Figure 6.

Enhancement vis-à-vis thermal counterparts of the ¹⁵N signals of ¹⁵N-alanine (a) and of natural abundance arginine ((b) and (c)), by polarization transfers from hyperpolarized water. The optimal delay in each case was extracted from simulations of the kind given in Figure 5. Hyperpolarized ¹⁵N NMR spectra in (a) and (b) were detected in single-scan experiments using a 90° ¹⁵N pulse, applied 43 and 18 sec after the injection of the water, respectively. Thermal acquisitions in (a) and (b/c) took ca. 2 and 14 hours, respectively. (c) Sum of the first 8 scans collected after injection of hyperpolarized water to an arginine sample, over a total time of 24 sec. An effective average enhancement ≥500× is observed. All measurements were done at ≈50 °C under conditions akin to those in Fig. 5. Notice that the NH of arginine has significantly lower enhancement than the other two peaks due to its slow k_ex at pH≈3.