title: Optimization of dynamic nuclear polarization experiments in aqueous solution at 15 MHz/9.7 GHz: a comparative study with DNP at 140 MHz/94 GHz.
abstract: Dynamic nuclear polarization is emerging as a potential tool to increase the sensitivity of NMR aiming at the detection of macromolecules in liquid solution. One possibility for such an experimental design is to perform the polarization step between electrons and nuclei at low magnetic fields and then transfer the sample to a higher field for NMR detection. In this case, an independent optimization of the polarizer and detection set ups is required. In the present paper we describe the optimization of a polarizer set up at 15 MHz (1)H NMR/9.7 GHz EPR frequencies based on commercial hardware. The sample consists of the nitroxide radical TEMPONE-D,(15)N in water, for which the dimensions were systematically decreased to fit the homogeneous B(1) region of a dielectric ENDOR resonator. With an available B(1) microwave field up to 13 G we observe a maximum DNP enhancement of -170 at room temperature by irradiating on either one of the EPR lines. The DNP enhancement was saturated at all polarizer concentrations. Pulsed ELDOR experiments revealed that the saturation level of the two hyperfine lines is such that the DNP enhancements are well consistent with the coupling factors derived from NMRD data. By raising the polarizing field and frequencies 10-fold, i.e. to 140 MHz (1)H/94 GHz EPR, we reach an enhancement of -43 at microwave field strengths (B(1) approximately 5 G). The results are discussed in view of an application for a DNP spectrometer.
authors.affiliation: Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany.
authors.forename: Maria-Teresa
authors.initials: MT
authors.lastname: Türke
authors.forename: Igor
authors.initials: I
authors.lastname: Tkach
authors.forename: Marcel
authors.initials: M
authors.lastname: Reese
authors.forename: Peter
authors.initials: P
authors.lastname: Höfer
authors.forename: Marina
authors.initials: M
authors.lastname: Bennati
mesh_terms: D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation* / Q000379:methods; D014216:Triacetoneamine-N-Oxyl / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water; D014216:Triacetoneamine-N-Oxyl