Self-assembly of discrete metal complexes in aqueous solution via block copolypeptide amphiphiles.

Keita Kuroiwa,Timothy J Deming,Yoshitaka Masaki,Yuko Koga

doi:10.3390/ijms14012022

Abstract

The integration of discrete metal complexes has been attracting significant interest due to the potential of these materials for soft metal-metal interactions and supramolecular assembly. Additionally, block copolypeptide amphiphiles have been investigated concerning their capacity for self-assembly into structures such as nanoparticles, nanosheets and nanofibers. In this study, we combined these two concepts by investigating the self-assembly of discrete metal complexes in aqueous solution using block copolypeptides. Normally, discrete metal complexes such as [Au(CN)2]−, when molecularly dispersed in water, cannot interact with one another. Our results demonstrated, however, that the addition of block copolypeptide amphiphiles such as K183L19 to [Au(CN)2]− solutions induced one-dimensional integration of the discrete metal complex, resulting in photoluminescence originating from multinuclear complexes with metal-metal interactions. Transmission electron microscopy (TEM) showed a fibrous nanostructure with lengths and widths of approximately 100 and 20 nm, respectively, which grew to form advanced nanoarchitectures, including those resembling the weave patterns of Waraji (traditional Japanese straw sandals). This concept of combining block copolypeptide amphiphiles with discrete coordination compounds allows the design of flexible and functional supramolecular coordination systems in water.

Highlights

Diblock copolypeptide amphiphiles are synthetic materials with many features that make them of interest to those working in the field of protein engineering, in applications such as drug delivery systems and tissue engineering [44,45,46,47,48,49,50]
A number of diblock copolypeptide amphiphiles were synthesized according to procedures previously published in the literature [44,45]: K96L1 (1), K183L19 (2) and K989L137 (3) (Figure 1)
All three copolypeptides exhibited low polydispersity values, ranging from 1.13 to 1.19, as measured by gel permeation chromatography (GPC) and 1H nuclear magnetic resonancy (NMR) integration data for the lysine (Lys) moieties. When these copolypeptide amphiphiles were dissolved in water at concentrations above 1 wt%, hydrogels were obtained

Summary

Introduction

There has, to date, been significant interest in the design and fabrication of low dimensional metal complexes, primarily because the electronic structures of such complexes are tunable via the formation of supramolecular architectures such as nanoparticles [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19], nanocrystals [20,21,22,23] and nanowires [24,25,26,27,28,29,30,31,32,33,34,35]. Diblock copolypeptide amphiphiles are synthetic materials with many features that make them of interest to those working in the field of protein engineering, in applications such as drug delivery systems and tissue engineering [44,45,46,47,48,49,50] Their unique properties are due to the propensity of these amphiphiles to form double-walled vesicles or biocompatible fibrillar nanostructures based on the self-assembly of their hydrophilic and hydrophobic blocks. We focus on the dynamic structural transformation of [Au(CN)2]−, achieved through the use of diblock copolypeptide amphiphiles having the general structural formula poly-L-Lysine-block-L-Leucine (KmLn) These amphiphiles are known to assemble into fibrillar structures, resulting in the formation of hydrogels and vesicles [52,53,54,55]. The nature of the systematic assembly of these materials in solution is discussed, based on the results of spectroscopic and microscopic measurements

Preparation of Copolypeptides

Materials and Instrumentation

General Polypeptide Synthesis

Conclusions