The possibility of all-
cis open-chain polypeptides is rarely addressed, owing to three main reasons, namely,(i) the extreme scarcity of cis peptide bonds in naturally occurring proteins and peptides, (ii) the lesserthermodynamic stability (by about 2.5 kcal/mol) of cis amide bonds with respect to their trans counterparts,and (iii) widely held preconceptions about the so-called "steric clash" between lateral chains borne bytwo successive carbons. Quantum-chemistry calculations performed on alanine tridecamers show howthe latter constraints can be efficiently relieved through proper / adjustments along the backbone,leading to several helical arrangements-presumably the only permitted regular structures. Four more-or-less regular helices were thus characterized, one of them, a superhelix, exhibiting intramolecularhydrogen bonds. Understanding and anticipating all-
cis open-chain structures not only make use of theclassical Ramachandran maps at each C
i, relating to
E =
f(
i,
i), but also require the profile of a newkind of conformational dependence, the
plaque maps, relating to
E =
f(
i,
i-1). The obvious couplingbetween two such maps enforces conformational dependence between two consecutive C
's, somewhatquestioning in this context the customary "local effects", and presumably reducing the whole chainplasticity. Whereas cis thermodynamic penalty cannot be abolished locally, energy clues indicate thatassembling cis-prepared building units is an exothermic process. Besides, once built up, the all-
cis backboneshould be difficult to unlock, thus affording reasonable kinetic stability.
