| Abstract
| - The information content of segmental order parameters inliquid-crystalline lipids is analyzed in terms oforientation distributions. First, we discuss the use of segmentalorder tensors, determined from motionallyaveraged NMR anisotropies, for obtaining approximations to theorientation distribution of the director in themolecular segment. In addition to the distribution consisting ofsecond-order spherical harmonics, twophysically reasonable five-parameter distributions are examined, andthe relation of the two shape parametersof these distributions to the principal order parameters is presented.The usefulness of these distributions forthe determination of torsional angles is also discussed.Furthermore, we introduce a novel approach forobtaining model-independent limits to bond orientation probabilitiesfrom bond order parameters. The analysisshows, for instance, that the C−H bonds of the glycerolsn-3 CH2 group in phosphatidylcholine, withbondorder parameters of −0.22, are oriented preferentially at anglesbetween 52° and 128° from the director. Theprobability that these bonds are oriented at angles smaller than 30°or larger than 150° is less than 25%.Assuming an upper limit to the ruggedness of the distribution,even more stringent probability limits can begiven. For instance, using a minimum standard deviation of 10°for any conformation, a C−H order parameterof −0.22 corresponds to a probability of less than 25% for C−Horientations below 51° and above 129°.The approaches described here are generally applicable to uniaxialliquid-crystalline systems.
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