Oligoamide-Based Molecular Duplex Strands

Strands 1 and 2 (Fig. 4.1), with self-complementary hydrogen bonding sites DADA and AADD, respectively, formed quadmply hydrogen-bonded duplexes with similar dimerization constants, = 4.4 x 10 and = 6.5 x 10 M, respectively, which were much higher than that for the two hydrogen-bonded complexes 3-3, = 25 M [12]. This suggests that there exists positive cooperativity. 

The above oUgoamide-based duplexes with adjustable stability found wide applications. For example, by attaching reactive olefin units into the above quadmply hydrogen-bonded duplex, Gong used it as a template for highly efficient and 

5 A molecular duplex based on urea and imide hybrid and its crystal stmctuie. Reprinted with the permission from Ref. [17], Copyright 2011 American Chemical Society 

Specific olefin metathesis reactions (Fig. 4.7) [22]. Formation of the duplex via quadmple hydrogen bonds brought the two olefin units into close proximity and thus lowered the AS of the reaction, which greatly facilitated the reactions. If the olefin units were present at the opposite side of the two individual strands, no cross-linked product could be isolated. A similar strategy was also adapted by Guan et al. to construct polymer materials with interesting contracting property [23]. 

In addition to catalytic metathesis reaction, dynamic disulfide bond formation reaction was also used by Gong s group to achieve hydrogen bonding in polar solvents [24]. When treated 1 1 mixed strands 19 and 20 in CH2CI2, CH3OH, and H2O with I2, cross-linked product 21 was detected in each case (Fig. 4.8). Mechanistic study revealed a thermodynamically controlled process [25] two hydrogen bonds were sufficient to shift the equilibria toward the formation of the cross-linked product in aqueous solution. 

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