The development of a voltmeter for solution EPR and its application to measure voltages of myosin biochemical states

This thesis establishes a method to measure electrostatic potential at spin labels on proteins with millivolt accuracy. Chapter 1 provides subject background and an outline of the work. Chapter 2 details the theory and presents small nitroxide experiments performed to calibrate a set of fast-relaxing paramagnetic metal complexes for these measurements. Chapter 3 applies the method to the spin-labeled protein myosin fragment S1. Chapters 2 and 3 are aimed at journal submission and have the structure of papers. The former is close to submission while the latter requires additional experiments.;The key feature of this research was the selection and calibration of 3 transition metal complexes that efficiently relax nitroxide spin labels in EPR accessibility measurements through collisional Heisenberg spin exchange. The tasks to establish this set of paramagnetic relaxation agents (PRAs) were to (1) select complexes with picosecond relaxation times carrying different charges that are the same size to null out steric accessibility, (2) determine that nitroxide accessibility measurements with these agents uniquely involve relaxation through collisional spin exchange, i.e. relaxation through dipolar coupling is insignificant, (3) measure/derive PRA spin exchange rates with respect to a neutral nitroxide to correct for differences in efficiency in real applications, (4) apply the set to a real application, namely myosin S1.;The development of this method depended critically on PRA calibration with small nitroxides of known structure and varying charge. This required the development of global lineshape fitting, a new analysis tool which adds the dimension of microwave power. Unlike ordinary lineshape fitting done on individual low power spectra, global lineshape fitting is performed on sets of progressive saturation spectra. By adding the dimension of power, global lineshape fitting measures an accurate spin-lattice relaxation rate ( R1) in addition to the transverse relaxation rate ( R2) and inhomogeneous linewidth (ΓG) already separable with ordinary lineshape fitting. Thus for the first time R1 has been separately measured from conventional EPR power saturation spectra in many nitroxide experiments reported here. R1 must be separated in this way for small nitroxides to accurately determine collisional spin exchange rate.