Kim K. W. Wong

Synthesis and characterization of hydrophobic ferritin proteins

Alkylated derivatives of the iron storage protein, ferritin, have been prepared by carbodiimide-activated coupling of long chain (C9, C12, C14) primary amines to surface carboxylic acid residues. In the case of a nonyl-derivatized protein, alkylation results in covalent modification of approximately 400 of the 520 amino acid carboxyl groups in the protein molecule. The hydrophobic proteins have a net positive charge in water and can be transferred from THF/water mixtures into dichloromethane, ethyl acetate and toluene by addition of small amounts of NaCl. Transmission electron microscopy, analytical ultracentrifugation, and polyacrylamide gel electrophoresis indicate that the hydrophobic proteins dissolve in the organic solvents as structurally intact, non-aggregated macromolecules which can be subsequently back-extracted into water.

Determination of the preexponential frequency factor for superparamagnetic maghemite particles in magnetoferritin

Magnetization and Mossbauer measurements on maghemite particles with an average particle diameter of 10 nm have been made in the temperature range from 5 K to 353 K spanning the superparamagnetic (SPM) and stable single domain (SD) regimes. The maghemite particles were produced within the iron-storage protein ferritin, resulting in a narrowly-sized, weakly interacting nanocomposite material called magnetoferritin. Experiments combining hysteresis measurements, low temperature remanence, and Mossbauer spectroscopy were used to characterize magnetoferritin and to provide experimental estimates of (1) the pre-exponential frequency factor ƒ0 in the Neel-Arhennius relaxation equation; (2) the SPM threshold size at room temperature for maghemite; and (3) the SD value of Hr/Hc at 0 K. The frequency factor was determined from the difference in blocking temperatures measured by dc magnetization and Mossbauer spectroscopy, yielding a value of f0≈109 Hz. This agrees well with the standard value and justifies the usually assumed superparamagnetic blocking condition of KV = 25 kT for remanence measurements. The SPM threshold size at room temperature for remanence measurements was estimated to be 20–27 nm and the extrapolated SD value at 0 K for Hr/Hc is 1.32. The latter value is slightly larger than the theoretical value of 1.09 but may be more appropriate for weakly interacting SD particles commonly found in sediments and soils. However, ƒ0 for ferrimagnetic magnetoferritin is a factor of 103 lower than was determined previously for native ferritin, which contains antiferromagnetic ferrihydrite cores. The difference in ƒ0 values between the two varieties of ferritin is probably related to the two different types of magnetic spin ordering of the core minerals and suggests that the higher value of ƒ0 is more appropriate for antiferromagnetic minerals like hematite and goethite, whereas the lower value is more appropriate for ferrimagnetic minerals like maghemite, magnetite, or greigite.

Properties of magnetoferritin: a novel biomagnetic nanoparticle

Abstract A soluble and biocompatible magnetic material would have many possible applications. An approach to producing biomagnetic nanophase particles with large magnetic moments is to reconstitute the iron-storage protein ferritin under conditions tailored to the synthesis of magnetite, rather than the normal ferrihydrite, in the core. The material produced in this way is called magnetoferritin. Mossbauer spectroscopy is an ideal technique for characterising such iron-containing nanophase systems. The Mossbauer spectra obtained at low temperatures and in large applied magnetic fields clearly show that the magnetoferritin core is quite different to that of ferritin. The changes in the spectra with increasing applied field indicate that the core is a ferrimagnet. Computer analysis of these spectra leads to the conclusion that the magnetoferritin core is very similar to the mineral maghemite (γ-Fe 2 O 3 ).

Classical and quantum magnetism in synthetic ferritin proteins

The magnetic properties of nanometer‐scale particles are studied using the protein‐complex ferritin as a vesicle for either an antiferromagnet or a ferrimagnet. For antiferromagnetic ferritin particles, the anisotropy energy is found to depend linearly on the particle volume, suggesting that bulk anisotropy dominates over surface anisotropy. Effects due to the bulk and surface spins are discerned at high magnetic fields (27 T). At very low magnetic fields (1 nT) and temperatures (20 mK), the tunneling frequency of the Neel vector is observed to scale exponentially with the particle volume, consistent with the linear dependence of the anisotropy barrier on volume and with theories of macroscopic quantum coherence. In the ferrimagnetic particles, the anisotropy barrier decreases for smaller particles while simultaneously displaying a slight increase in coercivity and a dramatic decrease in the remanence over three orders of magnitude.

Hydrophobic proteins: synthesis and characterisation of organic-soluble alkylated ferritins

Alkylated derivatives of the iron storage protein, ferritin, have been prepared by carbodiimide-activated coupling of long chain primary amines to surface carboxylic acid residues; the proteins are soluble in dichloromethane as intact, non-aggregated biomolecules.

Small-scale structures in biomineralisation and biomimetic materials chemistry

The study of biomineralisation and the biomimetic synthesis of inorganic materials with small-scale structures is a continuing area of growth because of the demand for inorganic colloids with ever decreasing particle sizes, complex morphological shape, hybrid structure and superlattice organisation. Some of the continuing advances are taking place at the interface between bio-inorganic and materials chemistry, with the focus on structures from single-celled biological systems that have dimensions from the supramolecular to those generally less than,a few micrometres in size.

Fullerates: interaction of divalent metal ions with Langmuir monolayers and multilayers of mono-substituted C60–malonic acid

Stable Langmuir monolayers of C60[C(CO2H)2] are formed on pure water and on solutions containing Ca2+ or Cd2+ ions; catioN–Headgroup interactions result in expanded monolayers and the transfer of Ca2+–fullerene multilayers onto quartz substrates.

Polymer-mediated crystallisation of inorganic solids: calcite nucleation on the surfaces of inorganic polymers

Nucleation of calcite from supersaturated calcium hydrogancarbonate solution has been achieved on the surfaces of poly(dimethylsiloxane) films formed by cross-linking in the presence of catalytic quantities of dibutyltindilaurate or zinc octanoate. Films containing high concentrations of the diorganotin catalyst produced, in addition to calcite, amorphous calcium silicate and calcium silanolate under similar conditions. Polar functional groups incorporated into the polysiloxane network served to promote calcite growth, but prolonged heating of all films at 90 °C rendered them inactive.