I believe some, may not be all, negative density may be due to the
choice of wavelength. C,O and N are not so sensitive to normally
used wavelength (1-1.54A), but S does. This is more true when you
have higher resolution and better quality data. You can estimate
this by f'.
By the way, although I am not a real supporter of "radicals from
damage", I believe that a photo-induced (here X-ray) chemical
reaction is much different from a heat-induced one. Low temperature
is not a problem. But this not for diffusion, as it is ~100% heat-
related. Please point out if I am wrong.
This "assignment" of free radicals to damage is often made
(flippantly) in the literature, but I feel a strong need to point
out that there is NO EVIDENCE of a free radical diffusion mechanism
for radiation damage below ~130K.
Lijun Liu, PhD
Institute of Molecular Biology
HHMI & Department of Physics
University of Oregon
Eugene, OR 97403
541-346-4080
