Combination of DQ and ZQ Coherences for Sensitive Through-Bond NMR Correlation Experiments in Biosolids under Ultra-Fast MAS
A double-zero quantum (DZQ)-refocused INADEQUATE experiment is introduced for J-based NMR correlations under ultra-fast (60 kHz) magic angle spinning (MAS). The experiment records two spectra in the same dataset, a double quantumsingle quantum (DQ-SQ) and zero quantumsingle quantum (ZQ-SQ) spectrum, whereby the corresponding signals appear at different chemical shifts in ?1. Furthermore, the spin-state selective excitation (S3E) J-decoupling block is incorporated in place of the second refocusing echo of the INADEQUATE scheme, providing an additional gain in sensitivity and resolution. The two sub-spectra acquired in this way can be treated separately by a shearing transformation, producing two diagonal-free single quantum (SQ-SQ)-type spectra, which are subsequently recombined to give an additional sensitivity enhancement, thus offering an improvement greater than a factor of two as compared to the original refocused INADEQUATE experiment. The combined DZQ scheme retains transverse magnetization on the initially polarized (I) spin, which typically exhibits a longer transverse dephasing time (T2') than its through-bond neighbour (S). By doing so, less magnetization is lost during the refocusing periods in the sequence to give even further gains in sensitivity for the J correlations. The experiment is demonstrated for the correlation between the carbonyl (CO) and alpha (CA) carbons in a microcrystalline sample of fully protonated, [15N,13C]-labelled CuII,?ZnII superoxide dismutase, and its efficiency is discussed with respect to other J-based schemes.