Protein Dielectrophoresis in Solution

Abstract

Proteins experience either pulling or repelling force from the gradient of an external electric field due to the effect known as dielectrophoresis (DEP). The susceptibility to the field gradient is traditionally calculated from the solution of the electrostatic boundary-value problem, which requires assigning a dielectric constant to the protein. This assignment is essential since the DEP susceptibility is proportional, in dielectric theories, to the Clausius-Mossotti factor, the sign of which is controlled by whether the protein dielectric constant is below (repelling) or above (pulling) the dielectric constant of water. The dielectric constant is not uniquely or even well-defined for a particle of molecular size and the Clausius-Mossotti factor is shown here to be inadequate for describing the dipolar response of the protein and hydration water. An alternative theory is developed from the standpoint of molecular properties of the protein solute and water solvent. The effective polarity of the protein molecule enters the theory in terms of the variance of its molecular dipole moment and its refractive index. Molecular dynamics (MD) simulations of the protein cytochrome c in solution are performed to calculate the dipolar susceptibilities entering the theory. We find that tumbling of the protein on the nanosecond time scale results in a positive DEP (pulling). The DEP susceptibility for cytochrome c acquired from MD simulations is 10-10 times higher than predicted by the Clausius-Mossotti factor. Nevertheless, this high DEP susceptibility is fully consistent with empirically confirmed Oncley's equation connecting the protein dipole to dielectric increments of protein solutions. For cytochrome c, high DEP susceptibilities calculated from MD are consistent with experimental dielectric data. We provide a general relation connecting the DEP susceptibility to the dielectric increment of solution.

Limitations of the Clausius‐Mossotti function used in dielectrophoresis and electrical impedance studies of biomacromolecules

“…Although it does not address the situation of a polar macromolecule suspended in a polar fluid, it does provide guidance as to how some key aspects can be tackled. Seyedi and Matyushov have calculated the effective polarizability of cytochrome‐c in aqueous solution using molecular dynamics simulations that incorporate both rotational relaxation of the protein dipole and the dipole moment of water coupled to the protein. The polarizability is found to be 10 3 −10 4 times higher than predicted by the standard electrostatic theory that leads to Eq.…”

“…. Correlations of the fluctuations of protein molecular charges with molecular dipoles of the hydration shell are found to be particularly important . Seyedi and Matyushov derive an expression for the dipole moment μ p that is equivalent to Eq.…”

“…Equation thus implies that in each case the field E i within the volume occupied by the protein is about 1.3‐times the field E o in the surrounding medium. The internal field E i is sensitive not only to the statistical formation of dipole charges at the protein‐water interface determined by local molecular structure , but also by the geometrical polarization factor A in Eq. .…”

AC electrokinetic immobilization of organic dye molecules

“…On the electrode surface, the electric field gradient was minimal and consequently the dielectrophoretic force was expected to be weak there and thus follows that accumulation of fluorescence dye molecules on the electrode surfaces cannot be the result of standard DEP alone, which requires a strong field gradient. In recent theoretical studies applying molecular dynamics simulations [29] and a more thorough derivation of the Clausius-Mossotti-factor [30], it has been argued that established DEP theory cannot simply be employed to molecules. As additional factors, the molecule's permanent dipole moment as well as correlations of the fluctuations of molecular charges with the hydration shell's dipoles should be taken into account.…”

3-Dimensional Electric Field Distributions of Castellated and Straight Dielectrophoresis Electrodes for Cell Separation

“…The separation of particles occurs when the particles respond toward different frequencies when an alternating current (AC) is applied. Examples of biological particles that can be separated are stem cells (Pethig et al 2010), red blood cells (Yunus et al 2017), proteins (Seyedi & Matyushov 2018), DNA (Nakano et al 2011), yeast cells (Çetin et al 2009), breast cancer cells (Becker et al 1995), bacteria (Pethig 2010;Stevens & Jaykus 2004), and platelets (Piacentini et al 2011a).…”