Opposite effects of successive hydration shells on the aqua ion structure of metal cations

Predicting the Thermodynamic Stability of Zirconium Radiotracers

The thermodynamic stability of a metal-ligand complex, as measured by the formation constant (log β), is one of the most important parameters that determines metal ion selectivity and potential applications in, for example, radiopharmaceutical science. The stable coordination chemistry of radioactive ⁸⁹Zr⁴⁺ in an aqueous environment is of paramount importance when developing positron-emitting radiotracers based on proteins (usually antibodies) for use with positron emission tomography. Desferrioxamine B (DFO) remains the chelate of choice for clinical applications of ⁸⁹Zr-labeled proteins, but the coordination of DFO to Zr⁴⁺ ions is suboptimal. Many alternative ligands have been reported, but the challenges in measuring very high log β values with metal ions such as Zr⁴⁺ that tend to hydrolyze mean that accurate thermodynamic data are scarce. In this work, density functional theory (DFT) calculations were used to predict the reaction energetics for metal ion complexation. Computed values of pseudoformation constants (log β') are correlated with experimental data and showed an excellent linear relationship (R² = 0.97). The model was then used to estimate the absolute and relative formation constants of 23 different Zr⁴⁺ complexes using a total of 17 different ligands, including many of the alternative bifunctional chelates that have been reported recently for use in ⁸⁹Zr⁴⁺ radiochemistry. In addition, detailed computational studies were performed on the geometric isomerism and hydration state of Zr-desferrioxamine. Collectively, the results offer new insights into Zr⁴⁺ coordination chemistry that will help guide the synthesis of future ligands. The computational model developed here is straightforward and reproducible and can be readily applied in the design of other metal coordination compounds.

QM/MM investigations of organic chemistry oriented questions

About 35 years after its first suggestion, QM/MM became the standard theoretical approach to investigate enzymatic structures and processes. The success is due to the ability of QM/MM to provide an accurate atomistic picture of enzymes and related processes. This picture can even be turned into a movie if nuclei-dynamics is taken into account to describe enzymatic processes. In the field of organic chemistry, QM/MM methods are used to a much lesser extent although almost all relevant processes happen in condensed matter or are influenced by complicated interactions between substrate and catalyst. There is less importance for theoretical organic chemistry since the influence of nonpolar solvents is rather weak and the effect of polar solvents can often be accurately described by continuum approaches. Catalytic processes (homogeneous and heterogeneous) can often be reduced to truncated model systems, which are so small that pure quantum-mechanical approaches can be employed. However, since QM/MM becomes more and more efficient due to the success in software and hardware developments, it is more and more used in theoretical organic chemistry to study effects which result from the molecular nature of the environment. It is shown by many examples discussed in this review that the influence can be tremendous, even for nonpolar reactions. The importance of environmental effects in theoretical spectroscopy was already known. Due to its benefits, QM/MM can be expected to experience ongoing growth for the next decade.In the present chapter we give an overview of QM/MM developments and their importance in theoretical organic chemistry, and review applications which give impressions of the possibilities and the importance of the relevant effects. Since there is already a bunch of excellent reviews dealing with QM/MM, we will discuss fundamental ingredients and developments of QM/MM very briefly with a focus on very recent progress. For the applications we follow a similar strategy.