Evidence of nonspecific surface interactions between laser-polarized xenon and myoglobin in solution
The high sensitivity of the magnetic resonance properties of xenon to its local chemical environment and the large 129Xe NMR signals attainable through optical pumping have motivated the use of xenon as a probe of macromolecular structure and dynamics. In the present work, we report evidence for nonspecific interactions between xenon and the exterior of myoglobin in aqueous solution, in addition to a previously reported internal binding interaction. 129Xe chemical shift measurements in denatured myoglobin solutions and under native conditions with varying xenon concentrations confirm the presence of nonspecific interactions. Titration data are modeled quantitatively with treatment of the nonspecific interactions as weak binding sites. Using laser-polarized xenon to measure 129Xe spin-lattice relaxation times (T1), we observed a shorter T1 in the presence of 1 mM denatured apomyoglobin in 6 M deuterated urea (T1 = 59 ± 1 s) compared with that in 6 M deuterated urea alone (T1 = 291 ± 2 s), suggesting that nonspecific xenon-protein interactions can enhance 129Xe relaxation. An even shorter T1 was measured in 1 mM apomyoglobin in D2O (T1 = 15 ± 0.3 s), compared with that in D2O alone (T1 = 506 ± 5 s). This difference in relaxation efficiency likely results from couplings between laser-polarized xenon and protons in the binding cavity of apomyoglobin that may permit the transfer of polarization between these nuclei via the nuclear Overhauser effect.