Cation-directed self-assembly of lipophilic nucleosides: the cation's central role in the structure and dynamics of a hydrogen-bonded assembly

This paper focuses on the cation's central role in controlling the self-assembly of a lipophilic nucleoside, isoguanosine (isoG) 2, in organic solvents. First, we use H-1 NMR spectroscopy to show that a Ba2+ cation directs a mixture of the isomers isoG 2 and guanosine (G) I to self-sort into separate assemblies, without any detectable G-isoG cross-association. Next, we use electrospray ionization mass spectrometry to show that the cation controls the reversible self-assembly of isoG 2. Final section focuses on the dynamic exchange of components between two different assemblies, namely, a (isoG 2)(5)-Li+ pentamer and a (isoG 2)(10)-Li+ decamer. Our H-1 and Li-7 NMR data is consistent with a cation-filled pentamer, (isoG 2)(5)-Li+, moving as a unit during a bimolecular pentamer-decamer exchange. These data highlight crucial aspects regarding the cation-templated self-assembly of lipophilic nucleosides: (1) the structural information encoded within each nucleoside dictates the size and shape of the hydrogen-bonded assembly; (2) a cation is required to template and stabilize these discrete hydrogen-bonded assemblies, and (3) dynamic exchange of cation-filled, hydrogen-bonded units is likely to be a hallmark of these multi-layered nucleoside assemblies. (C) 2002 Elsevier Science Ltd. All rights reserved.