A selective 15N-to-1H polarization transfer sequence for more sensitive detection of 15N-choline

The sensitivity and information content of heteronuclear nuclear magnetic resonance is frequently optimized by transferring spin order of spectroscopic interest to the isotope of highest detection sensitivity prior to observation. This strategy is extended to N-15-choline using the scalar couplings to transfer polarization from N-15 to choline's nine methyl H-1 spins in high field. A theoretical analysis of a sequence using nonselective pulses shows that the optimal efficiency of this transfer is decreased by 62% as the result of competing N-15-H-1 couplings involving choline's four methylene protons. We have therefore incorporated a frequency-selective pulse to support evolution of only the N-15-methyl H-1 coupling during the transfer period. This sequence provides a 52% sensitivity enhancement over the nonselective version in in vitro experiments on a sample of thermally polarized N-15-choline in D2O. Further, the N-15 T-1 of choline in D2O was measured to be 217 +/- 38s, the N-15-methyl H-1 coupling constant was found to be 0.817 +/- 0.001 Hz, and the larger of choline's two N-15-methylene H-1 coupling constants was found to be 3.64 +/- 0.01 Hz. Possible improvements and applications to in vivo experiments using long-lived hyperpolarized heteronuclear spin order are discussed. (C) 2010 Elsevier Inc. All rights reserved.