DYNAMIC INTERACTION OBSERVED IN HUMIDITY- INDUCED PHASE TRANSITIONS OF NUCLEOSIDES AND NUCLEOTIDES. Yoko Sugawara, School of Science, Kitasato University, Kitasato, Sagamihara, Kanagawa 228, Japan

We carried out dynamic analysis of humidity-induced phase transitions in nucleoside and nucleotide crystals, e.g. guanosine and disodium adenosine 5'-triphosphate(Na₂ATP), by the X-ray diffraction method and low-frequency Raman spectra.^1-3. There exist flexible or switchable interactions which would be called as dynamic interactions.

In the crystal exhibiting the phase transition, base moieties of nucleosides and nucleotides are stacked to form a columnar structure. There is a water region between the molecular columns. The number of crystal water molecules in this region is a function of humidity, and the reversible crystal transition proceeds.

Intermolecular interactions are classified into two types. The first type is a dynamic interaction. Intercolumnar hydrogen-bonding networks vary together with increase or decrease of water molecules. It is noteworthy that ribose puckering modes contribute to the reconstruction of hydrogen-bonding networks. The second type is, of course, a static or rigid interaction which plays a role in holding the crystal lattice.

It is well-known that transformation of hydrogen-bonding networks induces structural changes of nucleic acids and proteins. Intermolecular interactions of biomolecules should be characterized by coexistence of both types of interactions.

1. Y. Sugawara et al. J. Am. Chem. Soc. 113 5440(1991).

2. Y. Sugawara et al. J. Biomol. Struct. Dyn. 11, 721(1994).

3. H. Urabe et al. Phys. Rev. B51, 5666(1995).