DIRECT DETERMINATION OF PHOSPHOLIPID LAMELLAR STRUCTURE AT 0.34-NM RESOLUTION

Low-dose, high-resolution electron microscopy combined with conventional direct-phasing methods based on the estimates of triplet-structure invariants are used to determine phase values for all observed electron-diffraction-structure factor magnitudes from epitaxially oriented multilamellar paracrystals of the phosphospholipid 1,2-dihexadecyl-sn-glycerophosphoethanolamine. The reverse Fourier transform of these phased-structure factors is a one-dimensional electrostatic potential map that strongly resembles the electron-density maps calculated from similar x-ray-diffraction data. Determination of the phase values for the electron-diffraction data with structure invariants alone is nearly as successful as the combined use of two separate methods, assigning values to 13 of the 16 reflections-i.e., the electrostatic potential map closely resembles the one calculated with all data.