Through-hydrogen bond spin-spin coupling has been investigated computationa
lly in the formamide dimer and in fragments of the protein ubiquitin. The F
ermi-contact term was calculated by finite perturbation theory with the B3L
YP DFT method with several basis sets. The distance and angular dependence
of the (3)J(N,C) coupling constant (N-H ... O=C) in the hydrogen-bonded for
mamide dimer was firstly examined for a wide range of mutual arrangements,
also in relation to the stability of the dimer. The magnitude of (3)J(N,C)
is relatively insensitive to the dihedral angle between the two amide plane
s. whereas values between 1-2 Hz are calculated for a variety of arrangemen
ts, including non-linear hydrogen bonds, in agreement with the shape of som
e occupied, low-lying molecular orbitals which connect donor and acceptor.
Then, fragments of the ubiquitin protein (for which such coupling constants
are experimentally available) were generated by removing from the experime
ntal structure all amino acids except those involved in hydrogen bonding, a
nd coupling constants were calculated for such fragments. Although calculat
ed (3)J(N,C) values are sometimes overestimated, they generally correlate w
ith the corresponding experimental values.