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Structures of duplexes with parallel and antiparallel backbones. (a) Duplexes where the sequence of recognition units dictates the backbone arrangement: parallel for DDA·AAD and ADD·DAA and antiparallel for DDA·DAA and ADD·AAD. (b) Duplex where the sequence of recognition units allows parallel and antiparallel backbone arrangements to coexist in equilibrium: AAA·DDD.
