Single and Double Threading of Congo Red into γ-Cyclodextrin. Solution Structures and Thermodynamic Parameters of 1:1 and 2:2 Adducts, as Obtained from NMR Spectroscopy and Microcalorimetry

Abstract

Detailed NMR studies of aqueous solutions (pH 7) of γ-cyclodextrin (γCD) and the azo dye Congo Red (CR) show distinct, concentration-independent ¹H NMR signals for different species. A very stable 1:1 pseudorotaxane (K ¹¹=38,000±1100 M^-1) is formed. In addition, a second complex corresponding to a 2:2 adduct (K ²²=13±3 M^-1) is produced by dimerisation of the 1:1 species. The structure of the 1:1 pseudorotaxane involves fast motion of the γCD ring along the CR backbone, leaving the outer naphthalene rings free. This entity undergoes structural reorganisation and dimerises to form the 2:2 adducts. Variable-temperature spectra did not lead to coalescence and allowed for the calculation of K ¹¹ and K ²² at each temperature and also of the corresponding thermodynamic parameters. Therefore, formation of the 1:1 complex is favourable (ΔG=-26.1±0.1 kJ/mol) and exothermic (ΔH=-21.7±1.0 kJ/mol), whereas formation of the 2:2 entity is also favourable (ΔG=-6.36±0.58 kJ/mol) but endothermic (ΔH=+43.3±8.7 kJ/mol). The corresponding values for entropy change are both positive (ΔS ¹¹=+14.5±0.7 J/mol, ΔS ²²=+166±33 J/mol). Isothermal titration calorimetry studies confirm the NMR findings. For the 1:1 complexation, the dependence of K upon the concentration is indicative of the dimerisation to form the 2:2 complex. When CR is in excess, aggregation processes involving 2:2 complexes and CR molecules are observed by NMR and calorimetry.

Keywords:

• Congo Red
• γ-Cyclodextrin
• 1:1 Pseudorotaxane
• 2:2 Pseudorotaxane
• Aggregation
• NMR
• Microcalorimetry
• Thermodynamic parameters

Acknowledgements

The scholarship of NCSR “Demokritos” to N.M. is gratefully acknowledged.

Notes

The Future of Supramolecular ChemistrySupramolecular Chemistry, as the chemistry beyond the molecule, has become a basic discipline that combines and uses all methods and techniques of traditional chemistry “divisions”, and has also acquired the power and tools to comprehend and bridge other disciplines of science together. In this sense, of course, nearly everything can be characterised as “supramolecular”. What I value, however, the most, and I think as the central point, is the fact that Supramolecular Chemistry has brought to the foreground the crucial importance of the weak intermolecular interactions, the dynamic relationships of molecules within interacting systems, and the multitude of factors that drive a process to select one direction (or one structure) over several others available. It has been amazing to realise that such soft interactions make life what it is and even more fascinating to discover similar phenomena in the world of small molecules in solution as described herein. In this sense, Supramolecular Chemistry has a long future ahead, as more tools are becoming available to study systems, and more synthetic methodologies are being developed to prepare them. So, one does not have to despair with “Supra” being outdated and “nano” coming into scientific fashion (and proposal jargon). Supramolecular chemistry has diffused everywhere and underlies all aspects of today's research.Dina Yannakopoulou graduated from the Chemistry Department, University of Athens, and moved to the USA, where she earned an MS degree with Professor R. A. Abramovitch at Clemson University, SC (1985) and a PhD degree with Professor A. R. Katritzky at the University of Florida (1988). She joined the Institute of Physical Chemistry at NCSR “Demokritos” in 1989, where she still works as a staff scientist. Her interests include the characterisation of host--guest interactions mainly by NMR spectroscopy with emphasis on structure, stability and equilibria of cyclodextrin--guest systems. Another (recent) aspect is synthetic modifications of cyclodextrins, synthesis of new guests, and interactions of both with surfaces.