Dear Zbyszek Wouldn't ripples be the results of calculating maps with truncated Fourier summations (unavoidably), and, consequently, be more obvious around a sharp feature such as an heavy atom metal center?
The mathematic basis can be found here: http://en.wikipedia.org/wiki/Gibbs_phenomenon With best regards, Thierry Note: sent a 2nd time as it seems that it did not reach the BB the first time. Apologies if the message reaches you twice. -----Original Message----- From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of Zbyszek Otwinowski Sent: Wednesday, July 06, 2011 1:19 PM To: [email protected] Subject: Re: [ccp4bb] How to evaluate Fourier transform ripples The question about Fourier transformation ripples has a straightforward answer in a fairly typical situation: A) data are collected to the resolution limit of diffraction, B) phases are uniform in quality across the resolution range, which is equivalent to R-free being uniform with respect to resolution within a factor of 2 or so, C) maps are not sharpened. The ripples originate from not including unobserved structure factors. The intensity of diffraction decreases rapidly past the measurability limit, so, in the above situation, the unobserved diffraction contributes very little. Consequently, the answer is that typically one should not see ripples. Ripples should not be confused with the effect of electron density maps being smoothed by vibrations and other forms of disorder. Zbyszek Otwinowski > > Dear All, Hi. I was asked in a manuscript revision to discuss > about the possible effects of Fourier transformation ripples on the > crystallographic results. Specifically, the reviewers question whether > ripples may affect on the electron density around heavy metal center which > has a Mo-S-As connection. From which angle or in which way this problem > should be addressed most convincingly ? Thank you for any > suggestion.Best,Conan Zbyszek Otwinowski UT Southwestern Medical Center at Dallas 5323 Harry Hines Blvd. Dallas, TX 75390-8816 Tel. 214-645-6385 Fax. 214-645-6353 Notice: This e-mail message, together with any attachments, contains information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New Jersey, USA 08889), and/or its affiliates Direct contact information for affiliates is available at http://www.merck.com/contact/contacts.html) that may be confidential, proprietary copyrighted and/or legally privileged. It is intended solely for the use of the individual or entity named on this message. If you are not the intended recipient, and have received this message in error, please notify us immediately by reply e-mail and then delete it from your system.
