Hi there, At low resolution, the quality of the electron density can lead you to introducing errors in the modeling process. In particular, the density for the side chains degrades (and below a certain resolution, helices appear only as filled tubes - all detail has gone). I don't know what low resolution you are talking (writing) about, but I'd certainly try to position an "ideal" alpha-helix using a few large side chains (if you are lucky to have any in that helix), refine using secondary structure (helical) restraints, and see if it makes sense afterwards. And low resolution refinement is not an easy process, as you will find out. Fred. xinghua qin wrote:
hi everyone: Thanks for all the response! what I said last time may be chinglish and lead to some misunderstanding, the word I said fix a helix means that the model quality of the helix is not very good, I want to modify it manually.It is so hard to do this in low resolution.I modified the helix according to the electron density map, after refinement, large red areas appear in the difference map. That made me crazy! Are there any suggestions or any reviews about this point? I am new beginner to structural biology, any suggestions are welcomed !
Best regards
Xinghua Qin