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Figure 1
Schematic representation of the different tautomeric and isomeric states of the peptide bond. This figure illustrates the different tautomeric and isomeric states of the peptide bond, highlighting the interplay between keto–enol tautomerism and cis–trans isomerization. The first row represents the trans configuration, which is the predominant form in protein structures, while the second row depicts the cis configuration, a less common form that is sometimes observed in proline-containing peptides. The leftmost column shows the standard keto forms, where the carbonyl oxygen atom (C=O) retains its conventional bonding characteristics. The middle column illustrates resonance structures, demonstrating electron delocalization between the C=O and C—N bonds, which contributes to the partial double-bond character of the peptide bond. The rightmost column represents the enol forms, in which protonation of the carbonyl oxygen atom alters the electronic distribution, potentially affecting hydrogen-bonding interactions. The current understanding suggests that peptide bonds exist as a mixture between the keto and enol forms, with an estimated 60% keto character and 40% enol-like character. This schematic provides a conceptual framework for understanding how tautomerism and protonation influence peptide-bond chemistry, with implications for protein structure, stability and function.

IUCrJ
Volume 12| Part 3| May 2025|
ISSN: 2052-2525
https://doi.org/10.1107/S2052252525002106