Cooperative effects in α‐helices: An ab initio molecular‐orbital study

Some properties of α‐helices of polyclycine and polyalanine, up to the decapeptide, were investigated by ab initio molecular‐orbital calculations. These helices were found to be unstable relative to the corresponding “fully extended chain” conformation. The electric field of helices of 8–10 residues is about 20% stronger than that of models built from noninteracting monomers, for example. This is a result of cooperativity, which is essentially governed by the intramolecular hydrogen bonds. The cooperativity is manifest in all properties of the helices: relative stability, dipole moment, proton affinity, electrical potential. The electric potential of helices of three and four residues is such that their instability can be compensated for by a single charged group acting as an “initiator.” The computed proton affinity of the (Ala)8 α‐helix is about 45 kcal/mol larger than that of formamide, which confirms that long helices may be protonated at the carboxyl end in solution.