Optochemical disequilibrium to measure biomolecule charge

F. M. Möller, M. Kieß, C. Mast, D. Braun

Physical Review E https://doi.org/10.1103/PhysRevE.98.062601

Abstract

Measuring the effective charge of a molecule inside a conducting fluid is difficult. One of the commonly used techniques is electrophoresis in centimeter-sized gels. Here we implemented a miniaturized, electrophoresis-based method on the microscale using an optically driven photodissociation reaction. Inside a focused laser beam, oppositely charged species of different size are created which diffuse out of the beam with different velocities. This creates a local electrical field which moves charged biomolecules via electrophoresis in the diffusion-dominated regime of low Peclet numbers (Pe < 1). The steady state fluorescence in the center is used to determine the effective biomolecule charge even for large molecules with charge-independent electrophoretic mobilities. We tested the mechanism for DNA and proteins and used two different photochemical reactions to generate inward or outward pointing electrical fields. A master curve described the proton transfer from the photodissociation reaction to the buffer. The full mechanistic understanding offers many possibilities for the all-optical manipulation of molecules.