Raman spectroscopic studies of some biochemically relevant molecules

Abstract

Near-infrared (NIR) Raman spectra of the protein aprotinin, in both powder form and aqueous solutions, are presented. The amide I and amide III bands give information about the secondary structure. The conformation around the sulphur bridges and the environment of tyrosine were also studied. Due to the low scattering efficiency, only aqueous solutions in the concentration range 2–20% (w/w) were used. Use of a windowless cell improved the quality of the spectra, as compared to spectra obtained with quartz cells. Fluorescence can be a serious problem in Raman studies of biologically relevant molecules. Some examples are shown, which illustrate that the use of NIR excitation can frequently eliminate this fluorescence. Heating effects give rise to serious problems with excitation at 1064 nm in the NIR-FT-Raman spectrum of some strongly coloured macromolecules, like haemoglobin. In order to avoid complications due to both heating and fluorescence, an excitation wavelength around 800 nm is suggested. A preliminary surface enhanced Raman (SER) spectrum of a peptide nucleic acid (PNA) in aqueous silver colloid solution is shown. Low-frequency Raman spectra of aprotinin in aqueous solution are presented. The low-frequency limit in the NIR-FT-Raman spectrum is ∼80 cm −1 . Several models are used to describe the bands assigned to hydrogen bonding in the systems. The low-frequency modes can be of importance for the formation and breaking of hydrogen bonds, and thus may be of importance for biological activity.