Design And Research Of New Hyperpolarized ¹²⁹Xe Magnetic Resonance Contrast Agents

¹²⁹Xe is a non-toxic gas with a wide range of chemical shifts,sensitivity to the surrounding chemical environment,and good solubility in physiological conditions.By laser Spin exchange optical pumping?SEOP?,the polarization for ¹²⁹Xe can be increased by 10⁴¹0⁵ times,which makes hyperpolarized ¹²⁹Xe suitable for application in life sciences.However,the hyperpolarized ¹²⁹Xe single-atom biosensor could not recognize target specifically.Therefore,a cage providing the most suitable exchange kinetics and affinity for ¹²⁹Xe was selected.In order to recognize target specifically,the cage for ¹²⁹Xe must be functionalized or the host@guest interaction for the cage must be studied.In this work,the characteristics of the most common cages?cucurbit[6]uril and cryptophane-A?for ¹²⁹Xe biosensor design were studied systematically.Besides,we have designed two new kinds of ¹²⁹Xe biosensor based on cucurbit[6]uril.The details are as follows:?1?The characteristics of the most common cages?cucurbit[6]uril and cryptophane-A?for ¹²⁹Xe biosensor design have been studied systematically.The results show that the depolarization rate for cucurbit[6]uril is one order of magnitude higher than that of cryptophane-A.This indicates that ¹²⁹Xe@cucurbit[6]uril biosensor could realize more sensitive detection for targets;In addition,the relaxivity of cucurbit[6]uril for dissolved ¹²⁹Xe is two orders of magnitude higher than that of cryptophane-A for dissolved ¹²⁹Xe.This indicates that it is more advantageous to design“stimulus-responsive dissolved ¹²⁹Xe relaxivity“OFF-ON”biosensor”using cucurbit[6]uril than cryptophane-A;Moreover,the synthetic procedures of cucurbit[6]uril are simpler than that of cryptophane-A,which indicate that constructing¹²⁹Xe@cucurbit[6]uril biosensor could save time and money than ¹²⁹Xe@cryptophane-A biosensor.Therefore,cucurbit[6]uril has been applied to the design of ¹²⁹Xe biosensor in the following chapters.?2?An intracellular diamine oxidase?DAO?triggered hyperpolarized ¹²⁹Xe signal“OFF-ON”biosensor has been constructed.The experimental principle is as follows:A strong cucurbit[6]uril binding substrate was converted into a product that cannot bind to cucurbit[6]uril via an enzymatic reaction.Since the binding constant of substrate@CB6(2.62×10⁶±3.66×10⁵ M^-1)was much larger than that of ¹²⁹Xe@CB6,the product of the enzymatic reaction was the one that cannot bind to CB6,thus the CEST response emerged after the enzymatic reaction.This biosensor showed good selectivity for DAO in buffer and intestinal villus epithelial cell.Besides,we have realized the monitoring of enzymatic reaction by this biosensor using ¹²⁹Xe NMR.Reported approaches overcame the risks?Before and after biosenor@target interaction,the chemical shift difference is comparably small?for“stimulus-responsive ¹²⁹Xe chemical shift difference biosensor”.This reported approaches(stimulus-responsive ¹²⁹Xe signal“OFF-ON”biosensor)could recognize more disease-related targets specifically by ¹²⁹Xe Hyper-CEST.?3?An intracellular lysine decarboxylase?LDC?triggered hyperpolarized ¹²⁹Xe relaxivity“OFF-ON”biosensor has been constructed.The experimental principle is as follows:A weak cucurbit[6]uril binding substrate was converted into a strong cucurbit[6]uril binding product via an enzymatic reaction.Since the binding constant of substrate@cucurbit[6]uril was much larger smaller than that of ¹²⁹Xe@cucurbit[6]uril,the product of the enzymatic reaction was much larger than that of ¹²⁹Xe@cucurbit[6]uril,thus the relaxivity of biosensor for dissolved ¹²⁹Xe reduced sharply after the enzymatic reaction.This biosensor showed good selectivity for LDC in buffer.Compared with the¹H T_2exex contrast agent,sensitivity for this ¹²⁹Xe biosensor has been improved by nearly four orders.Besides,reported approaches overcame the risks(Before and after biosenor@target interaction,¹²⁹Xe signal is unstable)for“stimulus-responsive ¹²⁹Xe signal“OFF-ON”biosensor”.Moreover,this is the first example for ¹²⁹Xe relaxivity“OFF-ON”biosensor.