title: Structure of poly d(AI).poly d(CT) in two different packing arrangements.
abstract: X-ray diffraction analysis of poly d(AI).poly d(CT) in oriented and polycrystalline fibers has revealed the DNA structure to be a 10-fold, right-handed, antiparallel, Watson-Crick base paired double helix in two distinct packing arrangements corresponding to one and two helices, respectively, in the unit cell. The helix pitch is 32.1 A and 32.4 A in the two cases, almost 1.5 A shorter than in classical B-DNA. The resulting B'-DNA geometry, described in terms of a tetranucleotide repeat which is conformationally similar to B-DNA, has its minor groove closely shut and major groove correspondingly widened, thus striking a sharp morphological contrast to B-DNA. According to difference electron density maps, a spine of hydration along the minor groove connects both strands and provides structural stability; ordered sodium ions and water molecules are actively involved in bridging the phosphate groups of neighboring helices. The crystallographic R-values for these two allomorphs are 0.26 and 0.20, respectively, for data up to 3.0 A resolution.
authors.affiliation: Whistler Center for Carbohydrate Research, Purdue University, West Lafayette, IN 47907, USA. Chandrar@foodsci.purdue.edu
authors.forename: R
authors.initials: R
authors.lastname: Chandrasekaran
authors.forename: A
authors.initials: A
authors.lastname: Radha
authors.forename: H S
authors.initials: HS
authors.lastname: Park
mesh_terms: D018360:Crystallography, X-Ray; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D011089:Polydeoxyribonucleotides / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D011089:Polydeoxyribonucleotides