Evidence of nonspecific surface interactions between laser-polarized xenonand myoglobin in solution

The high sensitivity of the magnetic resonance properties of xenon to its l ocal chemical environment and the large Xe-129 NMR signals attainable throu gh optical pumping have motivated the use of xenon as a probe of macromolec ular structure and dynamics. In the present work, we report evidence for no nspecific interactions between xenon and the exterior of myoglobin in aqueo us solution, in addition to a previously reported internal binding interact ion. Xe-129 chemical shift measurements in denatured myoglobin solutions an d under native conditions with varying xenon concentrations confirm the pre sence of nonspecific interactions. Titration data are modeled quantitativel y with treatment of the nonspecific interactions as weak binding sites. Usi ng laser-polarized xenon to measure Xe-129 spin-lattice relaxation times (T -1), we observed a shorter T-1 in the presence of 1 mM denatured apomyoglob in in 6 M deuterated urea (T-1 = 59 +/- 1 s) compared with that in 6 M deut erated urea alone (T-1 = 291 +/- 2 s), suggesting that nonspecific xenon-pr otein interactions can enhance Xe-129 relaxation. An even shorter T1 was me asured in ? mM apomyoglobin in D2O (T-1 = 15 +/- 0.3 s), compared with that in D2O alone (T-1 = 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 polariz ation between these nuclei via the nuclear Overhauser effect.