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Hi, 1. the same image in jpg or png format occupies a mere 180 - 300 kB. It's nice to be considerate of other people's bandwidth especially since you're asking for help 2. how do you know it's a covalent modification? Anything on MS to confirm that? 3. this reminds me of http://www.jbc.org/cgi/content/full/278/3/2008/F1 4. in theory this could be a particularly stable calcium complex (what's the distance between the center of the fat blob in the middle and the N of His?) Cheers, Artem > Hi -- I am close to finishing up a protein structure of an aldo-keto > reductase but for a modification of a histidine residue shown here > (one in each molecule of the AU). Note the following electron density > in which this residue in both molecules of the AU was mutated to an > Ala, Refmac'd for 5 cycles and then 2fofc and fofc maps are shown. > Resolution goes to 1.6 A, and the Rwork/Rfree is 16/18.9%. However > this modification flumoxes me and I was hoping someone (someone!!) has > seen something like this before. This crystal was shot at LS-CAT but > I see something similar with a slightly lower resolution crystal I > shot on our homesourse Raxis4. Crystallization conditions are 0.2M Ca > Acetate, 9% PEG 8K,0.1M Na Cacodylate pH6.5, Cryo- 25%Et.Glycol. > Other people have suggested this may be a modification of the > cacodylate, however the cacodylate modifications I've seen in the > lit/pdb seem to refer to modifs of cysteine in which the arsenic > contains two oxygens attached to the arsenic atom which is in turn > bound to the cysteine sulfur. However in this case, there we seem to > have at least 4 atoms shown coming off the Nepsilon of the his residue. > > Thank you for your time. > > Alex Singer >