Figure 1.

¹³C NMR spectra of uniformly ¹⁵N,¹³C-labeled L-alanine in partially protonated glycerol/water, containing 10 mM of the triradical dopant DOTOPA-4OH (red, a, b, c, e), 15 mM of the biradical dopant TOTAPOL (green, a, b), 10 mM of the triradical dopant DOTOPA-Ethanol (black, d), or 0.2 mM DyEDTA (blue, a–e). (a) Cross-polarized spectra of samples 1, 2, and 3 at 23–28 K with MAS at 6.7–6.8 kHz. Natural-abundance ¹³C NMR signals from glycerol are labeled “gly.” (b) Cross-polarized spectra of samples 1, 2, and 3 at 87–107 K with MAS at 6.7–6.8 kHz. (c) Cross-polarized spectra of samples 1 and 3 at 18–19 K without MAS. (d) Cross-polarized spectra of samples 3 and 6 at 81–89 K without MAS. (e) Directly excited spectra of samples 4 and 5 at 24–29 K with MAS at 6.6–6.7 kHz. Spectra in each panel are normalized by the number of scans to show signal per scan. Sample compositions, temperatures, recycle delays for each spectrum are given in Tables 1, 2.