Increased bandwidth for dielectric spectroscopy of proteins through electrode surface preparation.

Dielectric spectroscopy measurements of liquids are often limited by electrode polarization. The influence of surface polishing and deposition of the conducting polymer polypyrrole/polystyrenesulfonate (PPy/PSS) on the polarization impedance is investigated. A quantitative description of the electrode polarization contribution to the real-valued permittivity spectrum is derived. This description explains the origin of the ω(-const). (const.>1) dependency commonly observed in permittivity measurements. Electrode surface roughness is correlated with both the magnitude and phase of the constant phase element. Generally, rougher electrodes have better performance, and an order of magnitude bandwidth improvement is achieved using PPy/PSS electrodes.

Method for estimating the internal permittivity of proteins using dielectric spectroscopy.

Protein charge organization is dependent on the low-permittivity region in the hydrophobic core of the molecule. We suggest a novel approach to estimate the dielectric constant of this region by comparing measured and simulated first- and second-order charge moments. Here, the dipole moment is measured as a function of pH using dielectric spectroscopy. The results are compared to dipole moments based on Poisson-Boltzmann estimates of pK(a) shifts calculated from structures in the Protein Data Bank. Structures are additionally refined using CHARMM molecular dynamics simulations. The best estimate for the internal permittivity is found by minimizing the root-mean-square residual between measured and predicted charge moments. Using the protein ß-lactoglobulin, a core dielectric constant in the range of 6-7 is estimated.