Abstract
Bioinspired macromolecules can aid nucleation and crystallisation of minerals by mirroring processes observed in nature. Specifically, the iron oxide magnetite (Fe3O4) is produced in a dedicated liposome (called a magnetosome) within magnetic bacteria. This process is controlled by a suite of proteins embedded within the liposome membrane. In this study we look to synthetically mimic both the liposome and nucleation proteins embedded within it using preferential orientation polymer design. Amphiphilic block co-polymers self-assemble into vesicles (polymersomes) and have been used to successfully mimic liposomes. Carboxylic acid residue-rich motifs are common place in biomineralisation nucleating proteins and several magnetosome membrane specific (Mms) proteins (namely Mms6) have a specific carboxylic acid motifs that are found to bind both ferrous and ferric iron ions and nucleate the formation of magnetite. Here we use a combination of 2 diblock co-polymers: Both have the hydrophobic 2-hydroxypropyl methacrylate (PHPMA) block with either a poly(ethylene glycol) (PEG) block or a carboxylic acid terminated poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) block. These copolymers ((PEG113-PHPMA400) and (PMPC28-PHPMA400) respectively) self-assemble in situ to form polymersomes, with PEG113-PHPMA400 displaying favourably on the outer surface and PMPC28-PHPMA400 on the inner lumen, exposing numerous acidic iron binding carboxylates on the inner membrane. This is a polymersome mimic of a magnetosome (PMM28) containing interior nucleation sites. The resulting PMM28 were found to be 246 ± 137 nm in size. When the PMM28 were subjected to electroporation (5 pulses at 750 V) in an iron solution, iron ions were transported into the PMM28 polymersome core where magnetic iron-oxide was crystallised to fill the core; mimicking a magnetosome. Furthermore it has been shown that PMM28 magnetopolymersomes (PMM28Fe) exhibit a 6 °C temperature increase during in vitro magnetic hyperthermia yielding an intrinsic loss power (ILP) of 3.7 nHm2 kg-1. Such values are comparable to commercially available nanoparticles, but, offer the added potential for further tuning and functionalisation with respect to drug delivery.
Highlights
Biological iron oxide precipitationThe presence of iron oxide particles has been found in various species of birds and dolphins.[1,2] While the true function and mechanism of action of these particles has never been properly explained, it is widely believed that in most cases they play an essential role in navigation by acting as “internal compasses”
The specific absorption rates (SAR) and the intrinsic loss power (ILP) are defined by equations 1 & 2 below, where ΔT/Δt is the initial gradient of the change in temperature over time, c is the heat capacity of water, MFe is the mass of iron per ml, H is the magnetic field and f is the frequency of the alternating field
PMM28 were prepared by polymerisationinduced self-assembly (PISA) via reversible addition– fragmentation chain transfer (RAFT) aqueous dispersion polymerisation of Hydroxypropyl methacrylate (HPMA) at pH 6.5 with a 70% : 30% ratio of PEG113-PHPMA400 to PMPC28-PHPMA400. 1H NMR studies indicated high monomer conversions (>99%) after 4 hours at 50 °C, indicated by the loss of the vinyl group peaks at 5.5 ppm (ESI 1†)
Summary
The presence of iron oxide particles has been found in various species of birds and dolphins.[1,2] While the true function and mechanism of action of these particles has never been properly explained, it is widely believed that in most cases they play an essential role in navigation by acting as “internal compasses”. It should be noted that carboxybetaine zwitterionic groups similar to phosphorylcholine have been shown to not interact with calcite mineralisation, to the extent they are not incorporated into calcite mineral whereas COOH groups alone do, so interaction with the negative charge on zwitterionic phosphorylcholine is improbable.[37] preferential location of these carboxylic acid liposome-mimicking groups on the polymersome inner leaflet should create a synthetic mimic of a nucleation protein within the magnetosome membrane (Fig. 1) This is achieved using a binary mixture of the PMPC-PHPMA chains with a second diblock copolymer, poly(ethylene glycol)-poly(2-hydroxypropyl methacrylate) (PEG-PHPMA). Using this di-block system we have designed and are able to produce synthetic polymersomes that mimic magnetosomes (PMM28)
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