is also unlikely since the same reaction occurs, and at a similar rate, with less than 1 equiv of LiMe. On the other hand, if the breaking of the P-C bond were the first step of the process, one would expect the benzyl group to migrate too, and even faster, in the benzyl-P analogue of 9; this was not observed. So far the only rearrangement reported for a (a-ally1)iron derivative, as in ~5-CpFe(CO)(PPh3)(ur-allyl), is its thermal U / K rearrangement, leading to the stable s5-CpFe(CO)(a-allyl), which implies the dissociation of the phosphane complex."Thus the mechanism of the process reported here most likely consists in the insertion of iron into an allylic or a terminal vinylic C-H bond followed by a 1,3-proton shift to the terminal olefinic or allylic carbon atom, respectively, with concomitant P-C bond cleavage. This reactivity pattern differs both from the previously found phenyl group migration between phosphorus and iron, which implies only the breaking of a P-C bond, and from an orthometalation reaction, which implies only the insertion of a metal into a C-H bond. The basicity of the metal is known to play a determining role in the activation of C-H bonds; low oxidation states are usually required. In the present case, the anionic phosphoranide ligand in 9 is likely to increase the charge density on iron, and hence its basicity.This unprecedented phenomenon is a further indication that the recently discovered phosphoranide ligands employed here promise to lead to new reactivity patterns.We have addressed the current controversy on whether the orientational order of acyl chains in unilamellar vesicles, whose surface is highly curved, is the same or different from acyl chains in multilamellar systems (where the surface has a much lower curvature). Finer,' Stockton et al.,* and Bloom et aL3 have stated that the order is essentially the same in both systems, whereas Petersen and Chan4 and, more recently, Fuson and Prestegards suggest that the order is lower in the vesicle system. We present evidence which shows that the C-D order parameter of =5 mol % selectively deuterated palmitic acids incorporated into unilamellar vesicles composed of 15% w/v egg phosphatidylcholine/bovine brain sphingomyelin (85: 15 w/w) in deuteriumdepleted water are significantly lower than the values2 found in multilamellar liposomes at comparable reduced temperatures.Fatty acids are considered to be reliable probes of the phospholipid acyl chain in model membranes.2*6 In fact, Pauls et al.' examined the fidelity of deuterated fatty acids used as probes of dipalmitoylphosphatidylcholine bilayers and concluded that, even at 20 mol % incorporation, the acids reflect the order of the membrane to within 10%.The unilamellar vesicles were prepared by dissolving 300 mg of egg phosphatidylcholine (isolated from fresh eggs8s9), 52 mg (1) Finer, E. G. J. Magn. Reson. 1974, 13, 76-86. (2) Stockton, G. W.; Polnaszek, C. F.; Tulloch, A. P., Hasan, F.; Smith, (3) Bloom, M.; Burnell, E. E.; Mackay, A. L.; Nichol, C. P.; Valic, M. I.; (4) Petersen, N. ...

The structure and motion of phospholipids in human plasma lipoproteins have been studied by using 31P NMR. Lateral diffusion coefficients, DT, obtained from the viscosity dependence of the 31P NMR line widths, were obtained for very low density lipoprotein (VLDL), low-density lipoprotein (LDL), high-density lipoproteins (HDL2, HDL3), and egg PC/TO microemulsions at 25 degrees C, for VLDL at 40 degrees C, and for LDL at 45 degrees C. At 25 degrees C, the rate of lateral diffusion in LDL (DT = 1.4 x 10(-9) cm2/s) is an order of magnitude slower than in the HDLs (DT = 2 x 10(-8) cm2/s). At 45 degrees C, DT for LDL increases to 1.1 x 10(-8) cm2/s. In contrast, DT for VLDL increases only slightly going from 25 to 40 degrees C. The large increase in diffusion rate observed in LDL occurs over the same temperature range as the smectic to disordered phase transition of the core cholesteryl esters, and provides evidence for direct interactions between the monolayer and core. In order to prove the orientation and/or order of the phospholipid head-group, estimates of the residual chemical shift anistropy, delta sigma, have been obtained for all the lipoproteins and the microemulsions from the viscosity and field dependence of the 31P NMR line widths. For VLDL and LDL, the anisotropy is 47-50 ppm at 25 degrees C, in agreement with data from phospholipid bilayers. For the HDLs, however, significantly larger values of 69-75 ppm (HDL2) and greater than 120 ppm (HDL3) were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

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The order of phosphatidylcholine (PC) acyl chains in the surface monolayer of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) has been determined from 2H nuclear magnetic resonance order parameters, SCD, using selectively deuterated PC or palmitic acids. From the computer simulated line shapes, we find two distinct phospholipid domains within the amphiphilic monolayer of both VLDL and LDL. In the more ordered domain of LDL, SCD was approximately 0.3 for the "plateau" chain region. The SCD values of VLDL particles are similar to those of LDL for the 5,6- and 11,12-positions, hence we suggest the organization of the more ordered region of VLDL and LDL are similar. The domain of low order in LDL comprises less than 10% of the phospholipid molecules (we do not distinguish between PC and sphingomyelin), having approximately the same order (SCD less than 0.1) as egg PC - sphingomyelin unilamellar vesicles. In VLDL, the domain of low order comprises between approximately 10 and approximately 20% of the phospholipid molecules and the entire acyl chain is in an essentially isotropic environment (SCD less than 0.02). We prepared VLDL-sized microemulsions composed of egg PC, deuterated PC, and triolein to test whether the apoproteins were responsible for creating the two differently organized domains in VLDL and LDL. Surprisingly, these protein-free particles also showed two domains of different order at two temperatures. The high order region, however, is less ordered than in VLDL and LDL. We explain two surface domains of PC in terms of lipid organization and the unique interactions of lipids in the various lipoprotein particles.