Influence of the C‐terminal Substituent on the Crystal‐state Conformation of Adm Peptides

The bis‐functionalized diamondoid α‐amino acid 2‐aminoadamantane‐2‐carboxylic acid (Adm) has been used as the building block of four Nα‐formyl homo‐dipeptide alkylamide sequences via a solution‐phase Ugi multicomponent reaction approach. The conformers of these peptides have been determined in the crystalline state by X‐ray diffraction to distinguish the influences of the C‐terminal substituent. One of the Adm peptides folds into an open and a hydrogen‐bonded γ‐turn geometry. Moreover, 3D‐structures have been observed featuring two consecutive γ‐turns in an incipient γ‐helical structure, a significantly distorted nonhelical β‐turn, as well as an S‐shaped conformation with opposite helical screw senses. A significant topological variety is thus exhibited by the ‐Adm‐Adm‐ sequences contingent on their C‐terminal substituents, illustrating both the broad conformational potential and the need for further characterization of this sterically bulky residue in explorations of its ϕ, ψ space.