Direct molecular level measurements of the electrostatic properties of a protein surface

Abstract

In this work, we used direct measurements with the surface force apparatus to determine the pHdependent electrostatic charge density of a single binding face of streptavidin. Mean field calculations have been used with considerable success to model electrostatic potential fields near protein surfaces, but these models and their inherent assumptions have not been tested directly at the molecular level. Using the force apparatus and immobilized, oriented monolayers of streptavidin, we measured a pI of 5-5.

Functionalized SnO2 nanobelt field-effect transistor sensors for label-free detection of cardiac troponin

“…Streptavidin caused a marked decrease (180 ns, 27%) in FET conductance at pH 9.0, a significant increase (350 ns, 130%) at pH 4.0, and negligible change at pH 7.2. These results are fully consistent with the pH-dependent variation in net charge of streptavidin (Sivasankar et al, 1998), which has an isoelectric point (pI) near 7.0 and net charge per molecule of −3.2e at pH 9.0, or +10.4e at pH 4.0. The decrease (increase) in FET channel conductance is caused by the negatively (positively) charged state of streptavidin at pH 9.0 (pH 4.0), which, upon binding biotin on the SnO 2 Fig.…”

Field-effect sensors – from pH sensing to biosensing: sensitivity enhancement using streptavidin–biotin as a model system

“…The pI of surfacebound streptavidin has been measured using Surface Force Apparatus (SFA) and Atomic Force Microscopy which demonstrated a pI of 5.0 ± 0.5. 79,83 Experimental determination of streptavidin pI is discussed further in ESI section 4. † Other biomolecules.…”

Nearly Instantaneous, Cation-Independent, High Selectivity Nucleic Acid Hybridization to DNA Microarrays

“…SA attached to a solid support has a pI near 5.5 (8). Therefore, SA is a highly anionic protein at neutral pH.…”