Disordered single crystal evidence for a quadruple helix formed by guanosine 5'-monophosphate

The disodium salt of guanosine 5'-monophosphate (5'-GMP) has been crystallized earlier in an orthorhombic array. We have obtained a new crystal form of 5'-GMP at pH 8 which reveals a clear helical nature, with guanine bases stacked perpendicular to the helix axis. Although the X-ray pictures show partial disorder, they can be indexed on a hexagonal net with a = b = 28.6 A, c = 9.8 A, V = 6942 A3 (1A = 0.1 nm). The probable space group is P6(4), and past experience with ca. 600 A3 per base in oligonucleotide crystals suggests that the cell contains 12 GMP molecules. The crystal packing parameters and the intensity distribution agree with a model of three hydrogen-bonded guanine tetrads in the unit cell, stacked so as to build a quadruple helix similar to that proposed earlier from fiber studies (Zimmerman, S.B., J. Mol. Biol. 106, 663-672 (1976)).