From owner-biophysics@net.bio.net Mon Nov 30 22:00:00 1998 From: "clive delmonte" Newsgroups: bionet.biophysics,bionet.molec-model,bionet.xtallography Subject: DNA Structure: Puzzle Number 2 Date: Tue, 1 Dec 1998 10:19:15 -0000 Lines: 32 X-Newsreader: Microsoft Outlook Express 4.71.1712.3 X-MimeOLE: Produced By Microsoft MimeOLE V4.71.1712.3 NNTP-Posting-Host: 195.7.227.184 Message-ID: <36643fd2.0@news.netdirect.net.uk> X-Trace: 1 Dec 1998 19:13:22 GMT, 195.7.227.184 Path: biosci!agate!newsfeed.berkeley.edu!btnet-peer!btnet!news-lond.gip.net!news.gsl.net!gip.net!demeter.clara.net!news.clara.net!peer.news.nildram.co.uk!news-peer.netdirect.net.uk!news.netdirect.net.uk!195.7.227.184 Xref: biosci bionet.biophysics:4608 bionet.molec-model:2334 bionet.xtallography:4500 It was Franklin's famous diffraction photograph of B-DNA (2), shown to him by Wilkins, that Watson sketched on the train back to Cambridge to rejoin Crick. One of Wilkins' collaborators, A R Stokes (3), working on the Fourier transform for a helix, showed that, defining as "delta" the angle between an arm of the "helical cross" and the Y-axis (meridian), tan delta = helical pitch / helical circumference Applying this result to Franklin's photograph of B-DNA, delta is measured as 40 degrees, and, with a pitch of 3.34 nm, the helical diameter is found to be some 1.3 nm. For a supposed double helical diameter of 2 to 2.2 nm, delta should be 28 - 26 degrees respectively, far removed from the actual value of 40. Interestingly, the helical diameter of 1.3 nm, found from Stokes equation applied to Franklin's photograph, coincides with the STM value of 1.3 nm measured by Lee et al. (Puzzle 1, ref. 1) and recorded in their Figure 3(D). 2 Molecular Configuration in Sodium Thymonucleate; R.E. Franklin & R.G. Gosling; Nature Vol 171 (1953) 740 - 741 3 The Theory of X-ray Fibre Diagrams; A.R. Stokes; Prog. Biophys. & Biophys. Chem. Vol 5 (1955) 140 - 167 -------------------------------------------------------------- Clive Delmonte .