Yo Kushida

Heating/Cooling Stimulus Induces Three-State Molecular Switching of Pseudoenantiomeric Aminomethylenehelicene Oligomers: Reversible Nonequilibrium Thermodynamic Processes

A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene (P)-tetramer and (M)-pentamer formed three states, namely, the heterodouble helices B and C and the random coil A. At high temperatures, A is the most stable. At low temperatures, C is the most stable, and the structural changes from A to the metastable state B to the product C occur, where B and C have pseudoenantiomeric helical structures. Heating then converts C to A. Essentially, all the molecules change their structure from A to B to C to A. Various nonequilibrium reversible thermodynamic responses appeared depending on thermal conditions: The metastable states A and B can be interconverted with thermal hysteresis without forming C in a far-from-equilibrium manner; three-state hysteresis occurs; states A and B can be frozen at low temperatures and defrosted by warming. An energy and population model for the three-state switching is given, involving inversion of thermodynamic stability and thermal hysteresis.

Molecular Thermal Hysteresis in Helix‐Dimer Formation of Sulfonamidohelicene Oligomers in Solution

Sulfonamidohelicene oligomers up to the nonamer level were synthesized by the repeated coupling reactions of a building block. A tetramer formed a helix dimer in 1,3-difluorobenzene, which unfolded to a random coil with heating. This structural change exhibited thermal hysteresis in which different thermal responses were observed in the course of temperature increase and decrease. The feature of the hysteresis was examined under different heating/cooling modes, and the mechanisms are discussed on the basis of the population change and the presence of an induction period. A proposal regarding the use of thermal hysteresis for sensing a temperature increase/decrease is also given.

Energy Aspects of Thermal Molecular Switching: Molecular Thermal Hysteresis of Helicene Oligomers

Molecular switching is a phenomenon by which a molecule reversibly changes its structure and state in response to external stimuli or energy. Herein, molecular switching is discussed from thermodynamic and kinetic aspects in terms of energy supply with an emphasis on the thermal switching exhibited by helicene oligomers. It includes the inversion of relative thermodynamic stability induced by temperature changes and molecular thermal hysteresis in a closed system. The thermal phenomenon associated with the oligomers involves population/concentration changes between metastable states under nonequilibrium thermodynamic control.

Molecular Function of Counting the Numbers 1 and 2 Exhibited by a Sulfoneamidohelicene Tetramer

The sulfoneamidohelicene tetramer in solution exhibits different molecular responses to the same cooling stimulus delivered once and twice under thermal hysteresis conditions. Its random-coil state at a high temperature was cooled and maintained at a given temperature for which its molecules remained in a random coil (first cooling); the resulting solution was heated and cooled, after which a helix dimer formed (second cooling). Such a property can be regarded as a molecular function of counting the numbers 1 and 2.

Concentration Threshold and Amplification Exhibited by a Helicene Oligomer during Helix-Dimer Formation: A Proposal on How a Cell Senses Concentration Changes of a Chemical

Sulfonamidohelicene tetramer (M)-1 exhibits a concentration threshold and amplification phenomena in solution during helix-dimer formation from a random-coil. The (M)-tetramer is a random-coil below a threshold concentration, and the concentration of the helix-dimer is irreversibly amplified once the threshold concentration is exceeded. For example, a 15% increase in (M)-tetramer total concentration from 0.6 to 0.7 mM induces an 8-fold increase in the concentration of the helix-dimer, being 8:0.15=53-fold amplification, under temperature oscillation conditions between 47 and 49 °C. Experiments without oscillation also exhibit concentration amplification. The threshold and amplification phenomenon involves concentration hysteresis, being away from equilibrium, and self-catalysis. On the basis of this study, a proposal on how a biological cell senses concentration changes of a chemical substance is provided.

Deterministic and Stochastic Chiral Symmetry Breaking Exhibited by Racemic Aminomethylenehelicene Oligomers

Racemic mixtures of aminomethylenehelicene (P)- and (M)-pentamers exhibited deterministic and stochastic chiral symmetry breaking during hetero-double-helix formation and self-assembly in solution. Heating a 50:50 mixture of (P)- and (M)-pentamers at 90 °C, and then cooling the mixture to 70 °C resulted in hetero-double-helix formation; a Cotton effect with negative Δε at λ=315 nm appeared. Chiral self-assembly occurred when the mixture was cooled to 25 °C. A strong tendency of deterministic chiral symmetry breaking appeared at the molecular and self-assembled levels, which was indicated by the negative Δε at λ=315 nm that appeared in most cases in repeated experiments. Mixtures containing 60:40 and 40:60 (P)-/(M)-pentamers also self-assembled with the same chirality. When a homo-double-helix (P)-/(M)-pentamer and a random coil (M)-/(P)-pentamer were mixed, the chiral self-assembly formed stochastically, and heating and cooling resulted in deterministic chiral symmetry breaking.

Spatially Heterogeneous Nature of Self-Catalytic Reaction in Hetero-Double Helix Formation of Helicene Oligomers.

A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene oligomers, (P)-tetramer and (M)-pentamer, in fluorobenzene show a self-catalytic phenomenon in the formation of hetero-double helices from random coils. This study visualizes the spatially heterogeneous nature of the self-catalytic reaction in dilute solution. UV/Vis imaging analysis of the mixture at 70 °C, containing random coils, exhibits a homogeneous bright area. When the solution is cooled from 70 to 30 °C and held at that temperature, dark domains of approximately 1 mm in size appear, which move approximately at a rate of 1 mm min-1 . The dark domains indicate that weaker UV/Vis absorption results from the formation of hetero-double helices, which is supported by circular dichroism (CD) imaging experiments. Then a homogeneous mixture is regenerated upon heating to 55 °C, as shown by CD imaging. Under self-catalytic conditions, a homogeneous solution spontaneously changed to a heterogeneous solution in the process of hetero-double-helix formation.