Glycine and Alanine Dipeptide Analogs

The glycine and alanine dipeptide analogs (GDA and ADA) have recently garnered theoretical attention, because they are the two simplest models of a polypeptide. Molecular mechanical force fields designed to model proteins are often parameterized to reproduce, as well as possible, the geometries and relative energies of the various GDA and ADA conformers. Since few experimental data are available, one must rely on theoretical studies. To date, DFT calculations on peptides have been limited to GDA and ADA.  

The two geometrical parameters of greatest interest are the C—N—C —C and N—C —C—N torsions, known respectively as f and tl . The series ot j and values for each residue in a protein ultimately describes that protein s secondary and tertiary structures. Table 4 contains the ) and values for the 

ADA has a slightly more complicated potential energy surface than GDA. Six local minima exist. Discrepancies of roughly 10° are found between the MP2 and DFT ) and t) values for five of the conformations C5, P2, 

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Table 4 HF , MP2 , DFT <>, i i Values (in Degrees) for the Glycine and Alanine Dipeptide Analogs (from Reference 12)...

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