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

**Kazushige Yokoyama \*, Kieran Brown, Peter Shevlin, Jack Jenkins, Elizabeth D'Ambrosio, Nicole Ralbovsky, Jessica Battaglia, Ishan Deshmukh and Akane Ichiki**

Department of Chemistry, The State University of New York at Geneseo College, Geneseo, NY 14454, USA; kb6627500@gmail.com (K.B.); PeterJShevlin@gmail.com (P.S.); jackjenk@iu.edu (J.J.); edambros@udel.edu (E.D.); nralbovsky@albany.edu (N.R.); jessicabattaglia120@gmail.com (J.B.); isd1@geneseo.edu (I.D.); ai7@geneseo.edu (A.I.)

**\*** Correspondence: yokoyama@geneseo.edu

Received: 10 September 2019; Accepted: 23 October 2019; Published: 28 October 2019

**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.

**Keywords:** amyloidogenic peptides; amyloid beta; alpha synuclein; beta 2 microglobulin; nano-gold colloids; peptide coverage; aggregation; adsorption orientation; spiking-out orientation; gyration
