A comparison of synchrotron micro-FTIR spectroscopic analysis of lipid composition in frozen-hydrated and air-dried mouse brain tissue

Sample preparation is a key consideration for FTIR spectroscopic analysis of biological cells and tissue and the effects of paraffin embedding and formalin fixation have been well studied. More recently, the effect of DNA and RNA hydration and its effect on nucleic acid absorbance bands has been studied and characterised. Surprisingly, although the effects of lipid hydration have been characterised with FTIR spectroscopy in pure lipid or model lipid bilayer systems, there has not yet been a study on the effects of lipid hydration on FTIR spectra collected from biological tissues. The X-ray fluorescence microscopy and X-ray absorption spectroscopy communities have commenced studies on the effect of tissue dehydration on the distribution and speciation of metal ions (and non-metal elements such as sulfur) in tissue samples. Therefore, the aim of this study was to investigate differences in FTIR spectra that exist when comparing frozen-hydrated tissues and air-dried dehydrated tissues, with a specific focus on lipid absorbance bands. The results highlight that not surprisingly, lipid dehydration is a key event that occurs when air-drying tissue sections, potentially removing valuable biochemical information. Through use of a temperature-controlled sample stage we demonstrate the tissues can be analysed with lipids still hydrated, in a frozen-hydrated state, which represents as close as possible to the in vivo condition currently achievable for organs such as brain tissue.