Sterically Controlled Recognition of Macromolecular Sequence Information by Molecular Tweezers

Sequence-specific binding is demonstrated between pyrene-based tweezer molecules and
soluble, high molar mass copolyimides. The binding involves complementary π−π stacking interactions,
polymer chain-folding, and hydrogen bonding and is extremely sensitive to the steric environment around
the pyromellitimide binding-site. A detailed picture of the intermolecular interactions involved has been
obtained through single-crystal X-ray studies of tweezer complexes with model diimides. Ring-current
magnetic shielding of polyimide protons by the pyrene “arms” of the tweezer molecule induces large
complexation shifts of the corresponding 1H NMR resonances, enabling specific triplet sequences to be
identified by their complexation shifts. Extended comonomer sequences (triplets of triplets in which the
monomer residues differ only by the presence or absence of a methyl group) can be “read” by a mechanism
which involves multiple binding of tweezer molecules to adjacent diimide residues within the copolymer
chain. The adjacent-binding model for sequence recognition has been validated by two conceptually different
sets of tweezer binding experiments. One approach compares sequence-recognition events for copolyimides
having either restricted or unrestricted triple−triplet sequences, and the other makes use of copolymers
containing both strongly binding and completely nonbinding diimide residues. In all cases the nature and
relative proportions of triple−triplet sequences predicted by the adjacent-binding model are fully consistent
with the observed 1H NMR data.