TOTAPOL: A Biradical Polarizing Agent for Dynamic Nuclear Polarization Experiments in Aqueous Media

Abstract

In a previous publication, we described the use of biradicals, in that case two TEMPO molecules tethered by an ethylene glycol chain of variable length, as polarizing agents for microwave driven dynamic nuclear polarization (DNP) experiments. The use of biradicals in place of monomeric paramagnetic centers such as TEMPO yields enhancements that are a factor of approximately 4 larger (epsilon approximately 175 at 5 T and 90 K) and concurrently the concentration of the polarizing agent is a factor of 4 smaller (10 mM electron spins), reducing the residual electron nuclear dipole broadening. In this paper we describe the synthesis and characterization by EPR and DNP/NMR of an improved polarizing agent 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL). Under the same experimental conditions and using 2.5 mm magic angle rotors, this new biradical yields larger enhancements (epsilon approximately 290) at lower concentrations (6 mM electron spins) and has the additional important property that it is compatible with experiments in aqueous media, including salt solutions commonly used in the study of proteins and nucleic acids.

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

“…This system permitted us to demonstrate DNP at 5T fields and to explore many important features of the experiments. For example, we established that cross effect DNP using biradical polarizing agents is the optimal mechanism [34][35][36][37][38][39][40][41] for high field experiments involving CW microwave radiation. Traditional approaches, based on the solid effect and thermal mixing, yield enhancements that are an order of magnitude smaller [38] or require high concentrations of polarizing agents that lead to significant electron-nuclear dipolar broadening [42].…”

“…For the latter we present in this paper DNP enhanced MAS spectra of bacteriorhodopsin (bR), a 26.6 kD membrane protein embedded in a lipid bilayer. This is a more challenging test case for the DNP method than small model compounds such as urea or proline that we have used extensively in other papers demonstrating DNP [36,49,50]. It is also a biologically important system that poses outstanding scientific questions and therefore addresses the applicability of DNP experiments to interesting systems.…”

50 & 100 years ago

“…Compared with experiments in the absence of DNP, 250 fold signal enhancements have been observed 16 using such biradicals. Currently, the favourite polarizing agent is a water soluble biradical 17 known as TOTAPOL, which typically yields 170 fold enhancements in model systems and in proteins.…”

Advances in mechanical detection of magnetic resonance

“…DNP efficiency in aqueous organic samples has shown dramatic improvement in recent years because techniques for generating high-power, very-high-frequency rf magnetic fields have advanced and because new biradical polarizing agents have been developed. 200 High frequency ͑94 GHz͒ irradiation has recently been employed in an ESR-MRFM experiment. 201 The advantage of DNP in a MRFM experiment is that it would allow detection of a spin polarization with a welldefined sign.…”