Crystallography of a Photocycle Intermediate

In their report “Energy transduction on the nanosecond time scale: Early structural events in a xanthopsin photocycle” (20 Mar., p. 1946), Benjamin Perman et al. discuss how proteins change when light energy is converted into a chemical signal in a halophilic phototrophic bacterium (1). It seems to us, however, that the crystallography in the report is in error. The most serious problem is that the proposed structure of the intermediate does not agree with the difference electron density map. The largest negative electron density feature (M) is appropriately ascribed to movement of the carbonyl oxygen, but the large positive density (L), which would logically be associated with the carbonyl, is incorrectly ascribed to movement of the carbon-carbon double bond.