Molecular structure of deoxyadenjlyl-3′-methylphosphonate-5′thymldine dlhydrate, (d-ApT-2H₂O), a dinudeodde monophosphate with neutral pbosphodiester backbone. An X-ray crystal study

dAPT, a modified deoxyribose dinucleoaide phosphate with an uncharged methylphosphonate group, crystallizes as dihy-drate in space group P2.2.2, a = 9.629(3), b = 20.884(6) and c = 14.173 (4)Å Z = 4. The structure has been determined using 2176 X-ray diffractometer reflections and refined to a final R of 0.105. Torsion angles about P-0(5′) and P-0(3′) bonds are -91.8° and 117-8°. The former is in the normal (-)gauche range while the latter is eclipsed. Bases are oriented anti, the sugar of adenosine is puckered T₃ (C(2′)endo) whereas that of thymidine displays puckering disorder with-major and minor occupancy sites. Major site is a half-chair ²T (C(2′ )endo-C(1′)exo) and minor site an envelope ^3kT² (C(3¹ )endo). Adenine and thymine bases of symmetry related molecules form reversed Hoogsteen type base pairs, water molecules are disordered in the crystal lattice.