Identification, classification, and signal amplification capabilities of high-turnover gas binding hosts in ultra-sensitive NMR†
Nuclear Magnetic Resonance (NMR) can be a powerful tool for investigating exchange kinetics of host–guest interactions in solution. Beyond conventional direct NMR detection, radiofrequency (RF) saturation transfer can be used to enhance the study of such chemical exchange or to enable signal amplification from a dilute host. However, systems that are both dilute and labile (fast dissociation/re-association) impose specific challenges to direct as well as saturation transfer detection. Here we investigate host–guest systems under previously inaccessible conditions using saturation transfer techniques in combination with hyperpolarized nuclei and quantitative evaluation under different RF exposure. We further use that information to illustrate the consequences for signal amplification capabilities and correct interpretation of observed signal contrast from comparative exchange data of different types of hosts. In particular, we compare binding of xenon (Xe) to cucurbituril (CB6) with binding to cryptophane-A monoacid (CrA) in water as two different model systems. The Xe complexation with CB6 is extremely difficult to access by conventional NMR due to its low water solubility. We successfully quantified the exchange kinetics of this system and found that the absence of Xe signals related to encapsulated Xe in conventional hyperpolarized 129Xe NMR is due to line broadening and not due to low binding. By introducing a measure for the gas turnover during constant association–dissociation, we demonstrate that the signal amplification from a dilute pool of CB6 can turn this host into a very powerful contrast agent for Xe MRI applications (100-fold more efficient than cryptophane). However, labile systems only provide improved signal amplification for suitable saturation conditions and otherwise become disadvantageous. The method is applicable to many hosts where Xe is a suitable spy nucleus to probe for non-covalent interactions and should foster reinvestigation of several systems to delineate true absence of interaction from labile complex formation.