3JC2/4-H1' and 3JC6/8-H1' scalar spin-spin coupling constants have been calculated for deoxyadenosine, deoxyguanosine, deoxycytidine, and deoxythymidine as functions of the glycosidic torsion angle c by means of density functional theory. Except for deoxythymidine, 3JC2/4-H1' depends little on the base type. On the contrary, 3JC6/8-H1' follow the usual trans to cis ratio (3JC-H(cis) < 3JC-H(trans)) for purine nucleosides but reveal the opposite relation (3JC-H(cis) > 3JC-H(trans)) for pyrimidine nucleosides. Our results compare well with the experiment for deoxyguanosine and predict a novel trend in the case of pyrimidine bases for which no NMR results are available in the syn region. A breakdown of the key Fermi contact part of 3JC6/8-H1' into MO contributions rationalizes this trend in terms of an unusual coupling mechanism in the syn orientation that is very effective for pyrimidine nucleosides and considerably weaker for purine nucleosides.