Associations of Water-Soluble Macrocyclic Hosts with 4-Aminoazobenzene: Impact of pH

Abstract

An investigation of the pH effect on the inclusion complexes of β-cyclodextrins and calixarenesulfonates with 4-aminoazobenzene was conducted both by experiments and molecular simulations. The whole thermodynamic characterizations of the association between hosts and 4-aminoazobenzene (K, Δr G 0, Δr H 0, and TΔr S 0) were determined by UV–visible spectroscopy. β-Cyclodextrin inclusion complexes are not affected by pH change unlike those obtained with calixarenes. All the studied systems were enthalpically favored. Nevertheless, the entropic behavior is different depending on the host. In order to interpret these experimental results, molecular simulations were used to calculate the number of atoms inserted into the cage-like host compounds and the number of water molecules expelled from the cavity.

Thermodynamics of Supramolecular Associations with Macrocyclic Water-Soluble Hosts

“…No significant changes are observed by changing the pH in concordance with our experimental results. 19 The Gibbs free energy profile in Figure 2 shows some local minima characterized by separation distances that clearly implies the insertion of the 4AA into the CD cavity in line with our previous results. Because the azobenzene moiety is longer than the cyclodextrin cavity, the 4AA is included longitudinally in the β-cyclodextrin cavity in perfect agreement with some reported NMR results.…”

“…The calculation of the free energy profiles obtained here and the presence of these barriers give a quantitative answer regarding this complexation and confirm that there is no association between the calixarene and the 4AA in water, in perfect agreement with our experimental and simulation results. 19 The same conclusions can be drawn for the (C6S, 4AA) association from the analysis of Figures S2–S4 (see the Supporting Information).…”

“…Computational Details. As shown by experimental results, 19 the stoichiometry of these complexes is 1:1. As a consequence, the simulation boxes were composed of one host, e.g., a calixarene (see Figure 1c for C4S) or a β-Cd (Figure 1a); one guest, e.g., a 4AAB (Figure 1b); and 2000 water molecules.…”

“…We previously reported experimental and computational investigations on the association of water-soluble macrocyclic hosts (β-cyclodextrin and p-sulfonatocalixarenes) with 4-aminoazobenzene (4AA) 19 in water (Figure 1). UV–visible spectroscopy was used to provide the whole thermodynamic characterizations of the association (Δ r G , Δ r H , and T Δ r S ).…”

Molecular Description of Grafted Supramolecular Assemblies on Gold Surfaces: Effect of Grafting Points and Chain Lengths

“…In acidic solution, all the sulfonate groups located on the upper rim are deprotonated. 11 Depending on the host, some ions such as Cl – or Na + can be added to maintain the neutrality of the simulation box. The details of the grafting methodology are given in Supporting Information S1.…”