35 GHz pulsed 13C and 15N ENDOR of HydA matured in the absence of lysate. Top, 13C Mims ENDOR spectrum at g=2.093 of HydA matured with [13C3,15N]-serine. Bottom, 15N Mims ENDOR spectra at g=2.004 of HydA matured with 15NH4Cl (blue) and with [13C3,15N]-serine (black). Mims holes are indicated with arrows. Microwave frequency, 34.605 GHz; rep time, 50 ms; T = 2 K. For a complete characterization of these signals, see ref 15b.