Conformational stability of a model macrocycle tetraamide: an ab initio study.

Ab initio calculations are carried out to investigate the conformational stability of a model macrocyle tetraamide. The four amide groups in the selected model are present in the sequence: -(O=CNH)-Ph-(NHC=O)-CH=CH-(O=CNH)-Ph-(NHC=O)-CH=CH-. In this sequence, two phenyl rings and two ethene groups act as bridges between the amide units. Each amide motif bonds to a phenyl ring through its amide nitrogen and to an ethene group through its amide carbon. Four clearly distinct minimum-energy conformations are found upon full geometry optimization using the B3LYP/6-31+G(d) method. Frequency calculations using the same method confirm that the four conformations are indeed minima in the macrocycle potential energy surface. Relative to the most stable conformer, the other conformations are higher in energy by 0.86, 2.09, and 9.17 kcal/mol, respectively, at the MP2/6-31+G(d,p) level. The stability of the macrocycle conformations is correlated primarily to the existence and strength of intramolecular N-H...O=C hydrogen bonds. Additional stability to the conformations is found to come from weak Ph-H...O=C hydrogen bonding between a carbonyl oxygen and a hydrogen atom of a phenyl group. Solvent effects play an important role in the relative energies of the various conformations, as indicated by the simple SCRF = dipole model calculations for the case of aqueous solution.