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Crystal structures of N(6),N(9)-dimethyladenine and N(6)-methyladenine hydrochloride were determined from three-dimensional x-ray diffraction data. The bases assume a conformation in which the N(6)-methyl group blocks one of the hydrogen-bonding sites normally used by adenine to form Watson-Crick pairs with thymine in double-helical DNA. When in this conformation, N(6)-methyladenine residues might alter the secondary structure of DNA. thereby preventing the scission of modified DNA's by restriction enzymes.

Crystals of 5-iodouracil are monoclinic, space group P21, with a= 14.210 (1), b = 4"458 (1), c= 4"899 (3) A, and r= 92.31 (1) °. There are two formula units per cell. Observed and calculated densities are 2-54 and 2.548 g cm -3, respectively. Intensity data for 591 reflections were collected on an automatic diffractometer. The structural parameters were refined by full-matrix least-squares calculations to an R index of 0.044. The molecules form planar, hydrogen-bonded ribbons, which are stacked with an inter-ribbon spacing of 3" 15 A and an 1. • • C(6) distance of 3"76/l, that is slightly shorter than a normal van der Waals contact. 1393

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