Cooperative properties of lipid bilayers : collective director fluctuations and the effects of hydrophobic mismatch in protein/lipid membrane systems
<p>Cooperative director fluctuations in lipid bilayers have been postulated for many years. ^2H-NMR T_1^(-1), T_(1P)^(-1) , and T_2^(-1); measurements have been used identify these motions and to determine the origin of increased slow bilayer motion upon addition of unlike lipids or protein...
| Summary: | <p>Cooperative director fluctuations in lipid bilayers have been postulated for
many years. ^2H-NMR T_1^(-1), T_(1P)^(-1) , and T_2^(-1); measurements have been used identify these motions and to determine the origin of increased slow bilayer motion upon
addition of unlike lipids or proteins to a pure lipid bilayer.</p>
<p>The contribution of cooperative director fluctuations to NMR relaxation in
lipid bilayers has been expressed mathematically using the approach of Doane et
al.^1 and Pace and Chan.^2 The T_2^(-1)’s of pure dimyristoyllecithin (DML) bilayers
deuterated at the 2, 9 and 10, and all positions on both lipid hydrocarbon chains
have been measured. Several characteristics of these measurements indicate the
presence of cooperative director fluctuations. First of all, T_2^(-1) exhibits a linear
dependence on S2/CD. Secondly, T_2^(-1) varies across the ^2H-NMR powder pattern
as sin^2 (2, β), where , β is the angle between the average bilayer director and the
external magnetic field. Furthermore, these fluctuations are restricted near the
lecithin head group suggesting that the head group does not participate in these
motions but, rather, anchors the hydrocarbon chains in the bilayer.</p>
<p>T_2^(-1)has been measured for selectively deuterated liquid crystalline DML hilayers
to which a host of other lipids and proteins have been added. The T_2^(-1) of
the DML bilayer is found to increase drastically when chlorophyll a (chl a) and
Gramicidin A' (GA') are added to the bilayer. Both these molecules interfere with
the lecithin head group spacing in the bilayer. Molecules such as myristic acid,
distearoyllecithin (DSL), phytol, and cholesterol, whose hydrocarbon regions are
quite different from DML but which have small,neutral polar head groups, leave
cooperative fluctuations in the DML bilayer unchanged.</p>
<p>The effect of chl a on cooperative fluctuations in the DML bilayer has been
examined
in detail using ^2H-NMR T_1^(-1), T_(1P)^(-1) , and T_2^(-1); measurements. Cooperative
fluctuations have been modelled using the continuum theory of the nematic state
of liquid crystals. Chl a is found to decrease both the correlation length and the
elastic constants in the DML bilayer.</p>
<p>A mismatch between the hydrophobic length of a lipid bilayer and that of
an added protein has also been found to change the cooperative properties of the
lecithin bilayer. Hydrophobic mismatch has been studied in a series GA' / lecithin
bilayers. The dependence of 2H-NMR order parameters and relaxation rates on
GA' concentration has been measured in selectively deuterated DML, dipalmitoyllecithin
(DPL), and DSL systems. Order parameters, cooperative lengths, and
elastic constants of the DML bilayer are most disrupted by GA', while the DSL
bilayer is the least perturbed by GA'. Thus, it is concluded that the hydrophobic
length of GA' best matches that of the DSL bilayer. Preliminary Raman
spectroscopy and Differential Scanning Calorimetry experiments of GA' /lecithin
systems support this conclusion. Accommodation of hydrophobic mismatch is
used to rationalize the absence of H_(II) phase formation in GA' /DML systems and
the observation of H_(II) phase in GA' /DPL and GA' /DSL systems.</p>
<p>1. J. W. Doane and D. L. Johnson, Chem. Phy3. Lett., 6, 291-295 (1970).
2. R. J. Pace and S. I. Chan, J. Chem. Phy3., 16, 4217-4227 (1982).</p>
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