Chemistry and Physics of Lipids, 27 (1980)237-250 © Elsevier/North-Holland Scientific Publishers Ltd. RELATION OF RAMAN ORDER PARAMETERS TO SPIN LABELING PARAMETERS L:,SZL6 I. HORV:kTHa, JULIUS CIRAK c and LASZLO VIGHb alnstitute of Biophysics, blnstitute of Biochemistry, Biological Research Center of the Hungarian Academy of Sciences, Szeged (Hungary) and CDepartment of Physics,"Electrw technical Faculty, Slovak Technical University, Bratisiava (Czechoslovakia) Received April 15th, 1980 accepted June 27th, 1980 For the quantitation of Raman and spin labeling data order parameters are commonly used. The spin label order parameter measured at any depth in the layer is a weighed sum of the segmental order since, due to fast conformational interconversions, each CH2 segment is partly in trails and partly in non-trims, e.g. gauche, kink, jog, etc., conformation during the measurement. The weighing factor, the trans finding probability, varies along the chain (cf. flexibility profile) but its mean value should be equal to the Ramim trims order parameter. This correlation is illustrated with the experimental data obtained for dipalmitoyi phospha- tidylcholine and n-alcohol mixtures, The rate of rotational diffusion, a dynamical parameter from spin labeling studies, is correlated with the lateral packing density as measured by the Raman lateral order parameter. For the obtained linear correlation a qualitative explanation is given. The effect of a series of long chain alcohols on the phase transition characteristics of dipalmitoyi phosphatidylcholine was investigated, The possible role of hydrogen bonding in the interfacial region is emphasized. Introduction The fluidity of phospholipid-bilayers is due to rotational isomerism in the hydrocarbon chains. Vibrational spectroscopy, particularly Raman spectroscopy, is a powerful method to measure the average conformation of the acyl chains [1,2] and can provide quantitative data by introducing order parameters as suggested by Gaber and Peticolas [3]. The concept of average order was first applied to the interpretation ot spin labeling data by Seelig [4,5] and has been commonly used ever since. Latest NMR results obtained by selectively deuterated lipids further refine that picture in certain respects [6]. It is somewhat confusing that the order parameters introduced by Gaber and Peticolas, what we will subsequently call Raman order parameters, have apparently different meaning as compared to magnetic resonance order parameters. Since deuterium order parameter values are not available for gel state lipid layers [6], a comparison over a wide termperature 237