Until recently, no effective experimental method has been developed to observe biomolecular polarization at the molecular scale. In general, polarization measurement requires a metal substrate to apply a well defined electric field to the target molecules.
This requirement can inactivate biomolecules and conflicts with the need for insulating substrates, such as mica, to be used to support biomolecules on surfaces while retaining the biomolecular functions.
In a recent study, which was published in Nanotechnology, researchers in Japan reported that the polarization of molecules could be imaged even on thick (millimeter-scale) insulating substrates. This development was achieved by using an inhomogeneous electric field around a tip apex and frequency-mode electrostatic force detection.
The figure shows capacitance images for DNA and DNA-T7 RNA polymerase complex on a mica substrate. The plots reveal the contrast inversion at the cross point of Vb = –2.0 V. The phenomenon is attributed to the differences in polarizability between DNA and transcription complex on a mica substrate. This suggests that the dielectric constant of the transcription complex is larger than that of DNA, indicating the large and flexible molecular structure of T7 RNA polymerase.
The results open up the possibility of studying the electrostatic properties of biological molecules and complexes on a one-by-one basis on commonly used insulator substrates with reduced denaturing. Using different responses to external electric fields, this method is potentially useful for discriminating between biological molecules and complexes, such as small proteins, DNA oligomers and knotted-string-like tangled regions of DNA, which have similar topographic appearances.