We have previously described that oligonucleotides containing phenylsulfoxide derivative of 2-amino-6-vinyulpurine nucleoside analog (1) are activated within duplex to form cross-link toward cytidine selectively at the target site. The new cross-linking motif with phenylsulfoxide structure (2) is characteristic in that the stable precursor may be transformed automatically within duplex to a reactive species. To search for more stable precursor susceptible for activation, we designed a series of substituted phenylsulfide analogs of 1. It has been demonstrated that introduction of an electron-donating group on the phenyl ring improved the cross-linking reaction. Particularly, 2-carboxyphenyl sulfide derivative exhibited cross-linking as effectively as phenylsulfoxide derivative without chemical oxidation prior to cross-linking.
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