title: Protein brownian rotation at the glass transition temperature of a freeze-concentrated buffer probed by superparamagnetic nanoparticles.
abstract: For applications from food science to the freeze-thawing of proteins it is important to understand the often complex freezing behavior of solutions of biomolecules. Here we use a magnetic method to monitor the Brownian rotation of a quasi-spherical cage-shaped protein, apoferritin, approaching the glass transition Tg in a freeze-concentrated buffer (Tris-HCl). The protein incorporates a synthetic magnetic nanoparticle (Co-doped Fe3O4 (magnetite)). We use the magnetic signal from the nanoparticles to monitor the protein orientation. As T decreases toward Tg of the buffer solution the protein's rotational relaxation time increases exponentially, taking values in the range from a few seconds up to thousands of seconds, i.e., orders of magnitude greater than usually accessed, e.g., by NMR. The longest relaxation times measured correspond to estimated viscosities >2 MPa s. As well as being a means to study low-temperature, high-viscosity environments, our method provides evidence that, for the cooling protocol used, the following applies: 1), the concentration of the freeze-concentrated buffer at Tg is independent of its initial concentration; 2), little protein adsorption takes place at the interface between ice and buffer; and 3), the protein is free to rotate even at temperatures as low as 207 K.
authors.affiliation: H. H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom.
authors.forename: J-C
authors.initials: JC
authors.lastname: Eloi
authors.forename: M
authors.initials: M
authors.lastname: Okuda
authors.forename: S E Ward
authors.initials: SE
authors.lastname: Jones
authors.forename: W
authors.initials: W
authors.lastname: Schwarzacher
mesh_terms: D001052:Apoferritins / Q000737:chemistry; D002021:Buffers; D005615:Freezing; D007700:Kinetics; D058185:Magnetite Nanoparticles / Q000737:chemistry*; D012399:Rotation; D013696:Temperature; D014783:Viscosity; D058989:Vitrification
chemical_list: D002021:Buffers; D058185:Magnetite Nanoparticles; D001052:Apoferritins