Fig. (1) Spectra of 0.5 M U-13C, 15N-proline in glycerol-d8–D2O–H2O (60 : 30 : 10 volume ratio) and 15 mM TOTAPOL biradical, 25 ml sample volume, with DNP (top trace) and without DNP (bottom traces). Reproduced from [8] with permission of The Royal Society of Chemistry.