Pulse sequence and sample formulation optimization for dipolar order mediated 1H→13C cross-polarization

Abstract

We have recently demonstrated the use of contactless radiofrequency pulse sequences under dissolution-dynamic nuclear polarization conditions as an attractive way of transferring polarization from sensitive ¹H spins to insensitive ¹³C spins with low peak radiofrequency pulse powers and energies via a reservoir of dipolar order. However, many factors remain to be investigated and optimized to enable the full potential of this polarization transfer process. We demonstrate herein the optimization of several key factors by: (i) implementing more efficient shaped radiofrequency pulses; (ii) adapting ¹³C spin labelling; and (iii) avoiding methyl group relaxation sinks. Experimental demonstrations are presented for the case of [1-¹³C]sodium acetate and other relevant molecular candidates. By employing the range of approaches set out above, polarization transfer using the dipolar order mediated cross-polarization radiofrequency pulse sequence is improved by factors approaching ∼1.65 compared with previous results. Dipolar order mediated ¹H→¹³C polarization transfer efficiencies reaching ∼76% were achieved using significantly reduced peak radiofrequency pulse powers relative to the performance of highly sophisticated state-of-the-art cross-polarization methods, indicating ¹³C nuclear spin polarization levels on the order of ∼32.1% after 10 minutes of ¹H DNP. The approach does not require extensive pulse sequence optimization procedures and can easily accommodate high concentrations of ¹³C-labelled molecules.