title: Direct calculation of trans-hydrogen-bond 13C-15N 3-bond J-couplings in entire polyalanine alpha-helices. A density functional theory study.
abstract: We present the first trans-H-bond 13C-15N 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)NNH2, 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...H distances and somewhat less well with N...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 13C-15N 3-bond J couplings in acidic solution should be interpreted with this context.
authors.affiliation: Department of Chemistry, City University of New York--Hunter College and the Graduate School, 695 Park Avenue, New York, New York 10021, USA.
authors.forename: Pedro
authors.initials: P
authors.lastname: Salvador
authors.forename: Robert
authors.initials: R
authors.lastname: Wieczorek
authors.forename: J J
authors.initials: JJ
authors.lastname: Dannenberg