Humidity-dependent structural transition of guanosine and disodium adenosine 5’-triphosphate crystals studied by low-frequency Raman spectroscopy

The structural transitions of guanosine and disodium adenosine 5’-triphosphate (${\mathrm{Na}}_2$⋅ATP) crystals controlled by relative humidity are monitored by low-frequency Raman spectroscopy. Although the water absorption-desorption curves against relative humidity show large hysteresis, the transition is reversible in both cases. Five well-defined states of guanosine and four well-defined states of ${\mathrm{Na}}_2$⋅ATP are distinguished, some of which are already known through x-ray analyses. In these states stacked bases form a long column. The transition schemes among well-defined states have the following characters. Microcrystals of guanosine undergo a phase transition, that is, the transition takes place cooperatively at a critical relative humidity. On the other hand, the transition of ${\mathrm{Na}}_2$⋅ATP in the humidity range from 10 to 50% is characterized as a gradual addition of water molecules, resulting in the coexistence of two states without long correlation length.