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Fluorescence Anisotropy Data Proc. Nati. Acad. Sci. USA Vol. 76, No. 12, pp. 6361-6365, December 1979 Biophysics Structural order of lipids and proteins in membranes: Evaluation of fluorescence anisotropy data (order parameter/lipid bilayers/lipid-protein interaction/protein dynamics) FRITZ JAHNIG Max-Planck-Institut fur Biologie, Corrensstrasse 38, D-74 Tubingen, West Germany Communicated by Manfred Eigen, October 1, 1979 ABSTRACT The limiting long-time value of fluorescence anisotropy in membranes is correlated with the orientational r,, order parameter, which characterizes the structural anisotropy of membranes. Existing experimental results for diphenylhex- atriene in lipid bilayers are evaluated for the order parameter of lipid order. Steady-state measurements of fluorescence an- rt isotropy can provide the order parameter in good approxima- tion. Proteins in a fluid lipid phase increase the lipid order pa- rameter so determined. Upon comparison with the order pa- rameter from deuterium magnetic resonance, it is concluded that proteins increase the order of the surrounding lipids in off-normal directions. Order parameters of protein order ob- tained from the limiting value of protein fluorescence anisot- ropy are discussed with respect to the influence of lipid order FIG. 1. Time dependence of FA. The dotted line applies to Per-. on protein order. nfI'slaw. Fluorescence depolarization is an extensively used technique of x7 was consonant with the general concepts of membrane in membrane research. A short pulse of polarized light falls on fluidity. a suspension of membranes containing fluorescent molecules, The derivation of Eq. 4 proceeds from the following concept: either extrinsic probes or intrinsically fluorescent proteins. The The polarized light excites dipole moments of a certain orien- intensity I of the fluorescence is recorded as a function of time tation that emit light over their lifetime r. Initially the emitted t for the two polarizations parallel and perpendicular to the light is polarized parallel, and rt is large. Due to rotational initial polarization. The fluorescence anisotropy (FA) is ob- diffusion the orientation of the emitting dipoles becomes in- tained as creasingly disordered, and rt decreases. Assuming the envi- ronment of the dipoles to be isotropic, their final distribution rt -III] will also be isotropic, and rt decreases to zero (Fig. 1). The larger X and therefore 0, the longer the initially created anisotropy Often measurements are performed under constant illumina- is preserved and consequently detected in steady-state mea- tion, yielding the steady-state FA surements yielding a large rs. III - I1L Recent time-resolved FA measurements with pure lipid rs = [2] membranes (2-4), lipid membranes containing cholesterol (5, + 6), and cell membranes (7, 8), as well as earlier measurements* in which IIl and IV are the time integrals of III and I-. The with excitable membranes (9), have shown rt not to decrease steady-state FA can be expressed by the time-resolved FA as to zero but to reach a finite level r. (Fig. 1). Thus the final distribution of emitting dipoles must be anisotropic. For mol- s= J dtrtIt/J dtIt, [3] ecules in membranes this is conceivable. So the final FA value furnishes information on the structural order in membranes, in which the total fluorescence intensity It = I II + 2Ij has been while the relaxation time provides information on kinetic introduced. properties such as microviscosity. Because both factors enter Since the work of Shinitzky (for a review see ref. 1) the the steady-state FA, the evaluation of rs has to be improved. steady-state FA has been interpreted in terms of the so-called The present paper deals with two questions: (i) What is the microviscosity 77 by applying the Perrin equation structural information provided by the experimental limiting =ro FA value re, and (ii) to what extent can the same information rs = T1+0, [4] be obtained from steady-state measurements-i.e., from rs? The answers will be utilized to gain insight into the problem of ro being the maximal FA value in the absence of any rotational lipid-protein interaction in membranes. motion, r the fluorescence life time, and X the rotational re- laxation time given by X = 77V/(kT), V the volume of the Fluorescence anisotropy and order parameter fluorophore, k the Boltzmann constant, and T the absolute To answer the first question a theoretical analysis is needed that temperature. The microviscosity 77 for lipid membranes so correlates r. to membrane properties. Such a study was carried determined was high in the ordered and low in the fluid phase, out by Kinosita et al. (10). For the case of both the absorption with an abrupt change at the phase transition. This behavior Abbreviations: FA, fluorescence anisotropy; LA, luminescence an- The publication costs of this article were defrayed in part by page isotropy; DMR, deuterium magnetic resonance; ESR, electron spin charge payment. This article must therefore be hereby marked "ad- resonance; DPH, diphenylhexatriene; Pam2PtdCho, dipalmitoyl vertisement" in accordance with 18 U. S. C. §1734 solely to indicate phosphatidylcholine; Myr2PtdCho, dimyristoyl phosphatidylcho- this fact. line. 6361 Downloaded by guest on September 30, 2021 6362 Biophysics: jihnig Proc. Natl. Acad. Sci. USA 76 (1979) and emission moment lying along the fluorophore axis their a lipid membrane as a function of temperature. The r., values result was for dipalmitoyl phosphatidylcholine (Pam2PtdCho) vesicles were measured by Kawato et al. (3) and by Lakowicz et al. (4), r= 2/5(P2(cos 0))2. [5] employing DPH as the fluorescence probe. Both sets of results Here P2(cos 0) = (3 cos20-1)/2 is the Legendre polynomial are presented in Fig. 2A and agree well. Because the absorption of second order; 6 is the angle between an instantaneous or- and emission moments of DPH are likely to lie along the mo- ientation of the fluorophore and the average orientation; and lecular axis, Eq. 7 can be utilized for evaluation, the result being the angular brackets denote the average over the orientations shown in Fig. 2B. The temperature behavior of the order pa- of the fluorophore within a certain time. This average and the rameter is as expected: the order is high in the ordered phase average orientation refer to the equilibrium state yielding r. and low in the fluid phase. The phase transition appears rela- The averaging time therefore is longer than the relaxation time tively broad, as known for vesicles. For comparison with the 0 needed to establish this equilibrium; it is, however, limited DMR order parameter, the results of Seelig and Seelig (17) for by the lifetime r because possible slower relaxation processes specifically deuterated Pam2PtdCho in liposomes are included. with >>» X are not detected. Introducing the probability dis- The FA result approaches the DMR order parameter in the tribution w(O) for the orientations, the average can be ex- fluid phase at the C12 position. According to the above argu- pressed as ments this is reasonable. In the ordered phase DMR order pa- rameters are difficult to obtain. For perdeuterated soaps, Mely (P2(cos 0)) = 3o dcos 6 P2(cos 0) w(cos 0) [6] and Charvolin (18) found values between 0.7 and 0.9, which are in the same range as the FA result. For complete order (all 0 = 0) one gets (P2) = 1, and for FA complete disorder [w(6) = 1] (P2) = 0. Therefore (P2) is a Steady-State measure of the orientational order of the fluorophores within To answer the second question we turn to a phenomenological their lifetime, which is of the order of 10-8 sec. way of reasoning. From the experiments of Kawato et al. (8) In order to evaluate the experimental r. values for lipid and of Lakowicz et al. (4) it follows that rt in lipid membranes membranes in terms of the distribution w(O), Kinosita et al. shows a simple exponential decay to the limiting value r., employed a cone model as done earlier by Wahl (11), w(6) rt = (ro - r.) + r., being constant between 6 = 0 and 6 = O,, and determined 6C. exp(-t/0) [8] Two points can be made about this evaluation. Recent theo- with ro = 0.395 ± 0.01 (3) or ro = 0.39 (4). Within the experi- retical results indicate W(6) to be better described by a Gauss- mental error these values for ro agree with the theoretical result ian-like distribution (12). More important, however, is the fact ro = 2/sP2(cos X) if the angle X between the absorption and that Eq. 6 represents the definition of the orientational order emission moments is zero, as expected for DPH. Because It t parameter S known for lipid membranes both from theory (12, exp(-t/r) the integrations in Eq. 3 are easily performed, 13) and experiment-e.g., deuterium magnetic resonance yielding (DMR) (14). The r. values may then simply be evaluated for the order parameter. This order parameter obtained from FA rs= + r. [9] differs, however, from the DMR order parameter in two re- spects: (i) DMR measures the order parameter S, of the indi- The first term represents the kinetic contribution, the second vidual methylene segments v along the lipid chains, whereas the structural one. The Perrin formula, Eq. 4, is obtained if the FA measures the order parameter of a probe between the lipid chains; (ii) the order parameter from DMR is an average over 0. A a time of about 10-4 sec, much longer than the FA averaging .3o o 0° time. The general problems connected with probe molecules 0. are the same for FA as with other methods such as electron spin resonance (ESR) (14). The widely used diphenylhexatriene rc 00..2 9 (DPH) molecules are known to be located deep in the lipid bi- layer and to be oriented parallel to the lipid chains (15).
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