Study on Macroscale 2D Self-assembly of Tyrosine-containing Helical Peptide and Proton Conduction in a Tyrosine/manganese Oxide Hybrid Nanofilm

Abstract

One of the important challenges in the development of protein-mimetic materials is to understand the sequence specific assembly behavior and the dynamic folding change. The discovery of laterally extended, untwisted, two-dimensional assembled structures in pure water is fundamentally challenging in peptide-based systems. Until now, most of the previous approaches for preparing 2D peptide assembled structures have been based on beta-sheet-like assembly or fibril-aggregated structures. In this study we present the assembly of a designed tyrosine-rich 5-mer peptide at the air/water interface, displaying facet formation of a water droplet. The identified peptides had a strong driving force to flatten the rounded top of droplet into a plane with a macroscopic 2D structure. The facet formation is driven by interactions of tyrosine and cross-linked stabilization by cysteine. We revealed the helical conformation of the peptide was stabilized by a disulfide bond, which was confirmed by circular dichroism and 2D-nuclear magnetic resonance spectroscopy. Futhermore, we created giant nanosheets decorated with gold nanoparticles at the air/water interface by introducing gold ion into the peptide. Due to the gold ions as an oxidant to rapidly induce a disulfide bridge between cysteines, the peptide assembled into the nanosheet and Au3+ were reduced to gold nanoparticles on the peptide sheet in the absence of other reducing agents simultaneously.<br/> In an effort to investigate the potential of tyrosine-rich peptide with redox active functions, we examine proton conductivity of the peptide films hybridized with manganese oxides. Proton conduction in biological systems has been an important issue for a better understanding of fundamental life mechanism. To understand and manipulate proton conduction in bio-system, several studies have investigated bulk proton conduction of biomaterials. However, little is known about the bulk proton conductivity of short peptides and their sequence-dependent behavior. We focus on tyrosine-rich peptide which has redox-active and crosslinkable phenol. Tyrosine has been known to play a critical role in PCET interplaying with a Ca-Mn cluster in the photosystem II. Moreover, it can be polymerized into cross-linked polymers, melanin, possessing a semiconductor-like behavior dependent on hydration. Inspired from this, we made proton conductor with tyrosine containing 2D peptide material by hybridization with manganese oxide (MnOx) via the tyrosine oxidation reaction. The peptide/MnOx hybrid films can efficiently transport proton and proton conductivity was 18.6 mS cm-1. The value is much higher than that of biomaterials and comparable with synthetic materials. These results suggest that peptide-based hybrid material can be a promising new class of proton conductor. In perspective of bioelectronics and further applications, it would be a good example for designing versatile platforms for bio-functionalized devices with biocompatible materials.<br/>