Dynamic Structure of Lipid Bilayers Studied by ... - glia.cu-tokyo.ac.jp

KAWATO. KINOSITA. AXD IKEGAMl“microviscosity” should be in the difference between the twoanalyses. In the present analysis, contributions of the anisotropicequilibrium on r(t) are considered separately; that is,qc is the viscosity only in the cone where probes can diffusepractically. The “microviscosity” was estimated from thesteady-state anisotropy rs without regarding the restrictionson the rotational motion. Rather higher values of the “microviscosity”reported for DPPC suggest restrictions in therotational motion of perylene in the bilayer. In fact, preliminaryresults show that r(t) of perylene in DPPC vesicles followseq 14 indicating restrictions.Fluorescence Lifetime and Steady-State Intensity. Thefluorescence lifetime and the quantum yield Q are directlyrelated to the rates of emission k, and internal quenching k,by the relation:Therefore the sharp decrease of 7 at Tt may be induced by theincrease in thermal motion and in polarity around DPH dueto the transition, because k, is generally affected by thesefactors.As the natural lifetime 70 Ilk, is supposed to be constantin these experimental conditions, the quantum yield Q is proportionalto 7 and the ratio IrS/7 gives the relative number ofDPH embedded in vesicles. The relative number of DPH in thebilayer increases about 10% at TI as shown in Figure 6.Dynamic Aspects of Lipid Bilayers. As DPH has a rod-likeshape, very similar to a hydrocarbon chain of lipid, DPH in thelipid bilayer should occupy just the same positions of chainsand their molecular motion should reflect directly the motionof hydrocarbon chains.The estimated values of 8, shown in Figure 4 are about 20’even at the crystalline state, sufficiently below T,, and about70° above TI. These values indicate that the fairly large spacemust be attributed to the cone used to describe the wobblingdiffusion. It is very unnatural to consider that such a largespace leaves a vacuum around each DPH. A major part of thespace should be occupied by hydrocarbon chains to reduce voidvolume. DPH is permanently in collision with these chains andwobbles around according to the thermal motion of hydrocarbonchains, and vice versa. The cone-like potential forwobbling diffusion is a conceptual one which indicates thetime-averaged arrangement of chains around DPH. Ratherlarge values of 8, at temperatures above Tr indicate a highlydisordered state of hydrocarbon chains.The wobbling diffusion constant is determined by the frequencyof collisions between DPH and hydrocarbon chains andby the mean free rotational angle between successive collisions.As the density of hydrophobic region is not so much changedby the transition, the temperature dependence of D, shownin Figure 5 mainly indicates the change in the frequency ofcollisions, that is, the rate of chain motion. It should be notedthat no drastic change in the rate of chain motion was apparentat Tr and its temperature dependence was like the exponentialfunction. However, the orientational freedom of chains, or O,,changed like a sigmoidal transition.Contrary to this wobbling diffusion constant, a sigmoidaltransition in the lateral diffusion constant of lipids in DPPCvesicles at Tr was reported recently (Naqvi et al., 1974). Therate of lateral diffusion of lipids may be determined mainly b)the number of structural defects in the two-dimensional arrangementsof lipids. Such defects should increase drasticallqat Tr corresponding to the increase of 0,.AcknowledgmentThe authors thank Professor S. Ebashi for kindly makinghis apparatus available for the experiment.ReferencesAndrich, M. P., and Vanderkooi, J. M. (1976), Biochemistr).15, 1257.Badley, R. A., Martin, W. G., and Schneider, H. (1973).Biochemistry 12, 268.Chapman, D. (1975), Q. Rec. Biophys. 8, 185.Cogan, U., Shinitzky, M., Weber, G., and Nishida, T. ( I973),Biochemistry 12, 52 1.Hubbell, W. L., and McConnell, H. M. (197 l), J. Am. Chem.Soc. 93, 314.Inesi, G., Millman, M., and Eletr, S. (I 973), J. Mol. Bid. 81,483.Israelachvili, J., Sjosten, J., Eriksson, L. E. G., Ehrstrom, M..Graslund, A,, and Ehrenberg, A. (1975), Biochim. Biophys.Acta. 382, 125.Jacobson, K., and Papahadjopoulos, D. 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