ELECTRON-DIFFRACTION FROM PHOSPHOLIPIDS - AN APPROXIMATE CORRECTION FOR DYNAMICAL SCATTERING AND TESTS FOR A CORRECT PHASE DETERMINATION

An approximate experimental correction of electron diffraction intensi ties from an epitaxically crystallized phospholipid bilayer for dynami cal scattering is described. This correction, which is useful for cert ain low-angle centrosymmetric data sets, compares intensities recorded at high and low electron-accelerating voltages to ascertain which ref lections are most affected by n-beam interactions. When applied to exp erimental intensity data from 1,2-dihexadecyl-sn-glycerophosphoethanol amine (DHPE), the correction facilitates a direct phase determination based on the probabilistic estimate of three-phase invariants because a more accurate estimate of the hierarchy of \E(t)\ values is obtained . When a multisolution technique is used, incorporating algebraic unkn owns for certain phase values, the best phase assignment can be assess ed by comparison of the single convolution of phased structure factors to the observed structure factor magnitudes for the low-voltage data. This approach exploits an approximate analogy made earlier by Moodie between the Sayre equation and the phase grating series and is valid a s long as the single convolution adequately models experimental low-vo ltage data (a condition favored by light-atom structures in a low-angl e region of reciprocal space). In real space, the correct structure ca n also be readily identified as the one having the smoothest density p rofile.