The radical formed by loss of the hemiacetal OH group from either of the two cyclic forms of ribose (yielding ribofuranosyl and ribopyranosyl compounds), by combination with an H of an -NH- or a -CH- group; the natural nucleosides are ribosyl compounds, not ribosides, as the bond between ribose and aglycon is C-N or C-C, not -C-O-X-.
