Consequences of 129Xe-1H cross relaxation in aqueous solutions

We have investigated the transfer of polarization from Xe-129 to solute protons in aqueous solutions to determine the feasibility of using hyperpolarized xenon to enhance H-1 sensitivity in aqueous systems at or near room temperatures, Several solutes, each of different molecular weight, were dissolved in deuterium oxide and although large xenon polarizations were created, no significant proton signal enhancement was detected in L-tyrosine, cu-cyclodextrin, P-cyclodextrin, apomyoglobin, or myoglobin. Solute-induced enhancement of the Xe-129 spin-lattice relaxation rate was observed and depended on the size and structure of the solute molecule, The significant increase of the apparent spin-lattice relaxation rate of the solution phase Xe-129 by cu-cyclodextrin and apomyoglobin indicates efficient cross relaxation. The slow relaxation of xenon in P-cyclodextrin and L-tyrosine indicates weak coupling and inefficient cross relaxation, Despite the apparent cross-relaxation effects, all attempts to detect the proton enhancement directly were unsuccessful, Spin-lattice relaxation rates were also measured for Boltzmann Xe-129 in myoglobin. The cross-relaxation rates were determined from changes in Xe-129 relaxation rates in the cy-cyclodextrin and myoglobin solutions. These cross-relaxation rates were then used to model H-1 signal gains for a range of Xe-129 to H-1 spin population ratios. These models suggest that in spite of very large Xe-129 polarizations, the H-1 gains will be less than 10% and often substantially smaller. In particular, dramatic H-1 signal enhancements in lung tissue signals are unlikely. (C) 1999 Academic Press.