Nanoscale materials provide new opportunities to interface solid-state electronics with biomolecules and biochemical activity. For example, single-walled carbon nanotubes (SWNTs) have the special property of electronic resistance that is sensitive to single electrons. We have exploited this sensitivity to build nanoelectronic biosensors that monitor the biochemical activity of individual proteins . As an attached protein moves, binds, or performs catalysis, its charged amino acid sidechains induce resistance fluctuations in the SWNT device that may be monitored with microsecond resolution .
Recently, we have used this measurement platform for single-molecule measurements of DNA polymerases . Polymerases are the key enzymes for converting single-stranded DNA to double-stranded helices, the primary step in DNA replication, amplification, and most sequencing technologies. When a polymerase processing DNA is also attached to a SWNT device, the electrical signal provides a high-resolution readout of single-nucleotide incorporations and exciting possibilities for high-density, high-throughput electronic DNA sequencing.
To investigate the feasibility of electronic DNA sequencing, we have compared single-molecule transduction by DNA polymerases from three different organisms. By working with multiple families of DNA polymerases, we have tested the applicability of the electronic technique while also generating detailed records of differences among the enzymes. For example, we observe an anomalous rate variability when measuring the polymerase from the bacillus phage φ29. Base incorporation rates average 20 s-1 for most the enzymes processing single-stranded DNA templates, but rates up to 200 and 400 s-1 occurred when φ29 encountered homopolymeric sequences of poly(dT) or poly(dC), respectively. When processing poly(dA) and poly(dG) sequences, on the other hand, φ29 had bursts of activity interrupted by pauses lasting 50 to 300 s. This sequence-dependent activity illustrates one way that single-molecule methods reveal information hidden in ensemble-based techniques.
Another workhorse protein in DNA sequencing technologies is the DNA polymerase derived from the thermophilic bacteria Thermus aquaticus (Taq). Anomalous stability at high temperatures makes Taq a unique enzyme for the polymerase chain reaction (PCR) and commercial amplification of DNA. In a first for single-molecule biophysics, SWNT devices have recorded Taq activity over a wide temperature range from 22 to 94 °C, including the typical PCR operating temperature of 72 °C. Even operating at this high temperature, the technique resolved Taq testing incoming nucleotides for complementarity and incorporating correct matches in the base-by-base construction of Watson-Crick pairs. The detailed recordings reveal the similarities of Taq’s operation to other, room temperature polymerases.
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