Knowledge Bank

“Application of infrared spectroscopy for establishment of molecular structure or even qualitative identification is empirical and based upon unique absorption bands caused by certain atomic groupings in the given molecules. Complex biologic molecules, however, frequently possess quite similar atomic groupings producing spectra of so great similarity that elucidation of structure or identification may become extremely difficult or impossible. These difficulties can be resolved in some instances by measurement of the intensities of infrared bands these complex compounds have in common. Two examples of application of intensity measurements for identification of complex lipids are given. (1) Lecithin and lysolecithin are glycero-phosphatidylcholines of greatly different biologic activity, which possess two or one fatty acid radicals in the molecule respectively. The infrared spectra are so similar that no definite clue as to the identity of the compounds can be derived from qualitative examination of the spectrum. Determination of the specific extinction of the ester carbonyl bands at about $1710cm-1$, however, clearly indicates the identity of the material. (2) Cerebrosides and sphingomyelins show rather great structural differences, but exhibit considerable overlap of infrared absorption bands. Determination of the specific extinction of the amide $\Pi$ band at $1550cm-1$ which is present in the spectra of both compounds assists in distinguishing of the two substances. This study was carried out in the solid state with a micro-KBr disk technique, which required relatively small amounts of material.”