Off-resonance NOVEL

Abstract

Dynamic nuclear polarization (DNP) is theoretically able to enhance the signal in nuclear magnetic resonance (NMR) experiments by a factor γ[subscript e]/γ[subscript n], where γ's are the gyromagnetic ratios of an electron and a nuclear spin. However, DNP enhancements currently achieved in high-field, high-resolution biomolecular magic-angle spinning NMR are well below this limit because the continuous-wave DNP mechanisms employed in these experiments scale as ω[superscript -n over subscript 0] where n ∼ 1-2. In pulsed DNP methods, such as nuclear orientation via electron spin-locking (NOVEL), the DNP efficiency is independent of the strength of the main magnetic field. Hence, these methods represent a viable alternative approach for enhancing nuclear signals. At 0.35 T, the NOVEL scheme was demonstrated to be efficient in samples doped with stable radicals, generating [superscript 1]H NMR enhancements of ∼430. However, an impediment in the implementation of NOVEL at high fields is the requirement of sufficient microwave power to fulfill the on-resonance matching condition, ω0I = ω1S, where ω[subscript 0I] and ω[subscript 1S] are the nuclear Larmor and electron Rabi frequencies, respectively. Here, we exploit a generalized matching condition, which states that the effective Rabi frequency, ω[superscript eff over subscript 1S], matches ω[subscript 0I]. By using this generalized off-resonance matching condition, we generate [superscript 1]H NMR signal enhancement factors of 266 (∼70% of the on-resonance NOVEL enhancement) with ω[subscript 1S]/2π = 5 MHz. We investigate experimentally the conditions for optimal transfer of polarization from electrons to [superscript 1]H both for the NOVEL mechanism and the solid-effect mechanism and provide a unified theoretical description for these two historically distinct forms of DNP.