Missing density: detergents. Two examples of detergent molecules placed into models of membrane proteins. (a) The plant SLAC1 anion channel structure, PDB entry 3m73
(Chen et al., 2010), shows two molecules (BOG A317/A318) that have clear density for the hydrophobic acyl chain but not for the head groups. (b) The presence of detergent molecules (BOG A700/A801 is shown) in the crystal structure of the Escherichia coli membrane enzyme glycerol-3-phosphate dehydrogenase, PDB entry 2qcu
(Yeh et al., 2008), appears to be entirely unsupported by the electron density. The 1.15 Å (a) and 1.75 Å (b) resolution mFo − DFc maps contoured at ±3σ (green/red) and 2mFo − DFc maximum-likelihood omit maps contoured at 1σ (blue) are shown. Maps were calculated for a refined model with the ligand atoms omitted.