Direct calculation of trans-hydrogen-bond 13C-15N 3-bond J-couplings in entire polyalanine α-helices.: A density functional theory study

We present the first trans-H-bond C-13-N-15 3-bond J couplings calculated from entire neutral and protonated alpha-helical polyalanines. The neutral helices considered are those of the capped peptides, acetyl(Ala)(N)NH2, where N = 8, 16, 17, and 18, while the protonated peptides are the uncapped (Ala)(17) protonated at three different positions. The calculated J values correlate well with O center dot center dot center dot H distances and somewhat less well with N center dot center dot center dot O distances, particularly if the terminal H-bonds are eliminated from the correlation. The J values calculated using the entire helix are about 6% lower in magnitude than those recently reported for H-bonding chains whose geometries were extracted from the same helices. Aqueous solvation favors protonation of the alpha-helix on the terminal COOH. Experimental measurements of the trans-H-bond C-13-N-15 3-bond J couplings in acidic solution should be interpreted with this context.