Intramolecular disorder and its relation to mesophase structure in lipid/water mixtures

NMR spin-half pair dipolar echo measurements are reported for the lamellar (dispersions and multibilayer stacks) and hexagonal phases of potassium palmitate/²H₂O mixtures. In the lamellar L_β and L_γ (gel) phases the alkyl chains are rigid and perfectly ordered, while in the lamellar L_α and hexagonal phases they are flexible and disordered. In particular, the measurements show that in the fluid lamellar L_α phase the chain is “bent” at the C₉–C₁₀ segment; but is “straight” in the hexagonal phase.     

Sugars favour formation of hexagonal (HII) phase at the expense of lamellar liquid-crystalline phase in hydrated phosphatidylethanolamines

The disaccharides, sucrose and trehalose, markedly decreased (up to 17-13C°) the temperature of the lamellar to hexagonal (L_α →H_II) phase transition and simultaneously increase by 2–4 C° the temperature of the lamellar gel to lamellar liquid-crystal (L_β →L_α) phase transition in hydrated dihexadecylphosphatidylethanolamine and distearoylphosphatidylethanolamine. These two transitions merge and convert into a single L_β-H_II phase transition when dispersed in 2.4 M sucrose. These results are inconsistent with recent reports by (8) and (9)) which suggest that trehalose stabilizes the L_α phase relative to the H_II phase and shifts upwards beyond detectability the L_α-H_II transition. The present results are considered as a manifestation of the Hofmeister effect in which the sugars act as kosmotropic reagents stabilizing the structure of bulk water. This tends to decrease the area of contact between the lipid and the aqueous phases and favours the H_II and L_β phases relative to L_α phase. This hypothesis is consistent with the effects of chaotropic reagents on the L_α-H_II phase transition (Yeagle and Sen (1986) Biochemistry 25, 7518–7522) and on the stability of the lamellar phase of dipalmitoylphosphatidylcholine (Oku and MacDonald (1983) J. Biol. Chem. 258, 8733–8738).     

Effects of cations on dipalmitoyl phosphatidylcholine/cholesterol/water systems

X-ray diffraction studies have been made on the effects of cations upon the dipamitoyl phosphatidylcholine/water system, which originally consists of a lamellar phase with period of 64.5 Å and of excess water. Addition of 1 mM CaCl₂ destroys the lamellar structure and makes it swell into the excess water. the lamellar phase, however, reappears when the concentration of CaCl₂ increases: a partially disordered lamellar phase with the repeat distance of 150–200 Å comes out at the concentration of about 10 mM, the lamellar diffraction lines become sharp and the repeat distance decreases with increasing CaCl₂ concentration. A small amount of uranyl acetate destroys lamellar phase in pure water. MgCl₂ induces the lamellar phase of large repeat distance, whereas LiCl, NaCl, KCl, SrCl₂ and BaCl₂ exhibit practically no effect by themselves. Addition of cholesterol to the phosphatidylcholine bilayers tends to stabilize the lamellar phaseThe high-angle reflections indicate that molecular arrangements on phosphatidylcholine bilayers change at CaCl₂ concentrations around 0.5 M. The bilayers at high CaCl₂ concentration seem to consist of two phases of pure phosphatidylcholine and of equimolar mixture of phosphatidylcholine and cholesterol.     

Peroxidase and Tyrosinase Are Present in Secondary Lysosomes That Degrade Photosensory Membranes of the Crayfish Photoreceptor: Possible Role in Pigment Granule Formation

Three enzymes (acid phosphatase, peroxidase, and tyrosinase) were localized by electron microscopy within the retina of crayfish Orconectes limosus. Peroxidase activity was observed only in lamellar bodies, which are secondary lysosomes and degrade photosensory membrane. After H₂O₂ was omitted from the reaction medium, peroxidase activity in lamellar bodies was partly inhibited but was not missing completely. After addition of sodium pyruvate, which inhibits endogenous generation of H₂O₂, staining of lamellar bodies was absent. Tyrosinase activity was found in lamellar bodies and in small vesicles within the rhabdoms similar to those found positive for acid phosphatase. Granules (500–700 nm in diameter) with an electron opaque matrix and mature screening pigment granules showed tyrosinase activity. Moreover, lamellar structures within membrane-bound organelles that additionally contained screening pigment-like granules were electron dense because of tyrosinase activity. After addition of phenylthiourea (PTU) to the incubation medium, lamellar bodies did not generally contain electron dense deposits, although weak staining of single membranes still was sometimes observed. After addition of sodium pyruvate in combination with PTU, no staining was detected. The possible role of tyrosinase in ommochrome synthesis within secondary lysosomes that degrade photosensory membrane is discussed.     

Single lamellar mechanics of the human lumbar anulus fibrosus

The mechanical behavior of the entire anulus fibrosus is determined essentially by the tensile properties of its lamellae, their fiber orientations, and the regional variation of these quantities. Corresponding data are rare in the literature. The paper deals with an in vitro study of single lamellar anulus lamellae and aims to determine (i) their tensile response and regional variation, and (ii) the orientation of lamellar collagen fibers and their regional variation. Fresh human body-disc-body units (L1–L2, n=11) from cadavers were cut midsagittally producing two hemidisc units. One hemidisc was used for the preparation of single lamellar anulus specimens for tensile testing, while the other one was used for the investigation of the lamellar fiber orientation. Single lamellar anulus specimens with adjacent bone fragments were isolated from four anatomical regions: superficial and deep lamellae (3.9±0.21 mm, mean ± SD, apart from the outer boundary surface of the anulus fibrosus) at ventro-lateral and dorsal positions. The specimens underwent cyclic uniaxial tensile tests at three different strain rates in 0.15 mol/l NaCl solution at 37°C, whereby the lamellar fiber direction was aligned with the load axis. For the characterization of the tensile behavior three moduli were calculated: E_low (0–0.1 MPa), E_medium (0.1–0.5 MPa) and E_high (0.5–1 MPa). Additionally, specimens were tested with the load axis transverse to the fiber direction. From the second hemidisc fiber angles with respect to the horizontal plane were determined photogrammetrically from images taken at six circumferential positions from ventral to dorsal and at three depth levels. Tensile moduli along the fiber direction were in the range of 28–78 MPa (regional mean values). Superficial lamellae have larger E_medium (p=0.017) and E_high (p=0.012) than internal lamellae, and the mean value of superficial lamellae is about three times higher than that of deep lamellae. Tensile moduli of ventro-lateral lamellae do not differ significantly from the tensile moduli of dorsal lamellae, and E_low is generally indifferent with respect to the anatomical region. Tensile moduli transverse to the fiber direction were about two orders of magnitude smaller (0.22±0.2 MPa, mean ± SD, n=5). Tensile properties are not correlated significantly with donor age. Only small viscoelastic effects were observed. The regional variation of lamellar fiber angle is described appropriately by a regression line ||=23.2+0.130× (r²=0.55, p<0.001), where is the polar angle associated with the circumferential position. The single anulus lamella may be seen as the elementary structural unit of the anulus fibrosus, and exhibits marked anisotropy and distinct regional variation of tensile properties and fiber angles. These features must be considered for appropriate physical and numerical modeling of the anulus fibrosus.     

Time-resolved x-ray diffraction and calorimetric studies at low scan rates: II. On the fine structure of the phase transitions in hydrated dipalmitoylphosphatidylethanolamine

The phase transitions of dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been examined by low-angle time-resolved x-ray diffraction and calorimetry at low scan rates. The lamellar subgel/lamellar liquid-crystalline (L_c → L_α), lamellar gel/lamellar liquid-crystalline (L_β → L_α), and lamellar liquid-crystalline/lamellar gel (L_α → L_β) phase transitions proceed via coexistence of the initial and final phases with no detectable intermediates at scan rates 0.1 and 0.5°C/min. At constant temperature within the region of the L_β → L_α transition the ratio of the two coexisting phases was found to be stable for over 30 min. The state of stable phase coexistence was preceded by a 150-s relaxation taking place at constant temperature after termination of the heating scan in the transition region. While no intermediate structures were present in the coexistence region, a well reproducible multipeak pattern, with at least four prominent heat capacity peaks separated in temperature by 0.4-0.5°C, has been observed in the cooling transition (L_α → L_β) by calorimetry. The multipeak pattern became distinct with an increase of incubation time in the liquid-crystalline phase. It was also clearly resolved in the x-ray diffraction intensity versus temperature plots recorded at slow cooling rates. These data suggest that the equilibrium state of the L_α phase of hydrated DPPE is represented by a mixture of domains that differ in thermal behavior, but cannot be distinguished structurally by x-ray scattering.     

Phase properties of the aqueous dispersions of n-octadecylphosphocholine

Properties of the aqueous dispersions of n-octadecylphosphocholine are examined by differential scanning calorimetry, fluorescence depolarization, light scattering, ³¹P-NMR, pig pancreatic phospholipase A₂ binding, and X-ray diffraction. On heating, these dispersions exhibit a sharp lamellar to micelle transition at 20.5°C. The lamellar phase consists of frozen (gel-state) alkyl chains which do not bind phospholipase A₂. The kinetics of the transition are asymmetric: the micelle to lamellar transition is very slow and the lamellar to micelle transition is fast. It is suggested that the lamellar phase is a frozen chain bilayer in which the chains interdigitate.     

Carapace repair in the green crab,Carcinus maenas (L.)

Formation of a circular hole 8–10 mm in diameter in the calcified layers of the carapace from crabs in stage C₄ of the molt cycle stimulates the tissue under and adjacent to the injury to deposit a unique calcified cuticular material below the intact membranous layer. Deposition was followed for 69 days using light microscopic histology, histochemistry, and scanning electron microscopy. Quantitative analyses of CaCO₃ were conducted using atomic absorption spectrophotometry and Gran titration. Spatial distribution of CaCO₃ was determined with X-radiography. A scab is formed by day two under the injury. At four days the epithelium changes from squamous to columnar and deposits a PAS-positive layer with an irregular lamellar fine structure, followed by highly organized lamellae structurally similar to normal exocuticle. Histochemically, however, these lamellae resemble normal endocuticle. CaCO₃ is evident external to the outermost lamellae by day eleven as a fused mass of aragonite granules. The lamellar region calcifies proximally from the outer surface and is amorphous CaCO₃. Repair cuticle is approximately 20%CaCO₃ by weight.     

Quantitative histopathology of Oreochromis niloticus gills after copper exposure

In order to test whether histopathological changes of gills demonstrated a good dose–response relationship with water copper levels, juvenile Nile tilapia Oreochromis niloticus, of both sexes and similar mass (36·3 + 7·7 g), were kept in dechlorinated tap water (temperature 25° C, range ±1° C; pH 6·5–7·5; hardness 74·5 mg l^?1 CaCO₃) and exposed to 40 and 400 μg l^?1 of copper. Gill samples were collected after 3, 7, 14 and 21 days. Six major histopathological changes (oedema, lifting, changes in filament epithelium thickness, lamellar fusion, vasodilatation and aneurisms) and three minor ones (proliferation of the lamellar epithelium, necrosis and adjacent lamellar fusion) were found and their prevalence estimated. The extent and severity of each histopathological change were used to develop a severity gradation scale (SGS). Semi‐quantitative analysis of the histopathological changes and measurements of gill copper deposition levels revealed a good dose– and time–response relationship. Oedemas and aneurisms were significantly correlated with acute exposure periods and lamellar fusion with chronic exposure. Epithelial lifting and changes in filament epithelial thickness were seen at lower and higher metal concentrations, respectively. The data also revealed that the SGS profile of each lesion was dependent of gill copper burden.     

ULTRASTRUCTURAL STUDIES OF SPERMATOGENESIS IN THE ANTHOCEROTALES. I. THE BLEPHAROPLAST AND ANTERIOR MITOCHONDRION IN PHAEOCEROS LAEVIS: EARLY DEVELOPMENT

Electron microscopic examination of thin sections showed that the blepharoplast of a young spermatid of Phaeoceros consists of two side-by-side centrioles and an accumulation of osmiophilic, granular matrix at their proximal ends. Lying between these nearly parallel organelles is a dark-staining body that will later disappear at the onset of flagellogenesis. For a brief period the centrioles are oriented perpendicular to the nuclear surface so that the granular matrix at their proximal ends is confluent with the nuclear envelope; furthermore, the nucleoplasm immediately in front of the centrioles becomes densely staining. The multilayered structure (MLS) develops directly under the centrioles. It comprises a band of 12 microtubules (the S₁ stratum) and three lower strata (S_2–4) whose constitutent lamellae are oriented at an oblique angle to the S₁ axis. While the S₁ tubules grow rearward over the nucleus which forms a beak adjacent to the posterior end of the lamellar strata, the centrioles are transformed into basal bodies with the distal growth of the axonemes and the proximal growth of the central cartwheels and lowermost triplets. The proximal ends of the basal bodies and the S₁ tubules overlying the lamellar strata are invested with osmiophilic matrix that extends down to the S₂ layer and may temporarily occlude the lamellar plates. At the onset of nuclear elongation an anterior mitochondrion becomes situated close beneath the lamellar strata which extend laterally beyond the S₁ tubules.     

Cubic topology in surfactant and lipid mixtures

Ternary systems of palmitoyl-oleoyl-phosphatidylcholine (POPC) and the non-ionic surfactant C₁₂EO₂ (di-ethylene-oxide-mono-dodecyl-ether) in water have been studied with optical microscopy, NMR, DSC and X-rays from ambient temperatures to 45 °C. Below 29 °C the system is in the lamellar liquid crystalline state. Between 30 and 32 °C it transforms into a cubic Ia3d structure which converts into the cubic Pn3m phase at 39 °C. The transitions are fully reversible. An epitaxial relationship between all three phases was found, which is an elegant and convenient way to rearrange molecules from lamellar bilayers to a network of curved surfaces. The la3d (Q²³⁰) to Pn3m (Q²²⁴) transition occurs without measurable enthalpy change. This, together with the metric relation of 1.60 between the cubic lattice constants is strong evidence for a Bonnet transformation, where the structural changes occur without change in curvature. The potential significance of the cubic phases as intermediate structures for biological processes, e. g. transport across a bilayer or fusion of membranes, are discussed.     

Dicyclohexylcarbodiimide and vanadate sensitive ATPase of lung lamellar bodies

Lung surfactant is synthesized in lung epithelial type II cells and stored in the lamellar bodies prior to its secretion onto the alveolar surface. The lamellar bodies, like other secretory organelles, maintain an ATP-dependent pH gradient that is sensitive to inhibitors of H⁺-ATPase. This report shows that the ATPase activity of lamellar bodies is enriched in a fraction prepared from lamellar bodies that were disrupted after isolation. The apparent V_max for this enzyme was 150 nmol ATP hydrolyzed per min per mg protein and apparent K_m for ATP was approximately 50 μM. The enzyme activity was sensitive to N-ethylmaleimide (NEM), dicyclohexylcarbodiimide (DCCD) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-C1) (all inhibitors of vacuolar-type H⁺-ATPase) and vanadate (inhibitor of phosphoenzyme-type ATPase). Besides, the activity could also be inhibited with diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and Ca²⁺. Two proteins (of approximately 45 kDa and 17 kDa) of this fraction showed acid-stable phosphorylation with ATP. The labeling of proteins with ATP (-γ-³²P) could be chased with unlabelled ATP, suggesting that phosphorylation and dephosphorylation of these proteins is associated with the ATPase activity. Our results on inhibition characteristics of the enzyme activity suggest that besides a vacuolar type H⁺-ATPase, the lamellar bodies also contain a phosphoenzyme type ATPase that is sensitive to inhibitors of vacuolar type H⁺-ATPase.     

The structural diversity of DNA–neutral phospholipids–divalent metal cations aggregates: a small-angle synchrotron X-ray diffraction study

We investigate the structure of aggregates formed due to DNA interaction with saturated neutral phosphatidylcholines [dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine] in presence of Ca²⁺ and Mg²⁺ cations using simultaneous synchrotron small- and wide-angle X-ray diffractions. For DPPC:DNA = 3:1 mol/base and in the range of 1–50 mM Ca²⁺, the diffractograms show structural heterogeneity of aggregates. We observe the coexistence of two lamellar phases in aggregates prepared at 1 mM Ca²⁺: L^x phase with the DNA strands (of unknown organization) intercalated in water layers between adjacent lipid bilayers and L_DPPC phase of DPPC bilayers without any divalent cations and DNA strands. Aggregates prepared in the range 2–50 mM Ca²⁺ show a condensed gel lamellar phase L_g^c with the lipid bilayer periodicity d ≈ 8.0 nm, and the DNA–DNA interhelical distance d _DNA ≈ 5.1 nm. The increase of temperature induces the decrease in the intensity and the increase in the width of the DNA related peak. In the fluid state, the condensed lamellar phase L_α^c gradually converts into L^x phase. The aggregates do not exhibit rippled P_β phase. The thermal behaviour of aggregates was investigated in the range 20–80°C. Applying heating–cooling cycles, the aggregates converted into energetically more favourable structure: a condensed lamellar phase L^c (or L^x) is preserved or we observe lateral segregation of the DNA strands and metal cations (L^x phase) in coexistence with L_PC phase of pure phospholipids. Dedicated to Prof. Dr Klaus Arnold on the occasion of his 65th birthday.     

Interaction of the C-Terminal Peptide of Pulmonary Surfactant Protein B (SP-B) with a Bicellar Lipid Mixture Containing Anionic Lipid

The hydrophobic lung surfactant SP-B is essential for respiration. SP-B promotes spreading and adsorption of surfactant at the alveolar air-water interface and may facilitate connections between the surface layer and underlying lamellar reservoirs of surfactant material. SP-B_63–78 is a cationic and amphipathic helical peptide containing the C-terminal helix of SP-B. ²H NMR has been used to examine the effect of SP-B_63–78 on the phase behavior and dynamics of bicellar lipid dispersions containing the longer chain phospholipids DMPC-d ₅₄ and DMPG and the shorter chain lipid DHPC mixed with a 3∶1∶1 molar ratio. Below the gel-to-liquid crystal phase transition temperature of the longer chain components, bicellar mixtures form small, rapidly reorienting disk-like particles with shorter chain lipid components predominantly found around the highly curved particle edges. With increasing temperature, the particles coalesce into larger magnetically-oriented structures and then into more extended lamellar phases. The susceptibility of bicellar particles to coalescence and large scale reorganization makes them an interesting platform in which to study peptide-induced interactions between lipid assemblies. SP-B_63–78 is found to lower the temperature at which the orientable phase transforms to the more extended lamellar phase. The peptide also changes the spectrum of motions contributing to quadrupole echo decay in the lamellar phase. The way in which the peptide alters interactions between bilayered micelle structures may provide some insight into some aspects of the role of full-length SP-B in maintaining a functional surfactant layer in lungs.     

Genetic Ablation of Calcium-independent Phospholipase A2�� Leads to Alterations in Hippocampal Cardiolipin Content and Molecular Species Distribution, Mitochondrial Degeneration, Autophagy, and Cognitive Dysfunction

Genetic ablation of calcium-independent phospholipase A₂γ (iPLA₂γ) results in profound alterations in hippocampal phospholipid metabolism and mitochondrial phospholipid homeostasis resulting in enlarged and degenerating mitochondria leading to autophagy and cognitive dysfunction. Shotgun lipidomics demonstrated multiple alterations in hippocampal lipid metabolism in iPLA₂γ^−/− mice including: 1) a markedly elevated hippocampal cardiolipin content with an altered molecular species composition characterized by a shift to shorter chain length molecular species; 2) alterations in both choline and ethanolamine glycerophospholipids, including a decreased plasmenylethanolamine content; 3) increased oxidized phosphatidylethanolamine molecular species; and 4) an increased content of ceramides. Electron microscopic examination demonstrated the presence of enlarged heteromorphic lamellar structures undergoing degeneration accompanied by the presence of ubiquitin positive spheroid inclusion bodies. Purification of these enlarged heteromorphic lamellar structures by buoyant density centrifugation and subsequent SDS-PAGE and proteomics identified them as degenerating mitochondria. Collectively, these results identify the obligatory role of iPLA₂γ in neuronal mitochondrial lipid metabolism and membrane structure demonstrating that iPLA₂γ loss of function results in a mitochondrial neurodegenerative disorder characterized by degenerating mitochondria, autophagy, and cognitive dysfunction.     

Shifts in chain-melting transition temperature of liposomal membranes by polymer-grafted lipids

The chain-melting transition temperature of dipalmitoyl phosphatidylcholine (DPPC) bilayer membranes containing poly(ethylene glycol)-grafted dipalmitoyl phosphatidylethanolamine (PEG-DPPE) was determined by optical turbidity measurements. The dependence on content, X_p, of PEG-DPPE lipid was studied for different polar headgroup sizes, n_p, of the polymer lipid, throughout the lamellar phase of the mixtures with DPPC. Mean-field theory for the polymer brush regime predicts that the downward shift in transition temperature should vary with polymer size and content as n_pX_p^5/3 (∼n_pX_p^11/6 for scaling theory). Any shift induced by the charge on PEG-lipids is independent of polymer size. These predictions are reasonably borne out for the longer polymer lipids (PEG molecular masses 750, 2000 and 5000 Da). Transition temperature shifts in the lamellar phase, before the onset of micellisation, are in the region of −1 to −2 °C (±0.1-0.2 °C) in reasonable accord with theoretical estimates of the lateral pressure exerted by the polymer brush. Shifts of this size are significant to the design of liposomes for controlled release of contents by mild hyperthermia.     

Influence of carbon dioxide supply on the chloroplast structure of Chlorella pyrenoidosa

Summary Electron microscopic investigations show that the structure of the lamellar system of Chlorella pyrenoidosa depends on the carbon dioxide supply in the culture medium. Chloroplasts of C. pyrenoidosa when grown with 0.03% CO₂, show typical grana structure whereas with 3% CO₂ the chloroplast structure is typical of that of the Chlorophyceae.     

Phase equilibria of mixtures of plant galactolipids. The formation of a bicontinuous cubic phase

The chloroplast galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were isolated from wheat leaves. The phase equilibria of galactolipid-water systems with MGDG / DGDG molar ratios equal to 0:1, 1:2, 1.2:1, 2:1 and 1:0 were investigated, using nuclear magnetic resonance (NMR) methods. MGDG and DGDG form reversed hexagonal and lamellar phases, respectively, at temperatures between 10 and 40°C at all water contents studied (up to about 14 mol ²H₂O per mol lipid). The galactolipid mixtures show a complex phase forming reversed hexagonal, lamellar and reversed cubic phases, depending on water content and temperature. It was found that the water hydration is similar for the lamellar and hexagonal phases formed by DGDG and MGDG, respectively. The non-lamellar phase areas increase with increasing content of MGDG. Small-angle X-ray measurements show that the cubic phase belongs to the Ia3d space group. From translational diffusion studies by NMR it is concluded that the structure of this cubic phase is bicontinuous.     

The transmembrane distribution of galactolipids in chloroplast thylakoids is universal in a wide variety of temperate climate plants

The transmembrane distribution of monogalactosyldiacylglycerol and digalactosyldiacylglycerol was determined in chloroplast thylakoids from a range of temperate climate plants. These plants included dicotyledons, monocotyledons, C_16:3 and C_18:3 plants and herbicide-resistant species. In all the thylakoids examined monogalactosyldiacylglycerol was enriched in the outer leaflet (53–65%) while digalactosyldiacylglycerol was highly enriched in the inner leaflet (78–90%). The non-bilayer forming monogalactosyldiacylglycerol represented 55–81% of the total acyl lipids of the outer monolayer. The relative acyl lipid composition of both leaflets of the thylakoid membrane indicates that the lamellar structure is strongly favored in the inner monolayer, whereas the outer one presents a metastable character which allows the probable coexistence of both lamellar and non-lamellar phases. The consequence of this asymmetry for the stability and function of the thylakoid membrane is discussed.     

New evidence for gel-liquid crystalline phase coexistence in the ripple phase of phosphatidylcholines

Experimental evidence supporting the hypothesis of gel-liquid crystalline phase coexistence in the stable ripple phase of diacylphosphatidylcholines has been obtained from time-resolved X-ray small- (SAXS) and wide-angle diffraction (WAXS) in the millisecond to second time domain. The pretransition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) exhibits a thin lamellar liquid crystalline intermediate phase (designated L_α*) if driven far away from equilibrium by an infrared temperature jump (T-jump) technique. The findings can be described by a two-step model. (1) Instantaneously with the T-jump, an anomalously thin lamellar liquid crystalline intermediate phase (d = 5.6–5.8 nm) forms, coexisting with the original gel-phase L_β′. Within the first seconds, the lamellar repeat distance of the intermediate increases to a value of about 6.7 nm. A closer examination of these kinetics reveals two relaxation components: a fast process, proceeding within tenths of a second, and a slow process, on the time scale of a few seconds. (2) Finally, both the liquid crystalline and the gel-phase relax into the stable ripple phase P_β′. The total process time of the transition is nearly independent of the addition of NaCl, but varies strongly with the chain length of the lecithin species. Received: 24 November 1999 / Revised version: 25 February 2000 / Accepted: 25 February 2000