Photoirradiation of dibenzoyldiazomethane (DBDM) produced highly electrophilic benzoylketene via Wolff rearrangement. DBDM derivative possessing an aminoalkyl side chain induced a DNA cleavage selectively at guanine (G) residues upon photoirradiation and subsequent piperidine treatment. In order to devise photochemical DNA cleavers that can specifically alkylate a guanine residue proximal to the target sequence of long DNA fragments, a new reagent, DBDM-OSu, which facilitates the connection of DBDM unit to various DNA binders, was developed. DBDM-oligonucleotide (ODN) conjugates 5 and 6 were obtained by the coupling of 5'-aminohexyl 8-mer [H2N(CH2)(6)-d(ACGTCAGG)-3'] and 15-mer [H2N-(CH2)(6)-d(ACGTCAGGTGGCACT)-3'], respectively, with DBDM-OSu in aqueous acetonitrile in the presence of sodium bicarbonate. Photoirradiation of 5 and 6 in the presence of 25-mer 5'-d(AGTGCCACCTGACGTCTG(18)CTCTCTC)-3' having a complementary sequence induced cross-linking of both oligomers. A distinct cleavage band at guanine residue (G(18)) was observed upon heating the cross-linked oligomers with piperidine. A similar DNA cleavage reaction of 5'-d(AGTGCCACCTGACG(14)TG(16)CG(18)TG(20)CG(22)-TCT)-3' having multiple guanine sites in the presence of DBDM-ODN conjugate 6 indicated that the most effectively cleaved site is G(16). These results demonstrated that DBDM-oligonucleotide conjugates can serve as a new class of photonucleases that can cleave single-stranded DNA at predetermined guanine sites. Furthermore, the reagent DBDM-OSu can be used as a convenient and effective photoinducible electrophile for the cross-linking or the modification of biopolymers.