Mims-ENDOR of Mn(II)-bound 15N-CP-Ser in the absence (red) and presence (green) of Ca(II). Spectrometer settings: νmw = 34 GHz, B0 ~ 1240 mT, π/2 = 8 ns, τ = 448 ns, T = 400 ns, temperature = 10 K. Protein sample conditions: 1 mM CP-Ser (–Ca) or 200 μM CP-Ser (+Ca), 180 μM Mn(II), pH 7.5 (75 mM HEPES, 100 mM NaCl, 20% (v/v) PEG 200), ± 2 mM Ca(II).