N. Camerman, J. Fawcett, A. Camerman
Nov 15, 1976
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Journal
Journal of Molecular Biology
Abstract
The crystal and molecular structure of the deoxyribose-dinucleotide sodium thymidylyl-(5′→3′)-thymidylate-(5′) hydrate (pTpT) at physiological pH has been determined by single-crystal X-ray diffraction. There are 4 dinucleotide molecules and approximately 52 water molecules in an orthorhombic unit cell of dimensions a =16·06, b =15·13, c =15·65 A, space group P 2 1 2 1 2. The two 5′-mononucleotide portions of the molecule have extremely similar conformations, with anti conformations of the bases with respect to the glycosidic bonds, C2′-endo deoxyribose rings, and trans arrangements of the P-O(5′) bonds with respect to the C(4′)-C(5′) bonds. This is the first experimental evidence of conformational consistency of mononucleotides in a larger molecule. The conformation about the phosphodiester bonds is (−)- gauche, trans , resulting in an extended dinucleotide molecule with base planes tilted away from being parallel by 39°. A thymidine polynucleotide structure has been generated by adding pTpT units of the same conformation; the result is a right-handed single helix in which the thymines point away from the helix axis and do not base-stack. Such a structure may be a model for poly(T). The pTpT crystal conformation fits the pTp electron density in a Staphylococcal nuclease-pTp-Ca 2+ complex very well; this and the high degree of hydration indicate that the observed dinucleotide conformation is maintained in biological milieus.