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Nonconventional C-H···F Hydrogen Bonds Support a Tetrad Flip in Modified G-Quadruplexes.

A G-quadruplex adopting a (3 + 1)-hybrid structure was substituted at its 5'-tetrad by 2'-deoxy-2'-fluoro-arabinoguanosine (FaraG) analogs. Incorporation of anti-favoring FaraG at syn-positions of the 5'-outer tetrad induced a reversal of the tetrad polarity without noticeably compromising the quadruplex stability. This conformational change is shown to be promoted by nonconventional C-H···F hydrogen bonds acting within the anti-FaraG residues.