[GRAPHICS] A method is presented for the synthesis of single compounds or small combinatorial libraries of oligonucleotides with 2'-acylamido-2'-deoxyuridine residues at the 3'-terminus. Selection experiments identified the residue of anthraquinone-2-carboxylic acid as a "molecular cap" that increases the UV melting point of the duplex (5'-ACGCGU-3')(2) by up to 28degreesC compared to the unmodified control duplex.