The role of enhanced aromatic -electron donating aptitude of the tyrosyl sidechain with respect to that of phenylalanyl in intramolecular interactions |
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Authors: | GA Chass S Lovas RF Murphy IG Csizmadia |
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Institution: | (1) Department of Biomedical Sciences, School of Medicine Creighton University, 2500 California Plaza, Omaha, NE 68178, USA, US;(2) Global Institute Of Computational Molecular and Material Science @ Velocet, 210 Dundas St. W., Toronto, Ontario, Canada M5G 2E8, CA;(3) Department of Chemistry, University of Toronto, Lash Miller Chemical Laboratories, 80 St. George St., Toronto, Ontario, Canada M5S 3H6, CA |
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Abstract: | An exhaustive ab initio and DFT search for energetically stable conformers from the topologically possible set was undertaken on the N-acetyl-phenylalanyl-N-methylamide
and N-acetyl-tyrosyl-N-methylamide systems. The geometries of all 81 phenylalanyl and 162 tyrosyl possible rotamers, described
under the rules outlined by Multi-Dimensional Conformational Analysis (MDCA), were attempted at each of the RHF/3-21G, RHF/6-31G(d)
and B3LYP/6-31G(d) levels of theory. A total of 32 and 66 stable conformational minima were found for the phenylalanyl and
tyrosyl amino acid diamides, respectively, at the B3LYP/6-31G(d) level. From the tyrosyl set, 33 unique conformers emerge
when the orientation of the A
i
3
dihedral angle (p-OH orientation) is disregarded. A total of 31 conformers were common to both sets and showed nearly identical geometries.
The comparison of the optimized DFT geometries of the two systems showed near by perfect linear fits with R2 values of 0.9997, 0.9994, 0.9997, and 0.9996 for the φ
i
, ψ
i
, A
i
1
, and A
i
2
dihedral angles, respectively. Relative energies of the matching 31 conformers also fitted to a linear plot with an R2 value of 0.9985. The geometric centroid of the aromatic ring in the sidechain of both systems was found to be within 4.1
?of the H and O atoms of the peptide groups, in 21 and 2 of the conformers, respectively. None of the non-matching conformers
showed any such interaction distance ≤4.1 ?.
Received 1st February 2002 / Received in final form 28 May 2002 Published online 13 September 2002 |
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Keywords: | PACS 31 15 Ar Ab initio calculations – 31 50 Bc Potential energy surfaces for ground electronic states – 33 15 Bh General molecular conformation and symmetry stereochemistry |
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