 NPs measured in electron microscopic images was 6.5 ± 0.5 nm, being commensurate with the length of R-PE tunnel cavity, and they were found to emit bright visible fluorescence.

RESULT

The initial aggregation temperature (T0) of freshly prepared Ag0⋅R-PE was decreased, compared to that of native R-PE (33 vs. 44°C). Upon the ripening of Ag0 NPs, their fluorescence was almost entirely quenched, and giant surface-enhanced Raman scattering was recorded. The bioconjugate self-assembled into nanorods 25 ± 5 nm long and could withstand heating to 90°C. Further heating (90°C → 100°C) resulted in the formation of filaments which are 0.5 ± 0.2 μm long and spherical aggregates 60 ± 10 nm in diameter.

CONCLUSION

These data can contribute to the development of practical applications for the Ag0⋅R-PE bioconjugate.

Acceleration of protein aggregation by amphiphilic peptides: transformation of supramolecular structure of the aggregates

Protein self-assembly and aggregation represent a special tool in biomedicine and biotechnology to produce biological materials for a wide range of applications. The protein aggregates are very different morphologically, varying from soluble amorphous aggregates to highly ordered amyloid-like fibrils, the latter being associated with molecular structures able to perform specific functions in living systems. Fabrication of novel biomaterials resembling natural protein assemblies has awakened interest in identification of low-molecular-weight biogenic agents as regulators of transformation of aggregation-prone proteins into fibrillar structures. Short amphiphilic peptides can be considered for this role. Using dynamic light scattering, turbidimetry, fluorescence spectroscopy, and transmission electron microscopy (TEM), we have demonstrated that the Arg-Phe dipeptide dramatically accelerates the aggregation of a model protein, α-lactalbumin, to generate morphologically different structures. TEM revealed transformation of spherical particles observed in the control samples into branched chains of fibril-like nanostructures in the presence of the peptide, suggesting that amphiphilic peptides can induce changes in the physicochemical properties of a protein substrate (net charge, hydrophobicity, and tendency to β-structure formation) resulting in accumulation of peptide-protein complexes competent to self-assembly into supramolecular structures. A number of other short amphiphilic peptides have also been shown to accelerate the aggregation process, using alternative complementary protein substrates for identification of molecular recognition modules. Peptide-protein assemblies are suggested to play the role of building blocks for formation of supramolecular structures profoundly differing from those of the individual protein substrate in type, size, and shape.