We have used site-specifically spin-labeled polyribonucleotides to extract motional information for single-and double-stranded polyribonucleotides. The spin-label is attached to either position 4 or 5 of an uridine analogue, and in the latter position the label is linked through various tethers to the base.

Local base motion and local structural base information are derived with a simple motional model from site specifically spin-labeled polyribo- and polydeoxyribonucleotides. The model was developed earlier for some nucleic acids and has now been applied to analyze 22 different nucleic acid systems. We conclude that the base motion of the spin-labeled nucleotide in single-stranded RNA, DNA, or non-base-paired bases in duplexes is of the order of 1 ns and that its base mobility decreases by about a factor of 4 upon base pairing. Also, the tether motion of the probe is slower in an RNA than in a DNA duplex.