Fig. 2: Crosslinks harbor the weakest bonds in collagen type I.

a Correlation between the bond dissociation energies (BDEs) and summed radical stabilization energies (RSEs) of homolytically cleaved bonds and formed radicals. All molecular structures used for calculations can be found in the SI. Bonds with BDEs <315 kJ mol⁻¹ are shown in more detail on the right. The small letters close to selected data points indicate respective bonds in the lower panels b and c. b, c Molecular structures of (b) investigated collagen crosslinks and (c) prevalent aminoacids in collagen. BDEs of bonds colored in black and orange (BDE <315 kJ mol⁻¹) were explicitly calculated. Residues R1, R1', R2 and R2' connect to the same collagen triple helix. The BDEs of the PYD bond denoted with the letter “a” was obtained after deprotonation of the pyridine-bound hydroxyl group. d Main electronic effects leading to the lowest found BDEs. The captodative effect describes the non-additive stabilization of the amino-carbonyl and carbonyl-amino group present at any C_α peptide radical. The lower part illustrates the increased radical delocalization upon deprotonation of PYD and DPD, lowering the BDE and RSE of bond “b” to that of bond “a”.