The fractionation factor of tritium between water and DNA as well as betwee
n water and the first hydration shell of DNA is determined. For this purpos
e the sublimation kinetics of water from DNA dissolved in water were determ
ined at -200 degreesC and tritium was measured in the remaining water free
DNA. The last sublimating water fractions showed a tritium level of about 1
.4, the residual water free DNA about 1.9 units above the bulk, water. The
tritium accumulation inside and near DNA is attributed to the thermodynamic
triton-proton exchange isotope effect existing between the strong hydrogen
bridges of water and weaker hydrogen bridges as well as inside DNA as betw
een the first hydration shell and DNA.