GUANINE RESIDUES IN D(T2AG3) AND D(T2G4) FORM PARALLEL-STRANDED POTASSIUM CATION STABILIZED G-QUADRUPLEXES WITH ANTIGLYCOSIDIC TORSION ANGLES IN SOLUTION

We report below on proton NMR studies of the G-quadruplex structure formed by the human telomere sequenced (T2AG3) and the tetrahymena telomere sequenced(T2G4) in K cation containing solution. We observe well-resolved proton NMR spectra corresponding to a G-quadruplex monomer conformation predominant at 50 mM K cation concentration and a G-quadruplex dimer conformation predominant at 300 mM K cation concentration. By contrast, d(T2AG3T) and d(T2G4T) form only the G-quadruplex monomer structures independent of K cation concentration as reported previously [Sen, D., & Gilbert, W. (1992) Biochemistry 31, 65-70]. We detect well-resolved resonances for the exchangeable guanine imino and amino protons involved in G-tetrad formation with the hydrogen-bonded and exposed amino protons separated by up to 3.5 ppm. The observed NOEs between the amino and H8 protons on adjacent guanines within individual G-tetrads support the Hoogsteen pairing alignment around the tetrad. The imino protons of the internal G-tetrads exchange very slowly with solvent H2O in the d(T2AG3) and d(T2G4) quadruplexes. The nature and intensity of the observed NOE patterns establish formation of parallel-stranded right-handed G-quadruplexes with all anti guanine glycosidic torsion angles. A model for the parallel-stranded G-quadruplex is proposed which is consistent with the experimental NOE data on the d(T2AG3) and d(T2G4) quadruplexes in solution. These results are in contrast to earlier studies on d(G(n)T(m)G(n)) sequences where the structures of the monovalent cation stabilized G-quadruplex exhibit alternating G(syn)-G(anti) glycosidic torsion angles along individual strands in solution [Wang, Y., de los Santos, C., Gao, X., Greene, K., Live, D., & Patel, D. J. (I 99 1) J. Mol. Biol. 222, 819-8321 and in the crystalline state [Kang, C. H., Zhang, X., Ratliff, R., Moyzis, R., & Rich, A. (1992) Nature 356, 126-131].