Minor nucleosides in RNA: Optical studies of dinucleoside phosphates containing dihydrouridine

Photoreduction with NaBH₄ was used to reduce the dinucleoside phosphates ApU, UpA, and GpU to the corresponding molecules containing dihydrouridine (H). Also obtained from this reaction are dinucleoside phosphates containing (β-N-ribosyl) ureido-propanol, an open ring form of dihydrouridine. The results of CD and ultraviolet absorption studies with these compounds imply that HpA is strongly stacked, but that ApH and GpH are only slightly stacked. The temperature dependence of the CD suggests that the stacking in HpA is unusual. The results with compounds containing open ring forms of dihydrouridine indicate that there is more intramolecular interaction when the ring is open than when it is closed.