聚腺嘌呤苷酸(Poly A)水化过程的傅里叶变换红外光谱研究 Ⅱ. 动态水化及脱水过程的特性

在研究核酸分子水化作用的工作中,前人及作者的前文都是将样品保持在恒温器中,在静态情况下实现湿度平衡。为了使样品能更充分地吸附水分子,我们设计了一个通气系统,使不同湿度的水蒸气连续通过密封样品池,让样品在动态情况下达到湿度平衡。除了水化过程外,对脱水过程的观察、分析也是研究核酸与水的相互作用的不可缺少的一环,但这方面的实验工作却很少。我们通过对Poly A薄膜动态水化和脱水过程中红外光谱的测定,证实了:(1)与静

FOURIER TRANSFORM INFRARED STUDY ON THE HYDRATION OF POLYADENYLIC ACID (POLY A) Ⅱ. DYNAMIC HYDRATION AND DEHYDRATION PROPERTIES

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.