The duplex-forming activities of oligonucleotides can be photomodulated by incorporation of an azobenzene unit. Upon isomerizing the trans-azobenzene to the cis form by irradiation with UV light, the T(m) value of the duplex (with the complementary DNA) is lowered so that the duplex is dissociated. The duplex is formed again when the cis-azobenzene is converted to the trans-azobenzene by irradiation with visible light. The photoregulation is successful irrespective of the position of the azobenzene unit in the oligonucleotides. The trans-azobenzene in the oligonucleotides intercalates between two DNA base pairs in the duplexes and stabilizes them because of a favorable enthalpy change. The nonplanar structure of a cis-azobenzene is unfavorable for such an interaction. These photoresponsive oligonucleotides are promising candidates for the regulation of various bioreactions.

When a DNA 22-mer ( 32 P-labeled at either the 3'-or the 5'-end) was cut by [Ce(NH 4 ) 2 (NO 3 ) 6 ], two types of fragments were formed for each of the internucleotide linkages. These fragments were separated by gel electrophoresis and treated with three natural enzymes (bacterial alkaline phosphatase, T 4 polynucleotide kinase, and terminal deoxynucleotidyl transferase). With these enzymatic treatments, all the fragments were quantitatively transformed into the expected forms. Thus the first direct evidence for the hydrolytic scission of DNA oligomers by the Ce IV salt has been provided. Both phosphate termini and OH termini are formed at the 3'-and the 5'-ends of the fragments by the hydrolysis of phosphodiester linkages. This work demonstrates the potential of the metal ion as a tool in molecular biology and biotechnology.

The sequence-selective scission of RNA has been attracting interest because of its potential applications, both in vivo and in vitro. [1] Artificial ribonucleases have been prepared by attaching catalytically active metal ions (mostly lanthanide ions) to DNA oligomers as sequence-recognizing moieties. [2±4] However, the following limitations remain: 1) divalent metal ions such as Zn II and Mg II , which are widely spread in vivo, [5] cannot be used, and 2) selective scission can be achieved only when free metal ions are absent in the reaction mixtures (otherwise, nonselective scission becomes dominant). Further progress is desirable.It has previously been shown that dinuclear Zn II complexes hydrolyze RNA under physiological conditions. [6,7] Although the Zn II ion itself is a rather poor candidate for RNA hydrolysis, a notable activity appears when two of them cooperate. Here we report that conjugates of a dinuclear Zn II complex and DNA oligomers selectively hydrolyze RNA at the target site, even in the presence of a considerable amount of free Zn II ions.Phosphoramidite monomer 1 containing a N,N,N',N'-tetrakis(2-pyridylmethyl)-3,5-bis(aminomethyl)benzene (TPBA) group was prepared according to Scheme 1. By use of 1 and CH 2 OH

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