Spin labeling was performed with 0.1 g of envelope samples in 10 mL of 50 mM MOPS, pH 7.5 containing 60 mM NaCl for 1 h at ro om temperature with 10 (for MTSL) or 50 (for M-DOTA-Gd 3+ ) µM of the spin label. The spin label was introduced into membrane vesicles by four rounds of freeze and thaw cycles in liquid nitrogen. The excess spin label was removed by three rounds of washing and centrifugation at 200,000xg for 1.5 h at 4°C with two cycles of freeze and thaw at each round. The membrane pellet was suspended in a final volume of ~ 30 µl using the same buffer. For PELDOR samples, the membrane was diluted in the same buffer with 20% d8-glycerol to an appropriate spin concentration (100 -200 M).