Hi Carlos Well, diffraction is a consequence of the crystal lattice repeat (or it could be a 1-D repeat in a fibre), for without the lattice there could be no diffraction, only scattering. But the lattice repeat is obviously directly related to the average distance between atoms and the number of atoms in the cell, the more atoms there are on average the bigger the cell, and the number of orders of diffraction you observe, and hence the number of reflections, is related to the lattice repeat and the wavelength. To get sufficient independent data for structure solution & refinement you need rather more unique reflections than atoms, so doing the detailed calculation you find that the lattice repeat (or cell volume), the number of atoms and the number of reflections cancel each other out, and you're left with the requirement that the wavelength must be equal to the average interatomic distance, plus or minus a factor of ~ 1.5.
-- Ian > -----Original Message----- > From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On > Behalf Of Carlos Frazao > Sent: 24 January 2007 09:09 > To: [email protected] > Subject: [ccp4bb] relation between wavelength and > inter-atomic distances > > Hi, > I have once heard and recently read that "the diffraction > event results > from the fact that both the X-rays wavelength and the atomic > distances > are of the same magnitude". Although such a relation seems > appealing I > am unsure if this is not a mere coincidence. Could someone clarify or > lead me to a relevant reading. > Cheers, > Carlos > > -- > ************************************** > Dr. Carlos Frazao > Crystallography Department > ITQB-UNL, Av Republica, Apartado 127 > 2781-901 Oeiras, Portugal > > Phone: (351)-214469666 > FAX: (351)-214433644 > e-mail: [EMAIL PROTECTED] > www.itqb.unl.pt > > Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). It may not be used or disclosed except for the purpose for which it has been sent. If you are not the intended recipient you must not review, use, disclose, copy, distribute or take any action in reliance upon it. If you have received this communication in error, please notify Astex Therapeutics Ltd by emailing [EMAIL PROTECTED] and destroy all copies of the message and any attached documents. Astex Therapeutics Ltd monitors, controls and protects all its messaging traffic in compliance with its corporate email policy. The Company accepts no liability or responsibility for any onward transmission or use of emails and attachments having left the Astex Therapeutics domain. Unless expressly stated, opinions in this message are those of the individual sender and not of Astex Therapeutics Ltd. The recipient should check this email and any attachments for the presence of computer viruses. Astex Therapeutics Ltd accepts no liability for damage caused by any virus transmitted by this email. E-mail is susceptible to data corruption, interception, unauthorized amendment, and tampering, Astex Therapeutics Ltd only send and receive e-mails on the basis that the Company is not liable for any such alteration or any consequences thereof.
