Enhanced Sensitivity and Resolution in Biomolecular CEST NMR Experiments Using the Extended Hadamard Encoding Scheme

By untangling information using deterministic frequency-domain linear combinations, the Hadamard Transform (HT) offers a robust way to extract spectral information. Hadamard-based encoding schemes can shorten the acquisition times of nuclear magnetic resonance (NMR) experiments, and lead to substantial sensitivity gains/unit time. However, the presence of spin-spin couplings, spin-spin cross-relaxation, or other mechanisms that break simple one-to-one relationships between a peak intensity and its frequency-domain position can complicate this approach. A case where manipulations at a given peak frequency position may affect the intensity of other peaks in the spectra arises in chemical exchange. Thus, in frequency-domain NMR experiments such as chemical exchange saturation transfer (CEST), saturation at one frequency position may bring about significant intensity changes at another frequency. This study shows that when based on classical frequency-domain Hadamard encodings, strong artifacts will then arise in NMR CEST experiments. The origin of these artifacts is explained and a way to eliminate them while preserving HT's desirable characteristics is proposed, utilizing an extended HT (eHT) scheme. CEST NMR experiments performed and processed using the eHT are shown to be free from artifacts, while leveraging Hadamard's sensitivity-enhanced performance over step-by-step frequency-domain implementations. Good performance is also observed when eHT CEST is compared against other accelerated versions of protein CEST NMR. A summary of the features and potential opened by these new experiments is provided.