Quantification of the π-π interactions that govern tertiary structure in donor-acceptor [2]pseudorotaxanes

Flexibility in pseudorotaxanes and interlocked molecules that rely on interactions between π-donor-acceptor subunits provides access to folded structures reminiscent of the tertiary structure of proteins. While they have been described before, only now have we been able to quantify one such tertiary structure by making use of pseudorotaxanes designed for the purpose. Here, the enhanced stability of a pseudorotaxane inside a folded structure is measured to be ΔG = ca. 0.5 kcal mol(-1). The tertiary structure is stabilized by a charge-transfer interaction between a tetrathiafulvalene-based π-donor that can situate alongside a π-accepting paraquat-based macrocycle by folding of a flexible linker. At room temperature, it was estimated that 70% of the pseudorotaxanes examined here exist in their folded state. This quantitative information is critical for the creation of interlocked molecular machines that have predictable energetics and structures and for revealing a complexity approaching biological molecules.