Reversible Diselenide Cross-links are Formed Between Oligonucleotides Containing 2’-Deoxy-6-selenoinosine

We have synthesized and characterized a phosphoramidite derivative of 2’-deoxy-6-selenoinosine (d6SeI) and incorporated this modification into an oligonucleotide by solid-phase synthesis. During cleavage from the solid-support and deprotection, spontaneous dimerization of this oligonucleotide occurs via formation of a diselenide cross-link between the modified nucleobases. This cross-link can be readily reduced to restore the single-stranded oligonucleotide. UV thermal denaturation and circular dichroism spectroscopy of duplexes with d6SeI paired against all four native nucleobases revealed minor differences in stability and structure relative to 2’-deoxyinosine. This selenium containing nucleobase modification may be useful for applications in DNA nanomaterials and X-ray crystallography

Stabilization of i-motif structures by 2'-β-fluorination of DNA

i-Motifs are four-stranded DNA structures consisting of two parallel DNA duplexes held together by hemi-protonated and intercalated cytosine base pairs (C:CH(+)). They have attracted considerable research interest for their potential role in gene regulation and their use as pH responsive switches and building blocks in macromolecular assemblies. At neutral and basic pH values, the cytosine bases deprotonate and the structure unfolds into single strands. To avoid this limitation and expand the range of environmental conditions supporting i-motif folding, we replaced the sugar in DNA by 2-deoxy-2-fluoroarabinose. We demonstrate that such a modification significantly stabilizes i-motif formation over a wide pH range, including pH 7. Nuclear magnetic resonance experiments reveal that 2-deoxy-2-fluoroarabinose adopts a C2'-endo conformation, instead of the C3'-endo conformation usually found in unmodified i-motifs. Nevertheless, this substitution does not alter the overall i-motif structure. This conformational change, together with the changes in charge distribution in the sugar caused by the electronegative fluorine atoms, leads to a number of favorable sequential and inter-strand electrostatic interactions. The availability of folded i-motifs at neutral pH will aid investigations into the biological function of i-motifs in vitro, and will expand i-motif applications in nanotechnology.This work is dedicated to the Memory of Alfredo Villasante, valuable collaborator and friend. FUNDING Funding for open access charge: NSERC Discovery grant (to M.J.D., A.K.M.); CIHR DDTP Training Grant (to H.A., R.H.V.); MINECO [BFU2014-52864-R to C.G.]; CSIC-JAE contract (to N.M.P.). Conflict of interest statement. None declaredS

Multiarm-star Polyisobutylenes by Living Carbocationic Polymerization

This article describes the synthesis of high molecular weight multiarm-star branched polyisobutylenes by living polymerization, using multifunctional initiators, and their initial characterization. First, macrointiators carrying tert-hydroxy function-alities were synthesized by the radical copolymerization of 4-(1-hydroxy-1-methylethyl)-styrene with styrene. This copolymerization system was found to be ideal with r1 ≡ r2 ≡ 1. Selected macroinitiators with average functionalities of 8–73 were then used to synthesize the star-branched polyisobutylenes. Polymers with molecular weights up to M̄n = 400,000 were obtained within 30–60-min reaction times, while under similar conditions the monofunctional 2-chloro-2,4,4-trimethylpentane initiator yielded M̄n ≈ 10,000 in 20 min. This can be viewed as an indirect proof that simultaneous multiple initiation took place with the macroinitiators. Under controlled conditions a branchedpolyisobutylene with M̄n = 375,000 and MWD = 1.2, and theoretically calculated 23 arms, with no detectable side products was obtained under living conditions in 60 min; the molecular weight of this polymer increased linearly with time. The branched structure of the polymers were demonstrated by SEC-LLS analysis and core destruction of selected samples. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 85–92, 199