Transition Dipole Orientation of Linear Polyenes: Semiempirical Models and Extrapolation to the Infinite Chain Limit

An Overview of Lutein in the Lipid Membrane

Lutein, zeaxanthin, and meso-zeaxanthin (a steroisomer of zeaxanthin) are macular pigments. They modify the physical properties of the lipid bilayers in a manner similar to cholesterol. It is not clear if these pigments are directly present in the lipid phase of the membranes, or if they form complexes with specific membrane proteins that retain them in high amounts in the correct place in the retina. The high content of macular pigments in the Henle fiber layer indicates that a portion of the lutein and zeaxanthin should not only be bound to the specific proteins but also directly dissolved in the lipid membranes. This high concentration in the prereceptoral region of the retina is effective for blue-light filtration. Understanding the basic mechanisms of these actions is necessary to better understand the carotenoid-membrane interaction and how carotenoids affect membrane physical properties-such as fluidity, polarity, and order-in relation to membrane structure and membrane dynamics. This review focuses on the properties of lutein.

Factors Differentiating the Antioxidant Activity of Macular Xanthophylls in the Human Eye Retina

Macular xanthophylls, which are absorbed from the human diet, accumulate in high concentrations in the human retina, where they efficiently protect against oxidative stress that may lead to retinal damage. In addition, macular xanthophylls are uniquely spatially distributed in the retina. The zeaxanthin concentration (including the lutein metabolite meso-zeaxanthin) is ~9-fold greater than lutein concentration in the central fovea. These numbers do not correlate at all with the dietary intake of xanthophylls, for which there is a dietary zeaxanthin-to-lutein molar ratio of 1:12 to 1:5. The unique spatial distributions of macular xanthophylls-lutein, zeaxanthin, and meso-zeaxanthin-in the retina, which developed during evolution, maximize the protection of the retina provided by these xanthophylls. We will correlate the differences in the spatial distributions of macular xanthophylls with their different antioxidant activities in the retina. Can the major protective function of macular xanthophylls in the retina, namely antioxidant actions, explain their evolutionarily determined, unique spatial distributions? In this review, we will address this question.

Fluorescence Anisotropy Detection of Barrier Crossing and Ultrafast Conformational Dynamics in the S2 State of β-Carotene

Carotenoids are usually only weakly fluorescent despite being very strong absorbers in the mid-visible region because their first two excited singlet states, S₁ and S₂, have very short lifetimes. To probe the structural mechanisms that promote the nonradiative decay of the S₂ state to the S₁ state, we have carried out a series of fluorescence lineshape and anisotropy measurements with a prototype carotenoid, β-carotene, in four aprotic solvents. The anisotropy values observed in the fluorescence emission bands originating from the S₂ and S₁ states reveal that the large internal rotations of the emission transition dipole moment, as much as 50° relative to that of the absorption transition dipole moment, are initiated during ultrafast evolution on the S₂ state potential energy surface and persist upon nonradiative decay to the S₁ state. Electronic structure calculations of the orientation of the transition dipole moment account for the anisotropy results in terms of torsional and pyramidal distortions near the center of the isoprenoid backbone. The excitation wavelength dependence of the fluorescence anisotropy indicates that these out-of-plane conformational motions are initiated by passage over a low-activation energy barrier from the Franck-Condon S₂ structure. This conclusion is consistent with detection over the 80-200 K range of a broad, red-shifted fluorescence band from a dynamic intermediate evolving on a steep gradient of the S₂ state potential energy surface after crossing the activation barrier. The temperature dependence of the oscillator strength and anisotropy indicate that nonadiabatic passage from S₂ through a conical intersection seam to S₁ is promoted by the out-of-plane motions of the isoprenoid backbone with strong hindrance by solvent friction.

Localization and Orientation of Xanthophylls in a Lipid Bilayer

Xanthophylls (polar carotenoids) play diverse biological roles, among which are modulation of the physical properties of lipid membranes and protection of biomembranes against oxidative damage. Molecular mechanisms underlying these functions are intimately related to the localization and orientation of xanthophyll molecules in lipid membranes. In the present work, we address the problem of localization and orientation of two xanthophylls present in the photosynthetic apparatus of plants and in the retina of the human eye, zeaxanthin and lutein, in a single lipid bilayer membrane formed with dimyristoylphosphatidylcholine. By using fluorescence microscopic analysis and Raman imaging of giant unilamellar vesicles, as well as molecular dynamics simulations, we show that lutein and zeaxanthin adopt a very similar transmembrane orientation within a lipid membrane. In experimental and computational approach, the average tilt angle of xanthophylls relative to the membrane normal is independently found to be ~40 deg, and results from hydrophobic mismatch between the membrane thickness and the distance between the terminal hydroxyl groups of the xanthophylls. Consequences of such a localization and orientation for biological activity of xanthophylls are discussed.

Tight binding model of conformational disorder effects on the optical absorption spectrum of polythiophenes

Semiconducting polymers are soft materials with many conformational degrees of freedom. The limited understanding of how conformational disorder affects their optoelectronic properties is a key source of difficulties that limits their widespread usage in electronic devices. We develop a coarse-grained approach based on the tight binding approximation to model the electronic degrees of freedom of polythiophene chains, taking into account conformational degrees of freedom. Particularly important is dihedral disorder, which disrupts extended electronic states. Our tight binding model is parameterized using density functional theory (DFT) calculations of the one-dimensional band structures for chains with imposed periodic variations in dihedral angles. The model predicts valence and conduction bands for these chain conformations that compare well to DFT results. As an initial application of our model, we compute the optical absorption spectrum of poly(3-hexylthiophene) chains in solution. We observe a broadening of the absorption edge resulting from dihedral disorder, just shy of the experimental broadening. We conclude that the effects of molecular disorder on the optoelectronic properties of conjugated polymer single chains can be mostly accounted for by torsional disorder alone.

Orientational ordering of carotenoids in myelin membranes resolved by polarized Raman microspectroscopy

We study orientational ordering of membrane compounds in the myelinated nerve fiber by means of polarized Raman microspectroscopy. The theory of orientational distribution functions was adapted to live-cell measurements. The obtained orientational distribution functions of carotenoids and lipid acyl chain clearly indicated a predominantly radial-like orientation in membranes of the myelin. Two-dimensional Raman images, made under optimal polarization of incident laser beam, corroborated the proposed carotenoid orientation within the bilayer. Experimental data suggested the tilted orientation of both carotenoid polyenic and lipid acyl chains. The values of maximum tilt angles were similar, with possible implication of carotenoid-induced ordering effect on lipid acyl chains, and hence change of myelin membrane properties. This study stages carotenoids of the nerve as possible mediators of excitation and leverages underlying activity-dependent membrane reordering.

Calorimetric studies of the effect of cis-carotenoids on the thermotropic phase behavior of phosphatidylcholine bilayers

Carotenoid geometry is a factor that determines their solubility and orientation in the lipid membrane as well as antioxidant capacities and bioavailability. The effects of the cis-isomers of carotenoids (zeaxanthin and beta-carotene) on the thermotropic properties of lipid membranes formed with dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) were investigated by means of differential scanning calorimetry. The results were compared with the effects caused by the all-trans-isomer. Both the trans and cis isomers of zeaxanthin shifted the main phase transition temperature to lower values and decreased the cooperativity of the phase transition. The effect of all-trans zeaxanthin on the physical properties of the lipid bilayers has been shown to strongly depend on the hydrocarbon chain length of the membrane. In the case of cis-zeaxanthin this relationship is weaker.

Dynamics of bolaamphiphilic fluorescent polyenes in lipid bilayers from polarization emission spectroscopy

The rotational motions of the biamphiphilic polyenes (bolapolyenes) dimethyl all-(E)-octacosa-10,12,14,16,18-pentaenedioate (DE28:5) and dimethyl all-(E)-tetratriaconta-13,15,17,19,21-pentaenedioate (DE34:5), with head-to-head distances of 34 and 42A, respectively, have been examined by fluorescence anisotropy methods. The membrane-spanning bolapolyenes, which contain a central emitting pentaene group tethered to two methoxycarbonyl opposite polar heads by symmetric C(8) (DE28:5) and C(11) (DE34:5) polymethylene chains, were dispersed in lipid bilayers of DPPC or DMPC, and the stationary and picosecond-resolved emission was recorded as a function of temperature. In fluid-phase DMPC bilayers, three relaxation times could be determined, assigned to fast (0.2 and 2ns) single-bond isomerization processes localized on the alkyl chains, and to whole-molecule oscillations ( approximately 11ns), respectively. The anisotropy decay parameters were further analyzed in terms of a diffusive model for wobbling in a Gaussian ordering potential, to assess the anchoring effect of the symmetric polar heads. In this way, the average rotational diffusion constant of the bolapolyenes, D( perpendicular), could be estimated as 0.022-0.026rad(2) ns(-1) (DMPC bilayers, 35 degrees Celsius), a value that is only 1/3 of that corresponding to the related pentaene fatty acid spanning a single membrane monolayer. In contrast, the amplitude of the equilibrium orientational distribution (theta(half-cone) approximately 50 degrees ) is very similar for both the transmembrane and the single-headed polyenes. The reorientational oscillations of the central emitting group in the bolapolyenes necessarily would produce large-amplitude (2-5A) and very fast (ns) translational motions of the polar heads.

The photobleaching sequence of a short-wavelength visual pigment

The photobleaching pathway of a short-wavelength cone opsin purified in delipidated form (lambda(max) = 425 nm) is reported. The batho intermediate of the violet cone opsin generated at 45 K has an absorption maximum at 450 nm. The batho intermediate thermally decays to the lumi intermediate (lambda(max) = 435 nm) at 200 K. The lumi intermediate decays to the meta I (lambda(max) = 420 nm) and meta II (lambda(max) = 388 nm) intermediates at 258 and 263 K, respectively. The meta II intermediate decays to free retinal and opsin at >270 K. At 45, 75, and 140 K, the photochemical excitation of the violet cone opsin at 425 nm generates the batho intermediate at high concentrations under moderate illumination. The batho intermediate spectra, generated via decomposing the photostationary state spectra at 45 and 140 K, are identical and have properties typical of batho intermediates of other visual pigments. Extended illumination of the violet cone opsin at 75 K, however, generates a red-shifted photostationary state (relative to both the dark and the batho intermediates) that has as absorption maximum at approximately 470 nm, and thermally reverts to form the normal batho intermediate when warmed to 140 K. We conclude that this red-shifted photostationary state is a metastable state, characterized by a higher-energy protein conformation that allows relaxation of the all-trans chromophore into a more planar conformation. FTIR spectroscopy of violet cone opsin indicates conclusively that the chromophore is protonated. A similar transformation of the rhodopsin binding site generates a model for the VCOP binding site that predicts roughly 75% of the observed blue shift of the violet cone pigment relative to rhodopsin. MNDO-PSDCI calculations indicate that secondary interactions involving the binding site residues are as important as the first-order chromophore protein interactions in mediating the wavelength maximum.