BASE-PAIR FORMATION OF FREE NUCLEOBASES AND MONONUCLEOSIDES IN THE GAS-PHASE

The native affinity of the nucleobases of DNA to form pairs is investi gated by studying the gas phase behavior of clusters of free unsubstit uted nucleobases and mononucleosides. The formation of neutral bimolec ular clusters of these compounds in the gas phase has been studied by IR laser desorption of the neutral molecules into a supersonic beam ex pansion. Complementary nucleobase pairs as found in DNA, these being a denine-thymine and cytosine-guanine pairs, have been found to be forme d in preference to noncomplementary base pairs. ''Pseudo association c onstants'' for the formation of the dimers of free nucleobases and nuc leosides in the gas phase are calculated from the experimental results . A strong influence due to side groups affecting the dimer formation of the nucleobases is shown.