Relation or Raman order parameters to spin labeling parameters

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 trans and partly in non-trans, 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 Raman trans order parameter. This correlation is illustrated with the experimental data obtained for dipalmitoyl phosphatidylcholine 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 dipalmitoyl phosphatidylcholine was investigated. The possible role of hydrogen bonding in the interfacial region is emphasized.