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Occasionally we have the good fortune of enjoying a paradigm shift in how a discipline performs its research. NMR spectroscopy has had a number of such paradigm shifts over the years, 2D NMR methods being probably the most significant in the past several decades. Two-dimensional NMR began with very simple pulse sequences that delivered relatively high sensitivity. As 2D NMR pulse sequences became more sophisticated, leading eventually to hyphenated 2D NMR techniques such as GHSQC-TOCSY and GHSQC-NOESY, sensitivity losses associated with the correspondingly more complex pulse sequences led to information rich spectra but with the penalty of correspondingly lower sensitivity. Consequently, despite the interpretational advantages of some of the hyphenated 2D NMR experiments, they tend to be less frequently used because of their lower sensitivity and consequently longer acquisition times. Unsymmetrical indirect covariance processing, an extension of recent development in covariance NMR methods, offers a potential high sensitivity alternative to access what are normally low sensitivity hyphenated 2D NMR correlation data. Given two coherence transfer experiments, A → B and A → C, it is possible to indirectly determine B → C coherence transfer data. The application of unsymmetrical indirect covariance processing methods to generate GHSQC-COSY and GHSQC-NOESY spectra from the more readily acquired GCOSY, NOESY, and GHSQC 2D NMR spectra and related examples will be described.
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