Robert M. J. Jacobs

Protein self-diffusion in crowded solutions

Macromolecular crowding in biological media is an essential factor for cellular function. The interplay of intermolecular interactions at multiple time and length scales governs a fine-tuned system of reaction and transport processes, including particularly protein diffusion as a limiting or driving factor. Using quasielastic neutron backscattering, we probe the protein self-diffusion in crowded aqueous solutions of bovine serum albumin on nanosecond time and nanometer length scales employing the same protein as crowding agent. The measured diffusion coefficient D(φ) strongly decreases with increasing protein volume fraction φ explored within 7% ≤ φ ≤ 30%. With an ellipsoidal protein model and an analytical framework involving colloid diffusion theory, we separate the rotational Dr(φ) and translational Dt(φ) contributions to D(φ). The resulting Dt(φ) is described by short-time self-diffusion of effective spheres. Protein self-diffusion at biological volume fractions is found to be slowed down to 20% of the dilute limit solely due to hydrodynamic interactions.

TOF-2: A Large 1D Channel Thorium Organic Framework

A new neutral 1D channel thorium organic framework material (TOF-2) has been synthesized under hydrothermal conditions. TOF-2 exhibits a hexagonal channel structure consisting of eight-coordinate ThO6F2 polyhedra and 1,3,5-benzentricarboxylate ligands. The channels run along the c-axis and are approximately 13 A in diameter. The single-crystal X-ray structure suggests that the amount of void space is 41%. The structure is stable to ca. 400 degrees C. Gas adsorption measurements show deferential gas uptake behavior.

Optical properties of pentacene and perfluoropentacene thin films.

The optical properties of pentacene (PEN) and perfluoropentacene (PFP) thin films on various SiO(2) substrates were studied using variable angle spectroscopic ellipsometry. Structural characterization was performed using x-ray reflectivity and atomic force microscopy. A uniaxial model with the optic axis normal to the sample surface was used to analyze the ellipsometry data. A strong optical anisotropy was observed, and enabled the direction of the transition dipole of the absorption bands to be determined. Furthermore, comparison of the optical constants of PEN and PFP thin films with the absorption spectra of the monomers in solution shows significant changes due to the crystalline environment. Relative to the monomer spectrum, the highest occupied molecular orbital to lowest unoccupied molecular orbital transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV). A second absorption band in the PFP thin film shows a slight blueshift (40 meV) compared to the spectrum of the monomer with its transition dipole perpendicular to that of the first absorption band.

Protein-protein interactions in ovalbumin solutions studied by small angle scattering: effect of ionic strength, and the chemical nature of cations

The influence of ionic strength and of the chemical nature of cations on the protein-protein interactions in ovalbumin solution was studied using small-angle X-ray and neutron scattering (SAXS/SANS). The globular protein ovalbumin is found in dimeric form in solutions as suggested by SANS/SAXS experiments. Due to the negative charge of the proteins at neutral pH, the protein-protein interactions without any salt addition are dominated by electrostatic repulsion. A structure factor related to screened Coulombic interactions together with an ellipsoid form factor was used to fit the scattering intensity. A monovalent salt (NaCl) and a trivalent salt (YCl(3)) were used to study the effect of the chemical nature of cations on the interaction in protein solutions. Upon addition of NaCl, with ionic strength below that of physiological conditions (150 mM), the effective interactions are still dominated by the surface charge of the proteins and the scattering data can be understood using the same model. When yttrium chloride was used, a reentrant condensation behavior, i.e., aggregation and subsequent redissolution of proteins with increasing salt concentration, was observed. SAXS measurements reveal a transition from effective repulsion to attraction with increasing salt concentration. The solutions in the reentrant regime become unstable after long times (several days). The results are discussed and compared with those from bovine serum albumin (BSA) in solutions.

Universality of protein reentrant condensation in solution induced by multivalent metal ions

The effective interactions and phase behavior of protein solutions under strong electrostatic coupling conditions are difficult to understand due to the complex charge pattern and irregular geometry of protein surfaces. This distinguishes them from related systems such as DNA or conventional colloids. In this work, we discuss the question of universality of the reentrant condensation (RC) of proteins in solution induced by multivalent counterions, i.e., redissolution on adding further salts after phase separation, as recently discovered (Zhang et al., Phys Rev Lett 2008; 101:148101). The discussion is based on a systematic investigation of five different proteins with different charge patterns and five different multivalent counterions. Zeta potential measurements confirm the effective charge inversion of proteins in the reentrant regime via binding of multivalent counterions, which is supported by Monte Carlo simulations. Charge inversion by trivalent cations requires an overall negative net charge of the protein. Statistical analysis of a representative set of protein sequences reveals that, in theory, this effect could be possible for about half of all proteins. Our results can be exploited for the control of the phase behavior of proteins, in particular facilitating protein crystallization. Proteins 2010.

Hydration and interactions in protein solutions containing concentrated electrolytes studied by small-angle scattering

During protein crystallization and purification, proteins are commonly found in concentrated salt solutions. The exact interplay of the hydration shell, the salt ions, and protein-protein interactions under these conditions is far from being understood on a fundamental level, despite the obvious practical relevance. We have studied a model globular protein (bovine serum albumin, BSA) in concentrated salt solutions by small-angle neutron scattering (SANS). The data are also compared to previous studies using SAXS. The SANS results for dilute protein solutions give an averaged volume of BSA of 91,700 Å(3), which is about 37% smaller than that determined by SAXS. The difference in volume corresponds to the contribution of a hydration shell with a hydration level of 0.30 g g(-1) protein. The forward intensity I(0) determined from Guinier analysis is used to determine the second virial coefficient, A(2), which describes the overall protein interactions in solution. It is found that A(2) follows the reverse order of the Hofmeister series, i.e. (NH(4))(2)SO(4) < Na(2)SO(4) < NaOAc < NaCl < NaNO(3) < NaSCN. The dimensionless second virial coefficient B(2), corrected for the particle volume and molecular weight, has been calculated using different approaches, and shows that B(2) with corrections for hydration and the non-spherical shape of the protein describes the interactions better than those determined from the bare protein. SANS data are further analyzed in the full q-range using liquid theoretical approaches, which gives results consistent with the A(2) analysis and the experimental structure factor.

An AFM Study of the Correlation of Lead Dioxide Electrocatalytic Activity with Observed Morphology.

PbO2 is widely employed as an electrocatalyst for anodic oxidation processes including the generation of oxygen and the degradation of various organic species in aqueous solution. However, despite extensive investigation, the precise mechanism of action remains obscure. In this paper we establish a previously unrecognized strong correlation between the morphology of the PbO2 deposits and their electrocatalytic activity. Cyclic voltammetric results are described for the electrodeposition of PbO2 on boron-doped diamond (BDD) electrodes from 2.5 mM solutions of lead(II) nitrate in nitric acid at pH values between approximately 0 and 7. A likely change in mechanism is observed around pH 4, consistent with the Pourbaix diagram of lead. The morphology of the PbO2 films is observed as a function of time and potential, using in situ AFM in each of the lead solutions. Information on the growth rates of the films is extracted, and the limitations of using AFM in such an application are discussed. It is shown that the deposit morphology depends strongly on the specific conditions used. The oxidation of a 100 mM glucose solution on electrodes modified by PbO2 deposition at a range of potentials and pH values is used as an indicator of the catalytic activity of the corresponding films, leading to the observation of the correlation between deposit morphology and catalytic activity mentioned above.

Charge-controlled metastable liquid–liquid phase separation in protein solutions as a universal pathway towards crystallization

We demonstrate that a metastable liquid–liquid phase separation (LLPS) in protein aqueous solutions can be induced by multivalent metal ions at room temperature. We determine the salt and protein partitioning in the two coexisting phases. The structure factor obtained by small angle X-ray scattering provides direct evidence for a short-ranged attraction, which leads to the metastability of the LLPS. An extended phase diagram with three control parameters (temperature, protein and salt concentration) provides a conclusive physical picture consistent with a criterion for the second virial coefficient. The presented isothermal control mechanism of the phase behavior opens new perspectives for the understanding of controlled phase behavior in nature. Furthermore, we discuss the application of this framework in predicting and optimizing conditions for protein crystallization.

Dopant-induced bandgap shift in Al-doped ZnO thin films prepared by spray pyrolysis

A series of 1 at. % Al-doped ZnO (AZO) films were deposited onto glass substrates by a spray pyrolysis technique. We find that the observed blue shift in the optical bandgap of 1% AZO films is dominated by the Burstein Moss effect. The Fermi level for an 807 nm thick AZO film rose by some 0.16 eV with respect to the edge of the conduction band. By controlling the film thickness, all AZO films exhibit the same lattice strain values. The influence of strain-induced bandgap shift was excluded by selecting films with nearly the same level of bandgap volume-deformation potentials, and the differences in out-plain strain and in-plain stress remained effectively constant.

Anion templated formation of pseudorotaxane and rotaxane monolayers on gold from neutral components.

The surface covalent attachment of indolocarbazole axles enables anion templation to be exploited in the formation of pseudorotaxane assemblies via the threading of neutral isophthalamide macrocycles from solution. The anion selectivity of this templating process can be monitored by a number of surface spectroscopic methods and shows subtle differences compared to the same process in solution. Though the fluxional and disordered nature of ethylene glycol extended axle adlayers prohibits detectable threading on the surface, rotaxane monolayers can be generated by a preassociation of the components and templating anion in solution. The threaded macrocycles therein can subsequently be released and detected by mass spectrometry by reductive stripping of the axle.