Figure 7.

(a)  ${}^{\mathrm{13}}$ C spectra of PBA with and without DNP. The enhanced spectrum (solid line) was obtained after performing the ISE sequence at a repetition frequency of 90 Hz for 600 s. (b) Buildup curves of ${}^{\mathrm{13}}$ C polarization in PBA for ISE repetition rates ranging from 10 to 90 Hz. Each data point represents the ${}^{\mathrm{13}}$ C polarization derived from the intensity of the NMR signal accumulated over 16 times. (c) Dependence of the ${}^{\mathrm{13}}$ C magnetization in PBA on the time interval between DNP and NMR measurement.