The image is actually from http://www.ruppweb.org/Xray/tutorial/vectordiag.htm
but the first image is missing. There you see that the 'vectors' are actually (general acentric) structure factors for a single anomalous atom, and each of those of course has a real and imaginary component. The real part of the anomalous structure factor f' has to be collinear with the fo part (in contrast to what the images insinuate, f' - being negative - points opposite to fo, but that makes those drawings even more confusing) The f" structure factor part then of course also has a real and imaginary part, but the f" contribution to the scattering is retarded by 90 degrees (i.e. -270 = +90, offset by pi). The key is to remember that these arrows are structure factors of a single atom, not the scattering factors themselves. BR -----Original Message----- From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of William Scott Sent: Wednesday, October 13, 2010 10:48 AM To: [email protected] Subject: [ccp4bb] embarrassingly simple MAD phasing question Hi Citizens: Try not to laugh. I have an embarrassingly simple MAD phasing question: Why is it that F" in this picture isn't required to be vertical (purely imaginary)? http://www.doe-mbi.ucla.edu/~sawaya/tutorials/Phasing/phase.gif (Similarly in the Harker diagram of the intersection of phase circles, one sees this.) I had a student ask me and I realized that there is this fundamental gap in my understanding. Many thanks in advance. -- Bill William G. Scott Professor Department of Chemistry and Biochemistry and The Center for the Molecular Biology of RNA 228 Sinsheimer Laboratories University of California at Santa Cruz Santa Cruz, California 95064 USA phone: +1-831-459-5367 (office) +1-831-459-5292 (lab) fax: +1-831-4593139 (fax)
