Binding calorimetry host-guests

With the advent of modem instrumentation that furnish all pertinent thermodynamic data from a single experiment within a few hours calorimetry can help to tackle problems in host-guest binding that are inaccessible (principally or on a quantitative basis) to any other method. The power of enthalpic measurements, however, is unleashed only if put in a well-defined structural context. As a corollary, the benefit of sound structure-energy correlation on this basis calls for a more intense synthetic input. 

Costa and co-workers have reported 80, a fluorescent squaramide-containing macrocyclic receptor for monitoring sulfate in water [97]. Isothermal titration calorimetry was employed to characterize the host-guest association of 80 with S04 , PhOPOs " and 204 dianions (titration carried out in methanol at 294 K). The data was fitted to a 1 1 binding model and it was foimd that 80 boimd S04 (4.6 1.0 x 10 M ) with the... 

A study of the complex formation of CB [5] with K ion in neutral water using titration calorimetry showed that CB [5] binds ion in a 1 2 stoichiometry. The utility of water-soluble CB [6J was also illustrated by formation of a stable 1 1 host-guest complex with acetylcholine in neutral water. The good solubilities of CB [5J and CB [6] in organic solvents make then-applications in ion-selective electrodes (ISEs) possible. Membrane electrodes prepared with CB [51 and CB [6J... 

The origin for the less-than-optimal repeatability of ITC titrations can be traced to systematic errors in sample treatment as well as to difficulties in data evaluation. First in line of the factors that interfere with the reproducibility of calorimetric results is the purity of the compounds used. Owing to the ubiquity of heat effects, even small deviations from a nontinal composition of a compound may result in dramatic differences in the calorimetric output. A frequent problem of this kind in abiotic host-guest binding is the presence of the solvent of crystallization. Ordinarily, this is not considered an impurity, as it is often present in a fixed stoichiometric ratio and can be accounted for in elemental analyses and in the spectroscopic evaluation. In calorimetry, however, solvent of crystallization adds a heat contribution of unknown size, which does not emerge from the interaction under study and thus tends to falsify the results. The problem is amplified by the polarity... 

The linkage of chemical processes in thermodynamic cycles arising from the definition of the state functions (the state of a system is independent on the way on which it was reached) is a great virtue that can immediately be exploited by calorimetry. In this sense, making use of the protonation properties is just a special case in the much broader field of competition analysis [25]. In fact, any molecular species that is expelled or taken up by the host as a result of binding, the guest of interest may serve as a probe in calorimetric determinations. Prominent examples include the binding or release of metal cations from proteins (e.g. Ca +, Mg +) which can be linked to their complexation by complexones (ethylenediaminetetra-acetic acid (EDTA) or similar). 

---