Abstract: By taking the N_2 as the carrier gas, the water vapour varying with different relative humidities passed through a sealed sample cell with a thin Poly A film coated on the BaF_2 window to achieve dynamic equilibrium. According to the asending and desending order of relative humidity (from 0% to 100%, then down to 0% again), a series of FT-IR and subtraction spectra have been studied in the region of 4000 to 800 cm~(-1) at (25±1) ℃ (Fig.1, 2). The results are concluded as following:
(1) In comparision with the static process studied by the authors′ previous work~[3], the essential features of hydration effect resemble each other. The strongest hydrating sites are on the —NH_2 and PO_2~- group. But the dynamic hydration behaves faster and stronger than that of static case. During the dynamic process the hydration usually begins at low humidity and then gradually increase; but in the static case, it begins almost around RH=75% then rises steeply (Fig.3). Besides, some new peaks around 1500 cm~(-1) have been observed.
(2) The hydration and dehydration process are irreversible. Both absorbance and peak position can not be restored (Fig.3). It indicates that the hydrating water molecules in Poly A have strong chemical bonding and cannot desorb easily at low humidity. The strongest hysteresis sites are ribose ring —NH_2 and PO_2~- group. So they are not exactly in correspondence with the strongest hydrating sites. This shows, the water molecules may redistribute among the different sites and it takes a quite long time to each equilibrium.
(3) The absorption and desorption isotherm of Poly A film is the well known sigmoidal shape (Fig.5). The number of absorbed water molecules per nucleotide unit at RH=100% is about 8.8. It is roughly approximate to the calculation by Clementi~[4] for the first solvation shell of B-DNA.