Scalar cou
pling constants and magnetic shieldings in the imino hydrogen-bonding region ofHoogsteen-Watson-Crick T·A-T and C
+·G-C tri
plets have been calculated as a function of the distancebetween
proton donor and acce
ptor nitrogen atoms. The Fermi contact contributions to
h2J(
15N-H···
15N),
1J(
15N-
1H), and
h1J(
1H···
15N) were com
puted using density functional theory/finite
perturbation theory (DFT/FPT) methods for the full base tri
plets at the unrestricted B3PW91/6-311G** level. Chemical shifts
(
1H) and
(
15N) were obtained at the same level using the gauge including atomic orbital (GIAO) method for magneticshielding. All three scalar cou
plings and all three chemical shifts are strongly interrelated and exhibit monotonicchanges with base
pair se
paration. These correlations are in conformity with ex
perimental data for a 32-nucleotide DNA tri
plex. The results suggest that both chemical shifts and cou
pling constants can be used togain information on H-bond donor-acce
ptor distances in nucleic acids. In addition to the DFT/FPT calculations,a sim
ple three-orbital model of the N-H···H bond and a sum-over-states analysis is
presented. This modelre
produces the basic features of the H-bond cou
pling effect. In accordance with this model and the DFTcalculations, a
positive sign for the
h2JNN cou
pling is determined from an E.COSY ex
periment.