Crystal structure of a naturally occurring dinucleoside phoaphate: uridylyl 3',5'-adenosine phosphate model for RNA chain folding.

The crystal structure of uridylyl 3’,5’-adenosine phosphate (UpA) has been determined and retied to an R value of O-062. There are two UpA molecules per asymmetric unit in 8 monoclinic unit cell with dimensions a = 16+91,6 = 12.37, c = 11-26 d and fi = 96.0'. The conformation of the phosphodiester linkage is different for the two molecules; one UpA suggests a model for a sharp turn in a single-stranded nucleic acid structure and the other for a helical structure. All the nuoleoside units have similar conformations. There is no complementary base pairing between the adenines and uracils but rather layers of base-paired adenines and base-paired uracils separated by 3.4 A. The predominant form of base stacking involves the ribosyl oxygen atom and the bases.