Enthalpies of hydrogen-bonds in alpha-helical peptides. An ONIOM DFT/AM1 study.
We report the enthalpy differences between alpha-helical and extended beta-strand conformations of acetyl(Ala)nNH2, for n = 8, 10, 12-17, calculated using molecular (MO) orbital theory from complete vibrational analyses of the optimized species. The calculations used the ONIOM method with B3LYP/D95(d.p) as the high and AM1 as the low levels. The incremental change in enthalpy upon addition of one Ala to a growing beta-strand defined using the hypothetical polycondensation reaction, n Ala + CH3COOH + NH3 --> acetyl(Ala)nNH2 + n H2O, reaches its asymptotic limit of -1.4 kcal/mol at n = 10, while that for the alpha-helix continues to increase in magnitude at n = 17. The asymptotic limit of the enthalpic preference of the alpha-helix over beta-strand is estimated to be about 3 kcal/mol, while that for n = 17 is 11.99 kcal/mol or about 0.8 kcal/mol/H-bond, which is similar to measured values for polyalanines of this size in aqueous solution.
Wieczorek
Robert
R
Department of Chemistry, City University of New York-Hunter College, New York, New York 10021, USA.
Dannenberg
J J
JJ
D003198:Computer Simulation; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary
D010455:Peptides