Internucleobase-Interaction-Directed Self-Assembly of Nanofibers from Homo- and Heteroditopic 1,ω-Nucleobase Bolaamphiphiles

The complementary 1,ω-thymine, 1,ω-adenine, and 1,ω-(thymine, adenine) bolaamphiphiles, [N,N‘-bis[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)propionyl]1,n-diaminoalkane [T - n -T (n = 10, 11, 12)], N, N‘-bis[3-(6-aminopurine-9-yl)propionyl]1,n-diaminoalkane [A - n -A (n = 10, 11, 12)], and N-[3-(2,4-dihydroxy-5-methylpyrimidine-1-yl)propionyl], N‘-[3-(6-aminopurine-9-yl)propionyl]1,n-diaminoalkane [T- n -A (n = 10, 11, 12)], respectively] have been synthesized. The spontaneous homo- and heteroassembly of these nucleobase-based bolaamphiphiles has been studied by light microscopy, energy-filtering transmission electron microscopy, FT-IR, and powder X-ray diffraction analyses. The achiral T-10-T bolaamphiphile produced in 10% ethanolic/aqueous solutions unprecedented double-helical ropes of 1−2 μm in widths and several hundred micrometers in length, whereas the complementary homologue A-10-A gave only microcrystalline solids of 1−10 μm in size. In contrast, an equimolar mixture of T-10-T and A-10-A yielded supramolecular fibers of 15−30 nm in width. 1H NMR, CD, and UV studies of solution photoreactions of T-10-T suggested that under natural light the chiral rope formation is triggered by photodimerization of trace amounts of the thymine moieties in the T-10-T assemblies. Complementary hydrogen bond formation between the thymine−adenine heterobase pairs was found to prevent such a photoreaction and resulted in no chiral rope formation. The heteroditopic T-12-A bolaamphiphile self-assembled to form supramolecular fibers. Multilamellar organization was proposed for the homo- and heteroassemblies made of T - n -T and A - n -A.