Structure of DNA hydration shells studied by Raman spectroscopy

Area of Science:

• Biophysics
• Spectroscopy

Background:

• DNA hydration is crucial for its structure and function.
• Understanding water's role in DNA requires precise quantification of hydration levels.

Purpose of the Study:

• To investigate the hydration of DNA films using Raman spectroscopy.
• To quantify the number of water molecules in the primary hydration shell and tightly bound water.
• To establish a method for measuring DNA hydration numbers.

Main Methods:

• Raman scattering spectroscopy was employed to analyze water O-H stretching modes in DNA films.
• Measurements were conducted across a range of relative humidity (r.h.) levels.
• Analysis focused on specific frequency bands (approx. 3450, 3220, and 3400 cm-1) related to water O-H stretching.

Main Results:

• The 3220 cm-1 band, indicative of the water hydrogen-bond network, disappears around 80% r.h., showing disruption in the primary hydration shell.
• Approximately 30 water molecules per nucleotide pair constitute the primary hydration shell.
• 5-6 tightly bound water molecules per nucleotide pair remain even at 0% r.h., evidenced by the persistent 3400 cm-1 band.
• The 3400 cm-1 O-H stretch mode frequency is lowered in the primary hydration shell compared to free water.
• Experimental results align with gravimetric measurements of water content.

Conclusions:

• The study provides a reliable method for measuring DNA hydration numbers.
• Disappearance of the 3200 cm-1 band and shifts in the 3400 cm-1 band are key indicators of hydration levels.
• DNA's primary hydration shell does not possess an "ice-like" structure.