Structure of DNA hydration shells studied by Raman spectroscopy

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Abstract
We have used Raman scattering to study the water O‐H stretching modes at ∼ 3450 and ∼ 3220 cm−1 in DNA films as a function of relative humidity (r.h.). The intensity of the 3220‐cm−1 band vanishes as the r.h. is decreased from 98% to around 80%, which indicates that the hydrogen‐bond network of water is disrupted in the primary hydration shell (which therefore cannot have an “ice‐like” structure). The number of water molecules in the primary hydration shell was determined from the intensity of the ∼ 3200‐cm−1 band as about 30 water molecules per nucleotide pair. The ∼ 3400‐cm−1 O‐H stretch band was used for determining the total water content, and this band persists at 0% r.h., implying that 5–6 tightly bound water molecules per nucleotide pair remain. The frequency of the ∼ 3400‐cm−1 O‐H stretch mode is lower by 30 to 45 cm−1 in the primary hydration shell compared to free water. The water content as a function of r.h. obtained from these experiments agrees with gravimetric measurements. The disappearance of the ∼ 3200‐cm−1 band and the shift of the ∼ 3400‐cm−1 O‐H stretch band provide a reliable way of measuring the hydration number of DNA.