Physical properties - optical and spectral characteristics

Much of our present knowledge of lipids is based on their spectral properties. Some of the well-established methods (ultraviolet absorption or X-ray diffraction) continue giving useful information, while a host of new ones, particularly in the fields of magnetic resonance and mass spectroscopy, provide us with a variety of previously unknown insights of lipid structure and dynamics finally, the incorporation of new elements into traditional techniques (e.g. interferometric optics and computerization to infrared spectroscopy) add new dimensions to 

Infrared (IR) spectroscopy has been used since the early days of lipid chemistry (Barcelo and Bellanato, 1953 Wheeler, 1954) and still finds frequent application in these studies (Fringeli and Gunthard, 1981 Amey and Chapman, 1983). 

Among the reasons for its popularity we shall mention its non-destructive and non-pertulrbing character. In addition, spectra can be obtained relatively rapidly, frequently with little or no sample preparation, and both qualitative and quantitative results can be obtained. IR spectroscopy is both sensitive and specific, it requires relatively small amounts of sample (in the milligram range) and can respond to subtle changes in molecular environment, particulary polymorphic transitions. There are drawbacks, especially when infrared is compared with ultraviolet spectroscopy IR absorptions are typically less intense than electronic spectra, thus higher concentrations of sample are required also IR 

The authors are grateful to Mrs Isabel Gonzalez for her secretarial assistance, and to Miss Cristina Otamendi for the artwork. Dr J. Hayward provided helpful assistance with the ESR section. This work was supported in part by a grant from CAICYT. Financial assistance from the British Council is also gratefully acknowledged. 

Infrared spectra arise from the interaction between matter and electromagnetic radiation of wavelength between about 2 and 50 jjm (respectively 5000 and 200 cm ). The energy associated with such radiations corresponds to transitions between vibrational levels of the ground electronic state of molecule. The absorbance is determined by Beer s law. 

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