Synthesis of Light-Responsive Bridged Nucleic Acid and Changes in Affinity with Complementary ssRNA
The control of molecular properties through the use of external stimuli such as pH,[1] temperature,[2] or change in redox potential[ 3] is an attractive area of research for the regulation of various biological phenomena. Among such stimuli, light is an ideal trigger because of its potential for spatiotemporal control of cellular chemistry.[4] The compounds that can release an active molecule upon light irradiation are called “caged compounds”.[ 5] In general, the properties of caged compounds, such as affinity for a specific molecule, can be changed only once (Figure 1A).[6] If another trigger could cause additional alteration after photoirradiation, it would be possible to achieve control of molecular properties in two stages; this would allow for stricter regulation of biological phenomena (Figure 1 B). ACHTUNGTRENUNGRecently, we reported the synthesis and properties of 2’-O,4’-Cmethyleneoxymethylene- bridged nucleic acid (2’,4’-BNACOC),[7] which is a 2’,4’-BNA/LNA[8] analogues (Scheme 1). It was observed that oligonucleotides containing 2’,4’-BNACOC show high affinity for complementary single-stranded RNA (ssRNA) because the sugar conformation of 2’,4’-BNACOC is prelocked in the N-type conformation, which is the major conformation in the A-form of RNA duplex structure.[9] Here, we focused on this property of 2’,4’-BNACOC and designed light-responsive BNA I that contains a 2’-hydroxy group and a 4’-hydroxymethyl group that are both protected by a photolabile 6-nitroveratryl group (Scheme 1).[10] This compound is expected to retain its binding ability to complementary ssRNA, despite the presence of a large hydrophobic group at the minor groove,[11] because its sugar conformation is restricted to N-type. Irradiation with light results in cleavage of the bridged structure of I; this transforms the analogue to the 4’-O-benzoyl form II, which contains a bulky substituent at the 4’-C position[11] and loses its sugar conformation restriction. Hence, oligonucleotides containing II lose their binding ability to complementary ssRNA. The 4’-O-benzoyl group of II may be removed by treatment with various nucleophiles; this results in the formation of 4’-Chydroxymethyl- RNA analogue III. 4’-C-hydroxymethyl-RNA analogue III restores the binding ability to complementary ssRNA, despite a flexible sugar conformation, because of a reduction in steric hindrance.[12] In other words, the affinity of oligonucleotides containing light-responsive BNA I for complementary ssRNA may be changed in two stages, unlike conventional caged compounds, triggered by light and treatment with nucleophiles.
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