The lipophilic nucleoside 3‘,5‘-didecanoyl-2‘-deoxyguanosine, dG 1, extracts potassium salts from
water into organic solvents. The K+ extraction drives the self-association of dG 1 to give G-quartet
structures. A series of 1H NMR experiments indicates that the identity of the assembled species in
CDCl3 is modulated by the amount of K+ extracted by dG 1. At an 8:1 dG 1 to K+ picrate ratio, the
octamer (dG 1)8−K+ predominates in solution. The (dG 1)8−K+ supramolecular complex, formed
by coordination of a single K+ ion by eight dG 1 monomers, is robust and structurally unique. The
1H NMR chemical shifts for both the exchangeable and nonexchangeable protons of (dG 1)8−KI in
CDCl3 were assigned from a combination of 2D 1H−1H and 13C−1H correlation experiments. One
set of 1H NMR signals corresponds to a dG 1 nucleoside with an anti conformation about the C(1‘)−N(9) glycosidic bond, whereas the other set of signals is due to 50% of the didecanoyl dG 1 adopting
a syn conformation. Although the possible arrangements of an octamer containing a 1:1 ratio of
anti dG 1 to syn dG 1 are many, the present NMR analysis leads to a defined single species composed
of two G-quartets. In one tetramer, all of the dG 1 components have a syn conformation about the
C(1‘)−N(9) glycosidic bond, while the other tetramer has an “all-anti” conformation. Moreover,
intertetramer NOEs are consistent with stacking of the “all-anti” tetramer in a “head-to-tail”
orientation on top of the “all-syn” tetramer, thus sandwiching a central K+ ion. This solution
structure is, to our knowledge, different from all of the assembled structures described so far for
guanine aggregates. Presumably, the K+-bound octamer represents the first observable stage of
the assembly process in the aggregation of dG 1.
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