英语翻译Diffraction by helices It may be shown (also stokes,unpu

英语翻译
Diffraction by helices
It may be shown (also stokes,unpublished)that the intensity distribution in the diffraction pattern of a series of points equally spaced along a helix is given by the squares of Bessel functions.A uniform continuous helix gives a series of layer lines of spacing corresponding to the nth layer line being proportional to the square of Ja ,the nth order Bessel function.A straight line may be drawn approximately through the innermost maxima of each Bessel function and the origin.The angle this line makes with the squator is roughly equal to the angle between an element of the helix and the helix axis.If a unit repeats n times along the helix there will be a meridional reflexion on the nth layer line.The belical configuration produces side-bands on this fundamental frequency,the effect being to reproduce the intensity distribution about the orgin around the new origin,on the nth layer line,corresponding to C in Fig.2.
We will now briefly analyse in physical terms some of the effects of the shape and size of the repeat unit or nucleotide on the diffraction pattern.First,if the nucleotide consists of a unit having circular symmetry about an axis parallel to the helix axis,the whole diffraction pattern is modified by the form factor of the nucleotide.Second,if the nucleotide consist of a series of points on a radius at right-angles to the helix axis,the phases of radiation scattered by the helices of different diameter passing through each point are the same.Summation of the corresponding Bessel functions gives reinforcement for the inner
Fig.2.Diffraction pattern of system of helices corresponding to structure of deoxypentose nucleic acid.The squares of Bessel functions are plotted about O on the equator and on the first ,second,third an fifth layer lines for half of the nucleotide mass at 20 A.diameter and remainder distributed along a radius,the mass at a given radius being proportional to the radius.About C on the tenth layer line similar functions are plotted for an outer diameter of 12 A .