Dissociation energies of X–H bonds in amino acids

Abstract

In biochemistry, free radicals are versatile species which can perform diverse functions including: signaling, synthesis, and destructive modification. It is of interest to understand how radicals behave within all biomolecules and specifically within peptides and proteins. The 20 standard amino acids contain a wide range of chemical structures, which give proteins their complexity and ultimately their functionality. Many factors influence how radicals interact with these complex molecules, including the bond dissociation energies (BDEs) for homolytically cleaving any X–H bonds. The BDEs provide a simple measure for comparing the thermodynamic favorability of abstracting hydrogen atoms from various sites within a protein. BDEs for abstractable hydrogen atoms have been calculated for each amino acid, the peptide backbone, and peptide termini in order to compile a roadmap of the relative thermodynamics which influence protein radical chemistry. With this information it is possible to gain insight into what contributions both kinetics and thermodynamics will make to various radical mediated reaction pathways.