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.

Influence of pK(a) shifts on the calculated dipole moments of proteins.

The protein dipole moment is a low-resolution parameter that characterizes the second-order charge organization of a biomolecule. Theoretical approaches to calculate protein dipole moments rely on pK(a) values, which are either computed individually for each ionizable residue or obtained from model compounds. The influence of pK(a) shifts are evaluated first by comparing calculated and measured dipole moments of ß-lactoglobulin. Second, calculations are made on a dataset of 66 proteins from the Protein Data Bank, and average differences are determined between dipole moments calculated with model pK(a)s, pK(a)s derived using a Poisson-Boltzmann approach, and empirically-calculated pK(a)s. Dipole moment predictions that neglect pK(a) shifts are consistently larger than predictions in which they are included. The importance of pK(a) shifts are observed to vary with protein size, internal permittivity, and solution pH.