The E2 porous protein cage.

<p>(A) and (B) E2 protein cage three-dimensional structure (adapted from PDB ID: 1b5s).[<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0162848#pone.0162848.ref042" target="_blank">42</a>] The inserts show typical electron microscopy images of the 5-fold axis (A) and 2-fold axis (B) orientations of the protein cage E2. The diameter <i>D</i> of the E2 protein cage is 25 nm. The diameter <i>d</i> of each pore is 6 nm. (C) E2 protein cage inner surface presenting RDGE loop sequences in blue. The diameter δ of the cage core is 13 nm.</p

Electron microscopy of wild-type and engineered E2 protein cages.

<p>Images of the E2 protein cages: (A, E) E2-WT; (B, F) E2-LH2; (C, G) E2-LH5; (D, H) E2-LH6. (A–D) scale bars are 100 nm. (E–H) scale bars are 50 nm.</p

Oligonucleotides for plasmid construction.

<p>Oligonucleotides for plasmid construction.</p

Method for specific internalisation of peptide coated gold nanoparticles into engineered E2 protein cages.

<p>(a) Site-directed mutagenesis of E2 protein cage’s core with oligohistidine sequences (blue) at the RDGE loop (green) on E2 protein subunits; (b) Surface coating of 3.9 nm gold nanoparticles with a self-assembled monolayer made of peptidols and thiolated alkane ethylene glycol (EG) ligands, functionalised with Ni²⁺ nitrilotriacetic moieties (NTA, Ni²⁺); (c) Specific internalisation by affinity binding of Ni-NTA-functionalised peptide coated gold nanoparticles into E2 protein cages presenting oligohistidine sequences.</p

Ni-NTA-functionalised peptide coated gold nanoparticles.

<p>(A) Size distribution of 212 gold nanoparticles with an average diameter of 3.9 ± 0.8 nm. The insert shows a typical electron microscopy image used for size measurement. Immobilisation of Ni-NTA-functionalised peptide coated gold nanoparticles on a hexa-histidine loaded resin. (B) NTA-functionalised SAM coated gold nanoparticles not loaded with Ni²⁺ present no non-specific binding to hexa-histidine resin. (C) 10% (mol:mol) Ni-NTA-functionalised peptide coated gold nanoparticles fully bind to hexa-histidine resin as no free gold nanoparticles are found in the clear supernatant.</p

Dynamic light scattering analysis of wild-type and engineered E2 protein cages with different oligohistidine sequences at the RDGE loop.

<p>Dynamic light scattering analysis of wild-type and engineered E2 protein cages with different oligohistidine sequences at the RDGE loop.</p

Transmission electron microscopy of samples of Ni-NTA-functionalised gold nanoparticles and E2 protein cages.

<p>Ni-NTA-functionalised gold nanoparticles mixed with (A) E2-WT, (B) E2-LH2, (C) E2-LH5, (D) E2-LH6. White circles indicate Ni-NTA-functionalised gold nanoparticles internalised into oligohistidine modified E2 protein cages. The samples were stained with 1% (w/v) phosphotungstic acid. Scale bars are 100 nm.</p

Computational and Experimental Investigation of the Structure of Peptide Monolayers on Gold Nanoparticles

The self-assembly and self-organization of small molecules on the surface of nanoparticles constitute a potential route toward the preparation of advanced proteinlike nanosystems. However, their structural characterization, critical to the design of bionanomaterials with well-defined biophysical and biochemical properties, remains highly challenging. Here, a computational model for peptide-capped gold nanoparticles (GNPs) is developed using experimentally characterized Cys-Ala-Leu-Asn-Asn (CALNN)- and Cys-Phe-Gly-Ala-Ile-Leu-Ser-Ser (CFGAILSS)-capped GNPs as a benchmark. The structure of CALNN and CFGAILSS monolayers is investigated using both structural biology techniques and molecular dynamics simulations. The calculations reproduce the experimentally observed dependence of the monolayer secondary structure on the peptide capping density and on the nanoparticle size, thus giving us confidence in the model. Furthermore, the computational results reveal a number of new features of peptide-capped monolayers, including the importance of sulfur movement for the formation of secondary structure motifs, the presence of water close to the gold surface even in tightly packed peptide monolayers, and the existence of extended 2D parallel β-sheet domains in CFGAILSS monolayers. The model developed here provides a predictive tool that may assist in the design of further bionanomaterials

Biocompatible Peptide-Coated Ultrasmall Superparamagnetic Iron Oxide Nanoparticles for <i>In Vivo</i> Contrast-Enhanced Magnetic Resonance Imaging

The biocompatibility and performance of reagents for <i>in vivo</i> contrast-enhanced magnetic resonance imaging (MRI) are essential for their translation to the clinic. The quality of the surface coating of nanoparticle-based MRI contrast agents, such as ultrasmall superparamagnetic iron oxide nanoparticles (USPIONs), is critical to ensure high colloidal stability in biological environments, improved magnetic performance, and dispersion in circulatory fluids and tissues. Herein, we report the design of a library of 21 peptides and ligands and identify highly stable self-assembled monolayers on the USPIONs’ surface. A total of 86 different peptide-coated USPIONs are prepared and selected using several stringent criteria, such as stability against electrolyte-induced aggregation in physiological conditions, prevention of nonspecific binding to cells, and absence of cellular toxicity and contrast-enhanced <i>in vivo</i> MRI. The bisphosphorylated peptide 2PG-S*VVVT-PEG4-ol provides the highest biocompatibility and performance for USPIONs, with no detectable toxicity or adhesion to live cells. The 2PG-S*VVVT-PEG4-ol-coated USPIONs show enhanced magnetic resonance properties, <i>r</i>₁ (2.4 mM^–1·s^–1) and <i>r</i>₂ (217.8 mM^–1·s^–1) relaxivities, and greater <i>r</i>₂/<i>r</i>₁ relaxivity ratios (>90) when compared to those of commercially available MRI contrast agents. Furthermore, we demonstrate the utility of 2PG-S*VVVT-PEG4-ol-coated USPIONs as a <i>T</i>₂ contrast agent for <i>in vivo</i> MRI applications. High contrast enhancement of the liver is achieved as well as detection of liver tumors, with significant improvement of the contrast-to-noise ratio of tumor-to-liver contrast. It is envisaged that the reported peptide-coated USPIONs have the potential to allow for the specific targeting of tumors and hence early detection of cancer by MRI