For the purpose of clarifying the magnetoorientation mechanism of several biological membrane systems, xylene-suspended thin-layered single crystals of lecithin, which is a major component of membranes, were subjected to magnetic fields of various strengths, and the orientation behavior of the crystals was analyzed. Lecithin crystals oriented in such a way that the directions of both the hydrocarbon chains and the phosphorylcholine group of the lecithin molecule were perpendicular to the magnetic field, suggesting that the crystals were diamagnetically biaxial. In contrast, hydrocarbon chains of phospholipids in many biological membranes are known to orient parallel to magnetic fields. The value of the volume diamagnetic susceptibility anisotro~py, Ax (i.e. the difference between susceptibilities parallel and perpendicular to the direction of the hydrocarbon chain in the lecithin crystals), estimated from the magneto-orientation behavior was about -9 X 10-8 cgs. This is distinctly larger than that of vesicles of egg yolk lecithin measured by Boroske and Helfrich [Boroske, E. & Helfrich, W. (1978) Biophys. J. 24,863-868] and also larger than that of single crystals of polyethylene, which was measured for comparison by the same technique as that used for lecithin in this study. The differences between these values of AX seem to result from the contribution of the polar head (i.e., the part of the lecithin molecule other than the two hydrocarbon chains), which is ordered, less ordered, or deleted in the respective cases above, although mainly from the contribution of the hydrocarbon chains, which are ordered in crystals and disordered in vesicles.Suspensions of various biological membrane systems such as retinal-rod outer segments (1, 2), chloroplasts (3-6), and purple membranes from Halobacterium halobium (7) have been known to show magnetic field-induced orientation. All these systems align with the membrane normal parallel to the field direction, and it seems that this orientation effect is a common feature of biological membrane systems. In this effect, however, lecithin molecules, which are a major component of membranes, orient with their hydrocarbon chains parallel to the field; this orientation is contrary to the reported anisotropy of the magnetic susceptibility of hydrocarbon chains (8). The orientation effect, therefore, is assumed to be primarily caused by the anisotropy that results from oriented membrane proteins (9-11). The aim of this paper is to clarify the role of lecithin molecules in the magneto-orientation effect of biological membranes and to put it on a more quantitative basis. In the course of structure analysis of lecithins by x-ray and electron diffraction methods, we succeeded in making single crystals of lecithin, and we studied the effect of a magnetic field on lecithin crystals suspended in xylene. In the magnetic field, the crystals oriented in such a way that hydrocarbon-chain and phosphorylcholine-group directions, which are orthogonal to each other, are perpendicular to ...
