Bis‐Gadolinium Complexes for Solid Effect and Cross Effect Dynamic Nuclear Polarization

Abstract

High-spin complexes act as polarizing agents (PAs) for dynamic nuclear polarization (DNP) in solid-state NMR spectroscopy and feature promising aspects towards biomolecular DNP. We present a study on bis(Gd-chelate)s which enable cross effect (CE) DNP owing to spatial confinement of two dipolar-coupled electron spins. Their well-defined Gd···Gd distances in the range of 1.2-3.4 nm allowed us to elucidate the Gd···Gd distance dependence of the DNP mechanism and NMR signal enhancement. We found that Gd···Gd distances above 2.1 nm result in solid effect DNP while distances between 1.2 and 2.1 nm enable CE for 1 H, 13 C, and 15 N nuclear spins. We compare 263 GHz electron paramagnetic resonance (EPR) spectra with the obtained DNP field profiles and discuss possible CE matching conditions within the high-spin system and the influence of dipolar broadening of the EPR signal. Our findings foster the understanding of the CE mechanism and the design of high-spin PAs for specific applications of DNP.Dynamic nuclear polarization (DNP) has experienced widespread application in structural biology and materials research for sensitivity enhancement of magic-angle spinning (MAS) NMR spectroscopy. [1] By transferring the large electron spin polarization to the surrounding nuclear spins through microwave (mw) irradiation of a frozen solution, the NMR signal intensity can be increased by up to three orders of magnitude. [2] Polarizing agents (PAs), typically based on nitroxides, provide the electron spin polarization. [3] Lately, complexes of the high-spin metal ions Gd III or Mn II have been introduced as alternative PAs. [4] A recent, intriguing development is the utilization of PAs site-directedly bound to biomolecules. Owing to the limited distance between PA and biomolecule, hyperpolarization can be selectively transferred to the site(s) of interest. [5] Loss of polarization to the bulk can be prevented either by matrix perdeuteration in the case of (indirect) 1 H DNP, [6] or by utilizing direct DNP of nuclei with small natural abundance in the matrix (e.g., 13 C, 15 N) so that spin-diffusion pathways are limited. [7] For the latter case the development of PAs with properties tailored for efficient, direct DNP of 13 C and 15 N is crucial. Bis-nitroxide tags can provide sufficient 1 H polarization but fail to provoke direct 13 C DNP, [6] while direct 13 C DNP was demonstrated with tags based on the paramagnetic metal ions Mn II or Gd III . [7,8] Furthermore, in contrast to nitroxides, Gd III and Mn II complexes are stable within cellular environments, which makes them suitable PAs for in-cell studies. [9] In the context of MAS NMR, two DNP mechanisms have been exploited so far: the solid effect (SE) [10] and the cross effect (CE). [11] The SE can be evoked by irradiating a nominally forbidden electron-nuclear double-or zero-quantum transition. For the CE, two dipolar-coupled electron spins need to be present and their frequency difference, Dw 0S , is required to match the nuclear Larmor frequency, w 0I [Eq. (1)]:DNP occurs with...

Dynamic Nuclear Polarization of Metal-Doped Oxide Glasses: A Test of the Generality of Paramagnetic Metal Polarizing Agents

“…To confirm the presence of the cross-effect, we have acquired the DNP field profiles for two different Gd 3+ concentrations in our ZnP glasses. Given that cross-effect requires the interaction between two Gd 3+ -centered spins, 24 it is expected to grow in significance as the Gd 3+ concentration is increased and the electron-electron interactions are strengthened. 26,30,48 As shown in Figure 1b and Table 1, the separation between the two extrema indeed decreased with an increase in Gd 3+ concentration, confirming that crosseffect plays a significant role in the nuclear hyperpolarization process.…”

The Clebsch–Gordan Coefficients and Their Application to Magnetic Resonance

“…e example described above was for a system of two identical spin-1/2 particles, but higher spin systems can be treated in a similar manner. For example, this could be the 3electron system of an N@C 60 endofullerene [29,30] or the 7electron DOTA-Gd 3+ complex [31].…”

“…Here, we have derived the Clebsch-Gordan coefficients for two tensors of rank 1 to form resultant rank 0, 1, and 2 tensors, determined the angular dependence and secular portion of the static, dipolar coupling Hamiltonian, and derived the triplet and singlet states of two identical spin-1/2 spins. Other uses include determining the spin states of high spin transition metal complexes such as those involving Gd 3+ [31] and endofullerenes [29,30] (such as N@C 60 ) and for the calculation of the fourth rank tensor components of second-order average Hamiltonians, such as those present in systems with large quadrupolar couplings [32], hyperfine couplings [33], and zero field splittings [34].…”