"Diffraction by helical structures"

Date:
1956-1957
Reference:
PP/CRI/H/2/17
Catalogue details

Licence: In copyright

Credit: "Diffraction by helical structures". Source: Wellcome Collection.

    14/98
    illuminâting« ït is probably the simplest to imagine geometrically*. We shall call it the basic helix since it leads to a very simple interpretation of the integer m f occurring in the selection rule (2) and characterising the various branches of the diffraction pattern. The equation to the basic helix is V- airi/p =- o \ This is a member of a family of space-filling helices all of the same pitch P t defined by r s const. 1 SP-^ÏTaysp s- coMfc. ® f Now we can imagine a helix passing through each atom, and can fix the position of each atom by a new set of co-ordinates, namely the azimuth of the helix passing through it, and the fraction of the helix screw measured along the helix. That is f we make the co ordination transformation ^ _ •% ^ 1 - V^y P «I - > Then the phase factor determining the contribution of the atom to JL .n. on the C layer line becomes jìì We thus see that m has the meaûing of an index for translational periodicity along the basic helix just as -£ is an index for ♦Mote that our basic helix corresponds to one of the two genetic helices of Bear ( iqSf> ). The choice above is that having the greatest pitch or smallest distance between equivalent points along the helix.