Radio frequency assisted homonuclear recoupling - A Floquet description of homonuclear recoupling via surrounding heteronuclei in fully protonated to fully deuterated systems

We present a Floquet theory approach for the analysis of homonuclear recoupling assisted by radio frequency (RF) irradiation of surrounding heteronuclear spins. This description covers a broad range of systems from fully protonated to deuterated proteins, focusing in detail on recoupling via protons and deuterons separately as well as simultaneously by the double nucleus enhanced recoupling (DONER) scheme. The theoretical description, supported by numerical simulations and compared to experimental results from a partially deuterated model compound, indicates that in perdeuterated systems setting the RF amplitude equal to the magic angle spinning (MAS) frequency is not necessarily optimal for recoupling via 1H and/or 2H nuclei and modified recoupling conditions are identified.

Simulation of the homonuclear recoupling process of two carbon nuclei coupled to three deuterium and two proton nuclei. Signal intensity was calculated as a function of the isotropic chemical shift difference between the carbons and the RF irradiation amplitudes applied simultaneously to the deuterium and proton channels. The extent of recoupling is color coded such that red corresponds to low recoupling and blue to high.130Research highlights► A Floquet theory analysis of the 1H and 2H RAD/DARR experiment is presented. ► Homonuclear recoupling conditions are derived for fully protonated to fully deuterated systems. ► In perdeuterated systems recoupling is not necessarily most efficient at rotary resonance conditions. ► The 1H and 2H RAD/DARR processes contribute simultaneously in DONER recoupling. ► Results are presented from a partially deuterated model compound and a perdeuterated protein.