A solid support bound polynucleotide, preferably a 3'-O- or 5'-O-substituted polynucleotide, is disclosed. The polynucleotides are of formula: ##STR00001## wherein Q is a substantially pure polynucleotide, each X is O or S; Y is an inert spacer group; R represents the solid support. Also disclosed is a method of isolating a target biomolecule, preferably a protein or a polynucleotide, from a mixture.