The relationship between molecular architecture and the nature of interactions with lipid bilayers has been studied for a series of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers using small-angle X-ray scattering (SAXS) and thermal analysis (differential scanning calorimetry, DSC). The number of molecular repeat units in the hydrophobic poly(propylene oxide), PPO, block has been found to be a critical determinant of the nature of triblock copolymer-lipid bilayer association. For dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-based biomembrane structures, polymers possessing a PPO chain length commensurate with the acyl chain dimensions of the lipid bilayer yield highly ordered, swollen lamellar structures consistent with well-integrated (into the lipid bilayer) PPO blocks. Triblock copolymers of lesser PPO chain length yield materials with structural characteristics similar to a simple dispersion of DMPC in water. Increasing the concentration (from 4 to 12 mol %) of well-integrated triblock copolymers enhances the structural ordering of the lamellar phase, while concentrations exceeding 16 mol % result in the formation of a hexagonal phase. Examination of temperature-induced changes in the structure of these mesophases (complex fluids) reveals that if the temperature is reduced sufficiently, all compositions exclude polymer and thus exhibit the characteristic SAXS pattern for hydrated DMPC bilayers. Increasing the temperature promotes better insertion of the polymers possessing PPO chain lengths sufficient for membrane insertion. No temperature-induced structural changes are observed in compositions prepared with PEO-PPO-PEO polymers that feature PPO length insufficient to permit full incorporation into the lipid bilayer.

We report Zn 2+ -dependent deoxyribozymes that ligate RNA. The DNA enzymes were identified by in vitro selection and ligate RNA with k obs up to 0.5 min −1 at 1 mM Zn 2+ and 23 °C, pH 7.9, which is substantially faster than our previously reported Mg 2+ -dependent deoxyribozymes. Each new Zn 2+ -dependent deoxyribozyme mediates the reaction of a specific nucleophile on one RNA substrate with a 2′,3′-cyclic phosphate on a second RNA substrate. Some of the Zn 2+ -dependent deoxyribozymes create native 3′-5′ RNA linkages (with k obs up to 0.02 min −1 ), whereas all of our previous Mg 2+ -dependent deoxyribozymes that use a 2′,3′-cyclic phosphate create non-native 2′-5′ RNA linkages. On this basis, Zn 2+ -dependent deoxyribozymes have promise for synthesis of native 3′-5′-linked RNA using 2′,3′-cyclic phosphate RNA substrates, although these particular Zn 2+ -dependent deoxyribozymes are likely not useful for this practical application. Some of the new Zn 2+ -dependent deoxyribozymes instead create non-native 2′-5′ linkages, just like their Mg 2+ counterparts. Unexpectedly, other Zn 2+ -dependent deoxyribozymes synthesize one of several unnatural linkages arising from reaction of an RNA nucleophile other than a 5′-hydroxyl group. Two of these unnatural linkages are the 3′-2′ and 2′-2′ linear junctions created when the 2′-hydroxyl of the 5′-terminal guanosine of one RNA substrate attacks the 2′,3′-cyclic phosphate of the second RNA substrate. The third unnatural linkage is a branched RNA resulting from attack of a specific internal 2′-hydroxyl of one RNA substrate at the 2′,3′-cyclic phosphate. When compared with the consistent creation of 2′-5′ linkages by Mg 2+ -dependent ligation, formation of this variety of RNA ligation products by Zn 2+ -dependent deoxyribozymes highlights the versatility of transition metals like Zn 2+ for mediating nucleic acid catalysis. † This research was supported by the Burroughs Wellcome Fund (New Investigator Award in the Basic Pharmacological Sciences to S.K.S.), the March of Dimes Birth Defects Foundation Many of the Mg 2+ -dependent deoxyribozymes mediate RNA ligation via reaction of a 2′,3′-cyclic phosphate, which may be opened by an attacking nucleophile with cleavage of either the P-O 2′ bond or the P-O 3′ bond. If the nucleophilic group is a 5′-hydroxyl, then these two reaction pathways lead to the isomeric native 3′-5′ linkage or non-native 2′-5′ linkage, respectively ( Figure 1). Of course, the attacking nucleophile is not necessarily a 5′-hydroxyl, but this functional group is typically the most nucleophilic of those present in RNA. Indeed, in our previous studies, all of the Mg 2+ -dependent deoxyribozymes that use a 2′,3′-cyclic phosphate RNA substrate were found to create only non-native 2′-5′ linkages (path ii in Figure 1), although the selection strategy itself did not compel this high selectivity against 3′-5′ linkages (46-50). Therefore, understanding the distribution of ligation products upon DNAcatalyzed reaction of a 2′,3′-cyclic phosphate RNA substrate i...

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