Fig. 4.19 Stacking interactions between electron-poor (Dan) and electron-rich (Ndi) can be used to direct the hetero-association of aromatic foldamers in buffered water. The mutual interaction of these two aromatic units leads to stable helical structure.
heteroduplexes increases more or less additively with each interacting pair of aromatic units contributing ca. AG = —1.9 kcal mol—1 in stability. Hence, no co-operativity is observed. Nevertheless, a tetramer has already a surprisingly large stability of K = 350000 M—1 in buffered water. The thermodynamic signature as determined by ITC shows an enthalpy driven association with a negative entropy contribution. The large negative entropy contribution most likely stems from the loss in the flexibility of the linkers. Otherwise simple aromatic stacking interactions are often also entropically favored due to the release of ordered water molecules from the interface (classical hydrophobic effect) . The heteroduplex also seems to be kinetically rather stable as it migrates as one sharp band in a poly-acrylamide gel electrophoresis experiment. Unfortunately, the exact structure of the heteroduplex is not known yet .
Similar zipper-shaped heteroduplexes were recently presented by Li and cow-orkers . Again, either electron rich dialkoxynaphthalenes or electron-deficient pyromellitic diimides were linked via simple alkylchains of variable length (Fig. 4.20). Concentration dependent NMR studies with two tetramers in chloroform indicate the formation of heteroduplexes of moderate stability (K < 3000 M—1). The chain length or chemical nature of the linker (ester or amide groups) only marginally affected the stability of the duplexes. However, the addition of polar solvents such as methanol or DMSO reduced the stability even so only moderately. This is in contrast to other aromatic stacking systems where solvophobic interactions play an important role . These however increase with increasing polarity of the solvent . Again, the exact structure determination of the duplex remains an open task.
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