Transfer of hyperpolarization from long T1 storage nuclei to short T1 neighbors using FLOPSY-8

Abstract

Nuclei with long T1s are optimal targets for dynamic nuclear polarization (DNP). Therefore, most of the agents used in metabolic imaging and spectroscopy studies are based on carboxylic acid moieties that lack protons, a strong source of dipolar relaxation. Metabolic flux information encoded into spectra of small molecule metabolites in the form of the 13C isotopomer data cannot be accessed using standard 13C hyperpolarization methods because protonated carbons relax too quickly through T1 dipolar relaxation. It is shown here that the longitudinal mixing sequence FLOPSY-8 can be used to transfer polarization from a long T1 storage nucleus to adjacent protonated carbons so that they may be detected with high sensitivity. We demonstrate that FLOPSY-8 allows a direct readout of isotopomer populations in butyrate and glutamate in vitro.

Figure 1

Enhancement of protonated butyrate carbons following FLOPSY-8 mixing. The ¹³C NMR spectra of hyperpolarized [U-¹³C]butyrate after a 30° excitation pulse (bottom) was acquired two seconds prior to the FLOPSY-8 enhanced spectrum (top). The top spectrum shows the impact of 13 cycles of evolution, followed by the same 30° excitation pulse. The absolute phase is not preserved due to the non-zero transverse components of the average Hamiltonian that describes the FLOPSY-8 sequence. In addition, the C3 resonance exhibits a change in phase for the central peaks of the multi-component resonance.

Figure 2

Absolute (top) and relative (bottom) enhancements of C2, C3, and C4 of [U-¹³C]butyrate as a function of FLOPSY-8 cycles. A total of n=4 enhancements were used to produce the error bars. * P <0.0001; ** P < 0.05 (except for C3 values).

Figure 3

¹³C NMR spectrum of hyperpolarized [U-¹³C]glutamate. The bottom panel shows the signal after an initial 30 degree excitation pulse. The top panel shows the spectrum after a 13 cycle FLOPSY-8 mixing period prior to a 30 degree excitation pulse, 2 seconds after the bottom spectrum was collected.