Steven R. Hunt

Single-Molecule Lysozyme Dynamics Monitored by an Electronic Circuit

Observing Protein Dynamics Following the dynamics of protein conformational changes over the relatively long periods of time typical of enzyme kinetics can be challenging. Choi et al. (p. 319; see the Perspective by Lu) were able to observe changes in lysozyme conformation, which changes its electrostatic potential, by using a carbon-nanotube field-effect transistor. Slower hydrolysis steps were compared with faster, but unproductive, hinge motion, and changes in lysozyme activity that occur with pH were shown to arise from differences in the relative amount of time spent in processive versus nonprocessive states. Changes in protein conformation can be detected via changes in electrostatic potential with a carbon nanotube transistor. Tethering a single lysozyme molecule to a carbon nanotube field-effect transistor produced a stable, high-bandwidth transducer for protein motion. Electronic monitoring during 10-minute periods extended well beyond the limitations of fluorescence techniques to uncover dynamic disorder within a single molecule and establish lysozyme as a processive enzyme. On average, 100 chemical bonds are processively hydrolyzed, at 15-hertz rates, before lysozyme returns to its nonproductive, 330-hertz hinge motion. Statistical analysis differentiated single-step hinge closure from enzyme opening, which requires two steps. Seven independent time scales governing lysozyme’s activity were observed. The pH dependence of lysozyme activity arises not from changes to its processive kinetics but rather from increasing time spent in either nonproductive rapid motions or an inactive, closed conformation.

Single Molecule Dynamics of Lysozyme Processing Distinguishes Linear and Cross-linked Peptidoglycan Substrates

The dynamic processivity of individual T4 lysozyme molecules was monitored in the presence of either linear or cross-linked peptidoglycan substrates. Single-molecule monitoring was accomplished using a novel electronic technique in which lysozyme molecules were tethered to single-walled carbon nanotube field-effect transistors through pyrene linker molecules. The substrate-driven hinge-bending motions of lysozyme induced dynamic electronic signals in the underlying transistor, allowing long-term monitoring of the same molecule without the limitations of optical quenching or bleaching. For both substrates, lysozyme exhibited processive low turnover rates of 20-50 s(-1) and rapid (200-400 s(-1)) nonproductive motions. The latter nonproductive binding events occupied 43% of the enzymes time in the presence of the cross-linked peptidoglycan but only 7% with the linear substrate. Furthermore, lysozyme catalyzed the hydrolysis of glycosidic bonds to the end of the linear substrate but appeared to sidestep the peptide cross-links to zigzag through the wild-type substrate.

Graphitic Electrical Contacts to Metallic Single Walled Carbon Nanotubes Using Pt Electrodes

We investigate electronic devices consisting of individual, metallic, single-walled carbon nanotubes contacted by Pt electrodes in a field effect transistor configuration, focusing on improvements to the metal-nanotube contact resistance as the devices are annealed in inert environments including ultrahigh vacuum. At moderate temperatures (T < 880 K), thermal processing results in high resistance contacts with thermally activated barriers. Higher temperatures (T > 880 K) achieve nearly transparent contacts. In the latter case, analytical surface measurements reveal the catalytic decomposition of hydrocarbons into graphene layers on the Pt surface, suggesting that improved electronic behavior is primarily due to the formation of an all-carbon nanotube-graphite interface rather than to the improvement of the nanotube-Pt one.

Scanning Gate Spectroscopy and Its Application to Carbon Nanotube Defects

A variation of scanning gate microscopy (SGM) is demonstrated in which this imaging mode is extended into an electrostatic spectroscopy. Continuous variation of the SGM probes electrostatic potential is used to directly resolve the energy spectrum of localized electronic scattering in functioning, molecular scale devices. The technique is applied to the energy-dependent carrier scattering that occurs at defect sites in carbon nanotube transistors, and fitting energy-resolved experimental data to a simple transmission model determines the electronic character of each defect site. For example, a phenolic type of covalent defect is revealed to produce a tunnel barrier 0.1 eV high and 0.5 nm wide.

Sensitivity of point defects in one dimensional nanocircuits

When tailored to contain a single resistive defect, one dimensional nanocircuits can realize high dynamic range, high bandwidth transduction of single molecule chemical events. The physical mechanisms behind this sensitive transduction, however, remain poorly understood. Here, we complement ongoing sensing measurements with scanning probe characterization of the electronic properties of defects. The high sensitivity of defect sites is directly probed, and is found to be in excellent agreement with a finite element model containing realistic device parameters for the defect sites. The model illuminates the most likely sensing mechanisms of these single molecule circuits, and fully supports the premise that further tailoring of the defect sites could enable the chemically selective interrogation of a wide range of complex molecular interactions.