Selectivity of Double-Quantum Filtered Rotational-Resonance Experiments on Larger-than-Two-Spin Systems

Abstract

Characterizing the orientation and molecular conformation of small organic molecules bound to the inner or outer surfaces of proteins represents an important step in drug design and in understanding the mechanisms of biochemical reactions, and similarly, of non-biological catalytic reactions. In a biochemical context, such molecular units or subunits may often contain only three or four carbon atoms, examples being the pyruvate anion, fumaric and maleic acid derivatives, or the phosphenolpyruvate moiety in differing degrees of ionization. Magic-angle spinning (MAS) NMR experiments, capable of delivering reliable information about the conformational properties of these molecular units, have to combine several properties in order to be able to fulfill these tasks in realistic application situations. First, the 13C resonances originating from the (fully or partially) 13C enriched substrate molecules of interest have to be separable from additional natural-abundance 13C resonances; this calls for the application of double-quantum filtration (DQF) techniques. Second, many of these small substrate molecules feature structural subunits that require using the orientation dependence of 13C chemical shielding as the source of information about molecular conformation; this calls for MAS NMR experiments where magnitudes and orientations of chemical shielding tensors are sensitively reflected. Third, for reasons of synthetic feasibility, the chosen MAS NMR techniques must be applicable in a quantifiable manner to larger-than-two-spin systems. The ease and robustness of the experimental and numerical implementations are an additional consideration.