| Sumario: | We have determined the structure of a PNA-DNA duplex to 1.7 A resolution by multiple- wavelength anomalous diffraction phasing method on a zinc derivative. This structure represents the first high-resolution 3D view of a hybrid duplex containing a contiguous chiral PNA strand with complete γ-backbone modification (`yPNA"). Unlike the achiral counterpart, which adopts a random-fold, this particular yPNA is already preorganized into a right-handed helix as a single strand. The new structure illustrates the unique characteristics of this modified PNA, possessing conformational flexibility while maintaining sufficient structural integrity to ultimately adopt the preferred P-helical conformation upon hybridization with DNA. The unusual structural adaptability found in the γPNA strand is crucial for enabling the accommodation of backbone modifications while constraining conformational states. In conjunction with NMR analysis characterizing the structures and substructures of the individual building blocks, these results provide unprecedented insights into how this new class of chiral γPNA is preorganized and stabilized, before and after hybridization with a cDNA strand. Such knowledge is crucial for the future design and development of PNA for applications in biology, biotechnology, and medicine.
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