Single-molecule electrometry

Abstract

Mass and electrical charge are fundamental properties of biological macromolecules. Although molecular mass has long been determined with atomic precision, a direct and precise determination of molecular charge remains an outstanding challenge. Here we report high-precision (<1e) measurements of the electrical charge of molecules such as nucleic acids, and globular and disordered proteins in solution. The measurement is based on parallel external field-free trapping of single macromolecules, permits the estimation of a dielectric coefficient of the molecular interior and can be performed in real time. Further, we demonstrate the direct detection of single amino acid substitution and chemical modifications in proteins. As the electrical charge of a macromolecule strongly depends on its three-dimensional conformation, this kind of high-precision electrometry offers an approach to probe the structure, fluctuations and interactions of a single molecule in solution.

A simple model for electrical charge in globular macromolecules and linear polyelectrolytes in solution

“…While it is not always clear how some of the previous definitions and calculations of $q_{\text{eff}}$ can be directly and quantitatively related to particular experimental situations, our interaction-energy-based definition of $q_{\text{eff}}$ can be readily related to measurements 34 . Importantly, our approach supports direct incorporation of geometric and chemical information on the molecule, e.g., shape, non-uniform charge distribution, finite length in polyelectrolytes, which is vital for comparison with experiments.…”

“…The quantity that is not only readily measurable in our experiments, 34 but of general importance in the interaction of a molecule with its environment, is an interaction free energy. We therefore introduce the electrostatic interaction free energy as a means to determining an effective or renormalized charge that can not only be directly measured in experiment but also readily calculated and therefore compared with predictions of effective charge calculated using other approaches.…”

“…Table I compares predictions of effective charge from our full model, including both charge regulation and renormalization, with experimental measurements of effective charge using our recently developed method, “escape-time electrometry” (ET e ), 34 for several classes of biomolecules such as DNA and intrinsically disordered and globular proteins.…”

Pneumatically Controlled Nanofluidic Devices for Contact‐Free Trapping and Manipulation of Nanoparticles

“…Recent research in nanofluidics shows great interest in developing tools and systems to confine and manipulate nano‐objects in a stable and noninvasive manner. The reliable detection and trapping of individual objects provide sensitive information on local dynamics, which lead to a fundamental understanding of molecular interactions and physical properties of the trapped objects . The precise manipulation of trapped objects found further applications in, for instance, surface patterning, nanoparticle assembly, or transporting and sorting of particles .…”

“…Electrostatic trapping was demonstrated for levitating negatively and positively charged gold nanoparticles (Au NPs), lipid vesicles, and even for angular depended trapping of silver nanorods . Further, it initiated concepts in digital information storage and applications in screening the electrostatic potential in salt gradient solutions and in determining the size and charge of Au NPs or biomolecules . In GIE trapping, the potential depth of the nanotraps is dependent on the surface charge density of the polydimethylsiloxane (PDMS) and glass, the charge of the particles, the ionic strength of the buffer solution and the geometrical parameters of the pockets.…”