Figure 2.

Xenon-129 NMR spectra monitoring the binding of biotin-functionalized xenon to avidin. The spectrum of functionalized xenon in the absence of protein is shown in full, with the peak at 193 ppm corresponding to xenon in water, and peaks at 70 ppm corresponding to cryptophane-bound xenon. Inset shows only the cryptophane-bound peaks. a shows the functionalized xenon before the addition of avidin, with the more intense peak corresponding to functionalized xenon and the smaller peak corresponding to xenon in the cage without linker and ligand, serving as both a chemical shift and signal intensity reference. b shows the spectrum on the addition of ≈80 nmol of avidin monomer. A third peak, corresponding to functionalized xenon bound to avidin, has appeared, and the unbound functionalized xenon peak has decreased in intensity. All chemical shifts are referenced to that of xenon gas.