Peptidyl-Prolyl Model Study: How Does the Electronic Effect Influence the Amide Bond Conformation?

The
triple-helical structure of collagen, the most abundant protein
in animal bodies, owes its stability to post-translationally installed
hydroxyl groups at position 4 of prolyl residues. To shed light on
the nature of this phenomenon, we have examined the influence of the
4-substituent on the amide isomerism in peptidyl-prolyl analogues.
The rigid bicyclic skeleton of 2,4-methanoprolines allowed us to follow
the through-bond impact of the substituent group (electronic effect)
without the side-chain conformation being affected by a stereoelectronic
effect. These proline analogues were prepared by [2 + 2] photocycloaddition
of (2-allylamino)­acrylic acid derivatives. Subsequent pKa studies demonstrated a remarkable electronic effect
of the 4-fluorine substitution, while the effect of the 4-methyl group
was negligible. The trans/cis amide
ratio was measured in model compounds under low temperature conditions.
The observed prevalence for a trans-amide is extraordinary,
and in this regard, 2,4-methanoproline is closer to primary α-amino
acids than to proline. At the same time the amide rotation velocities
were 3−4 orders of magnitude ​higher when compared to N-acetylprolyl. Finally, our results indicate that the electronic
effect of the 4-substituent only affects the kinetics of the amide
isomerization but not the thermodynamic prevalence for the trans-rotamer.