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The IR absorption spectra of protein, DNA, RNA, and phospholipid films as a function of the water content are reported. We find that the hydration of protein films affects the peak intensity of amide I and amide II bands and the shape of the amide III band. For nucleic acids, the symmetric (nu(S) PO(2) (-)) and antisymmetric (nu(AS) PO(2) (-)) stretching vibrations of the phosphate linkage are the most affected by hydration, because both intensity changes and frequency shifts are observed. The spectra of phospholipid films are also sensitive to hydration, and they exhibit changes in the peak intensities and frequencies of both nu(S) PO(2) (-) and nu(AS) PO(2) (-) vibrations. We interpret the spectral differences between water saturated and dried films both in terms of structural changes and the change in the local dielectric in the vicinity of the polar and solvent exposed groups. In addition, we observe that the most significant change in the absorption intensity, frequency, and shape of the water sensitive vibrations occurs at high hydration levels. The principal component analysis of hydration results and the kinetics of water removal from sample films are also discussed. In addition, protein spectra acquired using film and KBr pellet sampling techniques are compared.

The infrared absorption spectra of two sample proteins as dry, lyophilized powders in KBr pellets, as hydrated proteins in KBr pellets, as hydrated or dehydrated films, and in solution phase are reported. We find an enormous increase in the absorption intensities of the amide I and amide II vibrations between the dry and the hydrated phases. This intensity enhancement was observed for hydration by both H2O and D2O. Furthermore, we observe significant changes in the amide I/amide II intensity ratios when comparing solution spectra acquired in transmission and attenuated total reflection (ATR) modes. Observed intensity changes between dry and hydrated protein samples were interpreted in terms of variations in the dielectric constant of the immediate surroundings of the peptide linkages.