UC Berkeley

In this dissertation, the sensitivity of xenon relaxation to changes in its environment is used to both develop new types of biosensors and also to develop new techniques that make use of xenon’s intrinsic interactions with its environment. A proton based relaxation experiment is also discussed due to its similarity to relaxation experiments done with xenon biosensors.

Contrast agents are developed for xenon NMR. These agents consist of a cryptophane cage covalently attached to a DOTA chelating agent, allowing one to bring xenon close to chelated paramagnetic ions, enhancing the bulk relaxation of xenon. Both the T1 and T2 relaxivity of these contrast agents are tested. Adding paramagnetic metal ions seems to affect T1 more than T2 for most ions, possibly because the cage itself drastically affects the T2 of xenon because of the slow exchange rate and large chemical shift difference. In general, metal ions known to have long electronic relaxation times relax xenon more efficiently than ions with shorter electronic relaxation times. Gadolinium (III) and manganese (II) have the greatest effect on the T1 and T2 of xenon, with gadolinium (III) affecting T2 more and manganese (II) affecting T1 more. Adding gadolinium (III) increases the T1 relaxivity of M2 cages to 0.002 mM-1s-1 from 0.0009 mM-1s-1 and the T2 relaxivity to 92.5 mM-1s-1 from 26.1 mM-1s-1 .

After testing the effect of these contrast agents, a relaxation based xenon biosensor is developed. This sensor consist of a cryptophane cage attached to a DOTA chelating agent and a biotin. The sensor works by binding to avidin, thereby increasing the rotational correlation time of the xenon inside the cage. This increases the relaxation rate of xenon inside the cage. Upon binding of a biotin-containing sensor to avidin at 1.5 µM concentration, the free xenon T2 is reduced by a factor of 4. Changes in relaxation were more easily seen in T2 due to the strength of the field used in this experiment. At high magnetic fields, T1 hardly responds to changes in the rotational correlation time.

A proton based relaxation agent, developed by the IBS institute from the Republic of Korea, is discussed in this dissertation. This group developed a sensor consisting of two parts: a super paramagnetic nanoparticle quencher and a paramagnetic metal ion enhancer. When the two are close together, the paramagnetic enhancer cannot efficiently relax water. Separating the two, done by either cleaving the bond keeping them together or by a conformational change in the linker binding them, prevents the super paramagnetic nanoparticle from quenching the enhancer, making water relaxation extremely rapid. Cleaving the bond between the quencher and enhancer increases the R1 of water by 1.5 s-1. This sensor was used to detect MMP2, an enzyme seen in certain tumors, both invitro and invivo. Concentrations as low as 15 ng per mL of MMP2 were detected invitro. This sensor is less sensitive invivo, with a lowest detected concentration of MMP2 being 450 ng per mL.

After studying many varieties of sensors developed to functionalize xenon, the direct interactions between xenon and its target were studied. Xenon interacts with many substances, including proteins, leading to rapid relaxation of the entire xenon ensemble. This is due to both nonspecific interactions with the protein surface relaxing xenon and also because many proteins have hydrophobic pockets xenon can occupy. This leads to rapid xenon relaxation, which can be perturbed by the protein binding to another ligand. Adding a ligand to a solution of protein, such as a small molecule drug, alters the relaxation of xenon in that solution. This effect was exploited in order to develop a method for measuring the binding affinity of certain drugs for albumin by monitoring their effect on the relaxation of xenon. Of the drugs studied, warfarin, tenoxicam, and sodium salicylate had the strongest effects due to their high affinity for albumin, with warfarin lowering the T2 of xenon from 5 seconds to 2 seconds.