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Thomas Womack wrote: > . . . >There are a number of high-resolution structures deposited recently containing haems near cysteines, and in most of them a re-refinement gives substantial positive difference density about the CM atoms; there is even occasionally a sign of some kind of longer tail coming out from the CMB position. It seems to be purely positive density, rather than the dipole that tends to be diagnostic of anisotropy. > > My current thought is that even a 1.4A structure of a haem-containing protein (I'm looking at an autoBUSTER re-refinement of the 3fo3 deposition from EMBL Hamburg) may well come from a crystal sufficiently well-ordered that we're seeing hydrogens - one of these structures has at least one isoleucine with green blobs at positions which would be reasonable for every hydrogen on the side-chain - but I would be interested to know other peoples' experience and interpretation. In the case of non-covalently bound hemes, this can be indicative of promiscuity with respect to orientation around the pseudo-2-fold axis: In a few percent of the heme, CMB is really CAC, with CBC coming out of it. But I guess this is impossible for covalently-bound heme (which is inserted and ligated by an enzyme) and at that resolution CBC should be resolved, so I think your explanation is reasonable. Ed