Formation and dissociation of synthetic hetero-double-helix complex in aqueous solutions: Significant effect of water content on dynamics of structural change

A 1:1 mixture of the ethynylhelicene pseudoenantiomers (M)-tetramer and (P)-pentamer, which possess hydrophilic terminal tri(ethyleneglycol) (TEG) groups, changes their structures in the water-THF (10 μM) solvent system between dissociated random-coils and an associated hetero-double-helix upon heating and cooling. A small change in water content between 30 and 33% significantly affects the dynamics of structural changes. At 30% water content, heating to 60 °C causes rapid formation of random-coil and cooling to 10 °C causes the rapid formation of hetero-double-helix, accompanied by repeated changes in Δϵ at 369 nm between 0 and -2000 cm^-1 M^-1. Heating and cooling experiments at constant rates between 60 and 10 °C resulted in sigmoidal curves in Δϵ/temperature profiles, which indicate rapid structural changes. Different phenomena occurred at 33% water content. Heating to 60 °C and cooling to 0 °C initially induced changes in Δϵ between 0 and -2000 cm^-1 M^-1, and repeated cycles gradually reduced the range between 0 and -500 cm^-1 M^-1. Heating and cooling experiments at constant rates between 60 and 10 °C caused small changes in Δϵ, and repeated cycles at 10 °C gradually increased Δϵ to -500 cm^-1 M^-1. These phenomena involved rapid changes in molecular structure and slow structural changes in the water-THF solvent system. The sharp switching of the dynamics of structural changes at water content between 30 and 33% indicated discontinuous structural changes in the hydration of TEG and/or in water clusters in the vicinity of oligomer molecules.