Structural knowledge of telomeric DNA is critical for understanding telomere biology and for the utilization of telomeric DNA as a therapeutic target. Very little is known about the structure of long human DNA sequences that may form more than one quadruplex unit. Here, we report a combination of molecular dynamics simulations and experimental biophysical studies to explore the structural and dynamic properties of the human telomeric sequence (TTAGGG)(8)TT that folds into two contiguous quadruplexes. Five higher order quadruplex models were built combining known single human telomeric quadruplex structures as unique building blocks. The biophysical properties of this sequence in K(+) solution were experimentally investigated by means of analytical ultracentrifugation and UV spectroscopy. Additionally, the environments of loop adenines were probed by fluorescence studies using systematic single-substitutions of 2-aminopurine for the adenine bases. The comparison of the experimentally determined properties with the corresponding quantities predicted from the models allowed us to test the validity of each of the structural models. One model emerged whose properties are most consistent with the predictions, and which therefore is the most probable structure in solution. This structure features contiguous quadruplex units in an alternating hybrid-1-hybrid-2 conformation with a highly ordered interface composed of loop residues from both quadruplexes.
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