Geometries of the F:A pairs in (A) duplex 1 and (B) in duplex 2 with d1 and d2 indicated. Conformational consequences at the duplex level due to replacement of T with F. Superimposition of the central tetramer duplexes (AATF):(AATF) and (AATT):(AATT) from the crystal structures of the modified and native DDD in complex with BhRNase H, resp., viewed (C) along the dyad and into the major groove, and (D) rotated around the horizontal by 90° and viewed along the helical axis. Duplex DNA atoms are colored gray (C; pink, native DDD), red (O), blue (N), orange (P) and green (F).