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(a) A fragment of the electron-density map at the 1.5σ contour level calculated after P-SAD phasing and density modification by the programs SHELXD and SHELXE, with the final model of a Cyt–Gua pair shown. (b) The model encompassing a total of 243 atoms of the dodecamer obtained from direct-methods phasing by SHELXD (some fragments belong to symmetry-equivalent molecules).
