Mimicking protein environments with bioinorganic models
Through the design and application of a novel modular synthetic strategy, a diverse family of phosphate receptors has been synthesised. The receptors were based around a complexed metal centre such as zinc (II) or copper (II) within a polyazamacrocycle framework. The initial design was to couple the polyazamacrocycle directly to a crown ether system to give a heteroditopic scaffold where the two components were in close proximity to each other. Due to the synthetic difficulties encountered, a novel approach was developed, whereby the use of an amino acid linker was employed to couple the two species. The modular approach successfully allowed the variation of the metal centre through incorporation of different sized polyazamacrocycles. In the first instance we used a four-nitrogen donor host, cylen, to chelate zinc and in the second we attached a smaller triazamacrocycle, tacn to complex copper metal. The linker was changed to allow variations in polarity, functionality and structure through the selection of specific backbone residues. The synthesis and characterisation of glycine, aminodiacetic acid and glutamic acid derivatives proved that the methodology was effective and viable for a number of different linkers with varying structure. The use of iminodiacetic acid enabled the coupling of two cyclen metal hosts which enabled the synthesis of a bimetallic zinc complex and a novel tritopic system. Most importantly, the artificial receptors that we have created were shown to be excellent hosts for phosphate moieties. The proposed mode of binding in our systems was entropically driven through the efficient desolvation of the crown ether cavity upon phosphate binding. The interactions were in general, endothermic (+DH°) which was assigned to the reorganisation of solvent molecules upon expulsion which is in agreement with a growing body of evidence in the literature. Through isothermal titration calorimetry, 1H NMR and UV/vis titration experiments, we were able to determine that the ditopic systems acted as ion pair receptors. The binding of one ion affected the affinity of the ion partner. In the case of ZnL1.(OTf)2 it was shown that the individual binding of the cation is negligible but in the presence of phosphate the cation exerts a positive influence on the binding of the anion. We were able to calculate through simple control experiments that the binding of inorganic phosphate with a number of different metal counter ions exhibited positive cooperativity. This is one of the first examples of positive cooperativity between ion pairs in an aqueous environment at physiological pH.