I'm a postgraduate student and just started studying X-ray crystallography. I'd like some answers about solving protein/DNA and proten/RNA stuctures, if its possible. I have read that the phasing problem can be solved either by multiple isomorphous replacement (MIR) or multiple anomalous dispersion (MAD). I have read about protein/DNA and RNA complexes that have been solved by MAD, but I haven't found any that have been solved using MIR. Are there any complexes that have been solved using MIR solely or maybe by using both MIR and MAD?
From what I understand, in order to determine the structure of a complex containing DNA or RNA, you have to substitute thymidine or uracyl with bromo-U or iodo-U or synthesize a DNA with S in place of O and then bond a Hg to the S. Are these modifications done in order to perform MAD? Can you also do these (or other) modifications in order to perform MIR? What modifications would you do to the DNA/RNA in order to perform MIR?
Probably it sounds very simple to you, but it's not to me!