Examination of Adsorption Orientation of Amyloidogenic Peptides Over Nano-Gold Colloidal Particle Surfaces

Abstract

The adsorption of amyloidogenic peptides, amyloid beta 1–40 (Aβ1–40), alpha-synuclein (α-syn), and beta 2 microglobulin (β2m), was attempted over the surface of nano-gold colloidal particles, ranging from d = 10 to 100 nm in diameter (d). The spectroscopic inspection between pH 2 and pH 12 successfully extracted the critical pH point (pHo) at which the color change of the amyloidogenic peptide-coated nano-gold colloids occurred due to aggregation of the nano-gold colloids. The change in surface property caused by the degree of peptide coverage was hypothesized to reflect the ΔpHo, which is the difference in pHo between bare gold colloids and peptide coated gold colloids. The coverage ratio (Θ) for all amyloidogenic peptides over gold colloid of different sizes was extracted by assuming Θ = 0 at ΔpHo = 0. Remarkably, Θ was found to have a nano-gold colloidal size dependence, however, this nano-size dependence was not simply correlated with d. The geometric analysis and simulation of reproducing Θ was conducted by assuming a prolate shape of all amyloidogenic peptides. The simulation concluded that a spiking-out orientation of a prolate was required in order to reproduce the extracted Θ. The involvement of a secondary layer was suggested; this secondary layer was considered to be due to the networking of the peptides. An extracted average distance of networking between adjacent gold colloids supports the binding of peptides as if they are “entangled” and enclosed in an interfacial distance that was found to be approximately 2 nm. The complex nano-size dependence of Θ was explained by available spacing between adjacent prolates. When the secondary layer was formed, Aβ1–40 and α-syn possessed a higher affinity to a partially negative nano-gold colloidal surface. However, β2m peptides tend to interact with each other. This difference was explained by the difference in partial charge distribution over a monomer. Both Aβ1–40 and α-syn are considered to have a partial charge (especially δ+) distribution centering around the prolate axis. The β2m, however, possesses a distorted charge distribution. For a lower Θ (i.e., Θ <0.5), a prolate was assumed to conduct a gyration motion, maintaining the spiking-out orientation to fill in the unoccupied space with a tilting angle ranging between 5° and 58° depending on the nano-scale and peptide coated to the gold colloid.

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“…In this work we investigated whether the gold nano-particles possess adequate affinity to anchor the S protein [ 28 , 29 ]. In our previous work, the orientation of the amyloidogenic peptide over the nano‑gold surface was found to take a “spiking-out” orientation in order to maximize the possibility of the networking [ 30 , 31 ]. Under low pH (less than pH 4), the amyloidogenic peptides conduct protein unfolding and complete the networking to the other peptide over the other surrounding gold colloid.…”

“…As described in the previous work [ 31 ], nine different sizes of gold colloidal particles (diameter d = 10, 15, 20, 30, 40, 50, 60, 80, and 100 nm) purchased from Ted Pella Inc. (Redding, California, USA) The residual stabilizer contained in all sizes of gold colloids were negligible ( e.g. , citrate < 10 −5 %, tannic acid < 10 −7 % and potassium carbonate < 10 −8 %) [ 12 ] The reported values of the estimated diameters (d), measured diameter ( d ), and particles per mL for each gold colloid are shown in Table 1 b.…”

“…, citrate < 10 −5 %, tannic acid < 10 −7 % and potassium carbonate < 10 −8 %) [ 12 ] The reported values of the estimated diameters (d), measured diameter ( d ), and particles per mL for each gold colloid are shown in Table 1 b. The concentration of S protein was fixed as 360 pmol [ 31 ], and the ratio of [S protein] / [Gold Colloid] as 4, 19, 39, 136, 303, 665, 1269, 2479, and 3409 for the 10, 15, 20, 30, 40, 50, 60, 80, and 100 nm gold colloid, respectively. The attachment of protein to the gold colloidal surface was expected to be achieved almost instantaneously and considered to reach equilibrium within a minute.…”

“…The largest difference was found in 100 nm gold colloid, and no significant differences were found for 40 nm and 60 nm gold. Based on the same technique with amyloidogenic peptide coated gold nano-particles [ 31 ], all tested nano-size gold colloid showed the positive ΔpH o and indicates that peptides were coated. However, under the current study, several gold colloids exhibited negative ΔpH o , implying that the S protein coating over the nano‑gold colloid with δ– segment requiring extra addition of H 3 O + , more than what bare gold colloids need.…”

“…However, under the current study, several gold colloids exhibited negative ΔpH o , implying that the S protein coating over the nano‑gold colloid with δ– segment requiring extra addition of H 3 O + , more than what bare gold colloids need. Thus, |ΔpH o | can be used as the indicative of the surface character change, the same analysis of the d pH vs. |ΔpH o | conducted for amyloidogenic peptides [ 30 , 31 ]. However, the trend of d pH with the S protein was opposite to what was observed in amyloidogenic peptides.…”