Boronic-acid-appended poly(L-lysine) (1) can form anionic boronate esters with diol derivatives in aqueous solution. One can thus expect that when nucleosides are mixed with 1, the resultant complexes will satisfy the basic structural characteristics of RNA, viz., one-dimensionally organized nucleic bases and anionic charges. To obtain insights into this novel RNA mimic, the solution properties of adenosine (Ad) were estimated in the absence and the presence of 1. The large absorption and CD spectral changes were observed at pH 6-10 where the poly(L-lysine) main chain tends to adopt an α-helix-rich conformation. On the basis of the fact that the magnitude of the spectral changes is much larger than the fraction of complexed Ad molecules, the polymer-supported organization mode was proposed: that is, Ad molecules complexed by covalent bonds with 1 act as clusters to induce further organization of uncomplexed Ad molecules by noncovalent interactions. The proposal was further confirmed by 1H NMR spectroscopy and dynamic light scattering. On the other hand, none of these spectral changes were observed for 2′-deoxyadenosine. The 1-Ad complex thus formed can interact with complementary poly(U) but not with noncomplementary poly(C), indicating that it can act as a novel RNA mimic.
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