Major Cation-Dependent Polymorphisms

The reasons for the dramatic difference between the solution-state Na form and the crystal-state form of d[AGGG(TTAGGG)3] are yet to be fully elucidated major changes in structure have previously been noted in CD studies of the transition, although it had not been possible to assign structures to the individual species. On the other hand, several recent studies have demonstrated that an antiparallel G-quadruplex is formed by the human telomere repeat sequence in solution in the presence of both Na and 

Attempts to determine the high-resolution structure of the 22-mer human telomeric sequence G-quadruplex in the presence of ions by NMR in solution have been unsuccessful because of the presence of multiple conformations. The crystal structure of the related bimolecular G-quadruplex formed by the sequence d(TAGGGTTAGGGT) in the presence of has 

A more recent study concerning the formation of G-quadruplexes by oligonucleotides with the common sequence motif dG4-loop-dG4, where the loop consisted of non-nucleosidic residues, also demonstrated formation of a single structure in the presence of Na ions and several structures in the 

A systematic study of G-rich oligonucleotides containing runs of G residues of various lengths, and with varying numbers intervening residues, revealed a cation-dependent pattern for the G-quadruplex folds adopted by these sequences. It was shown that ions are required to stabilize chair type structures, that is, G-quadruplexes with lateral loops instead of diagonal loops. A minimum of three residues in the central loop and two residues in the two outer loops is required to form an intramolecular chair-type quadruplex, otherwise a mixture of tetramolecular parallel-stranded quadruplexes is 

Longer loops and more G-quartets favor G-quadruplexes with a 

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