Resumen de Structure and reactivity of hydrogen-bonded radical cations: application to amino acids and peptides
This work analyses the effects of ionisation on the structure and reactivity of different kind of systems, which can be involved in biological oxidative or mass spectrometry processes, by means of quantum chemical methods. In the first part of the work we focus on the changes induced on the binding energies of AHn systems ( A = Li-F and Na-Cl), along with their behaviour when interacting with H2O. The second part of this work includes an exhaustive theoretical study on the influence of ionisation in amino acids and peptides, the basic structural units of proteins.
A crucial point observed in this work is the large increase of acidity induced in a system upon removal of one electron. Moreover, ionisation can change the acidity relative order of different functional groups present in biological molecules. As a consequence, ionisation largely affects the hydrogen bonds in amino acids and peptides in such a way that important reorganisations such as isomerisations or proton-transfer processes, leading to distonic radical cations, are often observed. In addition, for amino acids and peptides, it is observed that the site of ionisation and subsequent rearrangements and reactivity largely depend on the nature of the side-chain and on the initial intramolecular hydrogen bonds.
Finally, this work shows that for open-shell systems, (case of radical cations) the most popular DFT method in the bibliography (B3LYP) may fail to predict the lowest energy ionised structure due to its tendency to overstabilise delocalised spin densities. It is observed that using the recent developed meta-hybrid MPWB1K functional with more percentage of exact exchange than B3LYP these problems are corrected.
