A highly sensitive analysis method for biological molecules is required because of recent developments in molecular biology. Conventional highly sensitive detection methods are based on labelling techniques using fluorescent dyes or enzymes. However, since those techniques involved some problems with signal instability, a new technology has been sought. Because the superconducting interference device (SQUID) is an extremely highly sensitive magnetic sensor, it can be applied to the highly sensitive detection of DNA labelled with small magnetic particles. The signal from SQUID is stable in contrast to fluorescent dyes and enzymes, therefore it permits highly sensitive measurement over long periods of time. Sample coverslips on which the small magnetic particles were anchored using biotin labelled DNA were prepared to demonstrate the availability of this method. Scanning the high-T_c SQUID sensor on the coverslip demonstrated that the magnetic flux on the coverslip agreed well with the pattern of labelled DNA anchored on the coverslip. This result suggests that SQUID can be applied for the specific detection of DNA molecules, especially for the detection of DNA chips.

DNA replication, repair, and recombination in the cell play a significant role in the regulation of the inheritance, maintenance, and transfer of genetic information. To elucidate the biomolecular mechanism in the cell, some molecular models of DNA replication, repair, and recombination have been proposed. These biological studies have been conducted using bulk assays, such as gel electrophoresis. Because in bulk assays, several millions of biomolecules are subjected to analysis, the results of the biological analysis only reveal the average behavior of a large number of biomolecules. Therefore, revealing the elementary biological processes of a protein acting on DNA (e.g., the binding of protein to DNA, DNA synthesis, the pause of DNA synthesis, and the release of protein from DNA) is difficult. Single-molecule imaging allows the analysis of the dynamic behaviors of individual biomolecules that are hidden during bulk experiments. Thus, the methods for single-molecule imaging have provided new insights into almost all of the aspects of the elementary processes of DNA replication, repair, and recombination. However, in an aqueous solution, DNA molecules are in a randomly coiled state. Thus, the manipulation of the physical form of the single DNA molecules is important. In this review, we provide an overview of the unique studies on DNA manipulation and single-molecule imaging to analyze the dynamic interaction between DNA and protein.

Continued advancement of protein array, bioelectrode, and biosensor technologies is necessary to develop methods for higher amount and highly oriented immobilization activity of proteins. In pursuit of these goals, we developed a new immobilization method by combining electrostatic transport and subsequent molecular diffusion of protein molecules. Our developed immobilization method is based on a model that transports proteins toward the substrate surface due to steep concentration gradient generated by low-frequency AC electric field. The immobilization of the maximum amounts can be obtained by the application of the AC voltage of 80 Vpp, 20 Hz both for His-tagged Green Fluorescent Protein (GFP) and Discosoma sp. Red Fluorescent Protein (DsRed), used as model proteins. The amounts of the immobilized His-tagged GFP and DsRed were approximately seven-fold higher than that in the absence of the application of low-frequency AC electric field. Furthermore, the positively and negatively charged His-tagged GFP at acidic and alkaline pH were immobilized by applying of low-frequency AC electric field, whereas the non-charged His-tagged GFP at the pH corresponding to its isoelectric point (pI) was not immobilized. Therefore, unless the pH is equal to pI, the immobilization of electrically charged proteins was strongly enhanced through electrostatic transport and subsequent molecular diffusion.