I see that the resolution of X-ray solved protein structures from the PDB (the whole of PDB) has a very high correlation with the size of the protein structures. I guess, this is nothing new but I would like to know if there is something else that you might be aware of regarding the inter-dependence of resolution and size of the proteins.
We observed that resolution (R) of X-ray protein structures is directly proportional to 1/log(N) where N is the number of residues in the protein. May be this (or something similar) is already a known standard term (or function) while calculation of X-ray structures' resolution.
Can someone explain?
more the size of protein, more are the number of interactions between protein molecules in the crystal. the interactions include hydrophobic, ionic etc. when you have high number of interactions, it does not always lead to good crystal packing and therefore results in poor resolution also. e.g. In case of salt crystals, the number of interactions are less involving ionic interactions only which results in very good crystal packing, so those crystals diffract well with high resolution. this is vice versa for macromolecules and their complexes.
Thanks Tahir for the comment. However I was looking for any well established mathematical relationship between the two (Resolution and size). I had found something close where an empirical relation between the mass of the protein, crystal size, and resolution has been suggested for X-ray structures (Holton and Frankel KA, 2010). Refer the link for this publication.
http://journals.iucr.org/d/issues/2010/04/00/ba5148/index.html
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