| Abstract
| - First-principle computations were carried out on the conformational space of trans and cis peptide bond isomersof HCO−Thr−NH2. Using the concept of multidimensional conformational analysis (MDCA), geometryoptimizations were performed at the B3LYP/6-31G(d) level of theory, and single-point energies as well asthermodynamic functions were calculated at the G3MP2B3 level of theory for the corresponding optimizedstructures. Two backbone Ramachandran-type potential energy surfaces (PESs) were computed, one each forthe cis and trans isomers, keeping the side chain at the fully extended orientation (χ = χ = anti). Similarly,two side chain PESs for the cis and trans isomers were generated for the (φ = ψ = anti) orientationcorresponding to approximately the βL backbone conformation. Besides correlating the relative Gibbs freeenergy of the various stable conformations with the number of stabilizing hydrogen bonds, the process oftrans → cis isomerization is discussed in terms of intrinsic stabilities as measured by the computedthermodynamic functions.
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