Free radical mediated x-ray damage of model membranes

The damaging effects of synchrotron-derived x rays on aqueous phospholipid dispersions have been evaluated. The effect of degree of lipid hydration, phospholipid chemical structure, mesophase identity, aqueous medium composition, and incident flux on the severity and progress of damage was quantified using time-resolved x-ray diffraction and chromatographic analysis of damage products. Electron spin resonance measurements of spin-trapped intermediates generated during irradiation suggest a free radical-mediated process. Surprisingly, radiation damage effects revealed by x-ray diffraction were imperceptible when the lamellar phases were prepared under water-stressed conditions, despite the fact that x-ray-induced chemical breakdown of the lipid occurred regardless of hydration level. Of the fully hydrated lipid systems studied, saturated diacyl-phosphatidylcholines were most sensitive to radiation damage compared to the ester- and ether-linked phosphatidylethanolamines and the ether-linked phosphatidylcholines. The inclusion of buffers or inorganic salts in the dispersing medium had only a minor effect in reducing damage development. A small inverse dose-rate effect was found when the x-ray beam intensity was changed 15-fold. These results contribute to our understanding of the mechanism of radiation damage, to our appreciation of the importance of monitoring both structure and composition when evaluating biomaterials radiation sensitivity, and to the development of strategies for eliminating or reducing the severity of damage due to an increasingly important source of x rays, synchrotron radiation. Because damage is shown to be free radical mediated, these results have an important bearing on age-related accumulation of free radicals in cells and how these might compromise membrane integrity, culminating in cell death.

Stability study of Rhodobacter capsulatus ferrocytochrome c2 wild-type and site-directed mutants using hydrogen/deuterium exchange monitored by electrospray ionization mass spectrometry

To estimate the stability of Rhodobacter capsulatus ferrocytochrome c2 wild-type and site-directed mutants, charge state distributions and hydrogen/deuterium exchange rates were monitored by electrospray ionization mass spectrometry. The relative stability of the mutants was observed with the order: V11 insert > Y75F > wild-type = K32E > K12D = K14E > or = K52E > K14E/K32E > W67Y > P35A > I57N > G34S. (A preliminary account has been presented for mutants G34S and P35A [Jaquinod et al. (1995) Rapid Commun. Mass Spectrom. 9, 1135-1140].) This approach is shown to be a useful tool for rapid characterization of mutational effects on protein conformation.

Polymorphism, mesomorphism, and metastability of monoelaidin in excess water

The polymorphic and metastable phase behavior of monoelaidin dry and in excess water was studied by using high-sensitivity differential scanning calorimetry and time-resolved x-ray diffraction in the temperature range of 4 degrees C to 60 degrees C. To overcome problems associated with a pronounced thermal history-dependent phase behavior, simultaneous calorimetry and time-resolved x-ray diffraction measurements were performed on individual samples. Monoelaidin/water samples were prepared at room temperature and stored at 4 degrees C for up to 1 week before measurement. The initial heating scan from 4 degrees C to 60 degrees C showed complex phase behavior with the sample in the lamellar crystalline (Lc0) and cubic (Im3m, Q229) phases at low and high temperatures, respectively. The Lc0 phase transforms to the lamellar liquid crystalline (L alpha) phase at 38 degrees C. At 45 degrees C, multiple unresolved lines appeared that coexisted with those from the L alpha phase in the low-angle region of the diffraction pattern that have been assigned previously to the so-called X phase (Caffrey, 1987, 1989). With further heating the X phase converts to the Im3m cubic phase. Regardless of previous thermal history, cooling calorimetric scans revealed a single exotherm at 22 degrees C, which was assigned to an L alpha+cubic (Im3m, Q229)-to-lamellar gel (L beta) phase transition. The response of the sample to a cooling followed by a reheating or isothermal protocol depended on the length of time the sample was incubated at 4 degrees C. A model is proposed that reconciles the complex polymorphic, mesomorphic, and metastability interrelationships observed with this lipid/water system. Dry monoelaidin exists in the lamellar crystalline (beta) phase in the 4 degrees C to 45 degrees C range. The beta phase transforms to a second lamellar crystalline polymorph identified as beta* at 45 degrees C that subsequently melts at 57 degrees C. The beta phase observed with dry monoelaidin is identical to the LcO phase formed by monoelaidin that was dispersed in excess water and that had not been previously heated.

Kinetics of lipid phase changes

In the past two years, the kinetics of transitions involving the assorted lamellar and inverted hexagonal and cubic phases in bulk hydrated lipid systems have been established using a variety of physical techniques. In several cases, the kinetic data have lead to a transition mechanisms being deciphered.

Characterization of the dynamic properties of Rhodobacter capsulatus ferricytochrome c'--a 28 kDa paramagnetic heme protein

The cytochrome c' are paramagnetic heme proteins generally consisting of two identical 14 kDa subunits. The recent assignment of the 1H and 15N resonances of the Rhodobacter capsulatus ferricytochrome c' has allowed characterization of the dynamic properties by measurement of the heteronuclear NOE for each resolved amide group. The relative importance of fast local motion and paramagnetic effect on nuclear relaxation were distinguished by comparison of the measured heteronuclear NOE with that of the overall experimental average. We show that the average experimental value of -0.16 corresponds to the rigid body motion expected for a spherical complex of 28 kDa. Residues 3-5, 50-55 and 69-70 exhibit decreased heteronuclear NOE due to local motions on a fast time scale with respect to molecular tumbling. Based on the X-ray crystal structure of the homologous cytochrome c' from Chromatium vinosum, the mobile regions correspond to the N-terminus of helix-1 and 2 regions of nonregular secondary structure located between helices-2 and -3.

Membrane-structure studies using X-ray standing waves

The principle and unique features of X-ray standing waves as a means for investigating membrane structure are described in this review. Thus far, X-ray standing waves have been used in structural studies of Langmuir-Blodgett and self-assembled monomolecular lipid films. Most recently, the method has been used in studies of supported membranes hosting the peripheral membrane protein, cytochrome c. Structural rearrangements occurring in membranes and at surfaces driven by temperature and composition changes have been monitored as well. Finally, ion distribution in the diffuse-double layer next to a charged membrane has been determined using this approach. The review addresses the manner in which these and related measurements were made. What is not covered in the review is a critical appraisal of the limitations of the X-ray standing wave method. Such limitations (which are pronounced and introduce significant mensuration ambiguities at short distances on silver mirrors as used in the membrane protein topology study) have just recently come to our attention and will be reported on separately (S Kirchner, Z Yin, J Wang & M Caffrey, in preparation).

1H and 13C NMR assignment and secondary structure of Chlorobium limicola f. thiosulfatophilum ferrocytochrome c555

The 1H resonances of the ferrocytochrome c555 from the anaerobic green sulfur bacterium Chlorobium limicola f thio-sulfatophilum (strain Tassajara) have been assigned. Identification of spin systems and sequential assignment of 1H was accomplished by automated assignment computer programs followed by manual verification. In addition, 13C resonances have been extensively assigned by HSQC experiments at natural abundance. As determined by short-range NOE connectivities, 13C alpha chemical shifts, and HN exchange experiments, the secondary structure consists of 3 helices ranging from residues 3-13, 43-53 and 70-86. Interestingly, the second helix is significantly longer than observed by X-ray crystallography [1977, Proc. Natl. Acad. Sci. USA 74, 5244-5247]. A topological model of the cytochrome c555 is presented based on a small number of long-range NOE contacts. The helices are shown to pack onto the heme according to the pattern common to all class I cytochromes c.

NMR assignment of Rhodobacter capsulatus ferricytochrome c', a 28 kDa paramagnetic heme protein

The cytochromes c' are paramagnetic heme proteins generally consisting of two identical 14 kDa subunits. The 1H and 15N resonances of the ferricytochrome c' from the purple phototrophic bacterium Rhodobacter capsulatus have been extensively assigned by the TOCSY-HSQC, NOESY-HSQC, HSQC-NOESY-HSQC, and HNHA 3D heteronuclear experiments performed on an 8 mM sample labeled with 15N. In addition, the 13C alpha and 13CO resonances were assigned by the HNCA and multiple-quantum HNCOCA 3D experiments performed on a 0.5 mM sample labeled with 13C and 15N. The assignment of the backbone 13C resonances was used to confirm the 1H and 15N assignments and to better define secondary structure. On the basis of medium-range NOEs, 3JHN alpha coupling constants, and backbone 13C chemical shifts, the secondary structure consists of four helices: helix-1 (3-29), helix-2 (33-49), helix-3 (78-97), and helix-4 (103-117). On the basis of long-range NOE contacts, the Rb. capsulatus ferricytochrome c' is a four-helix bundle protein in which consecutive helices are antiparallel with respect to one another.

Phases and phase transitions of the sphingolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Herein, we present a review of the LIPIDAT data subset referring to sphingolipids together with an analysis of these data. It includes data collected over a 40-year period and consists of 867 records obtained from 112 articles in 25 different journals. An analysis of these data has allowed us to identify trends in hydrated sphingolipid phase behavior reflecting differences in fatty acyl chain length, saturation and hydroxylation, head group type, and sphingoid base identity. Information on the mesomorphism of biologically-derived and dry sphingolipids is also presented. This review includes 161 references.

High-resolution 3D HNCOCA experiment applied to a 28 kDa paramagnetic protein

A new triple-resonance 3D HNCOCA pulse scheme is presented, designed to identify the backbone nuclei (H(N), N, CO, C(α)) of doubly labelled proteins. The two carbon frequencies are labelled along the same indirect dimension and the corresponding dwell times can be independently scaled in order to account for the relaxation properties and chemical shift ranges of the CO and C(α). If one takes advantage of the symmetry properties of the spectra in the course of the peak picking, this 3D scheme has the same sensitivity as the 4D experiment, but with an improved resolution. The sequence is illustrated on a 0.5 mM sample of Rhodobacter capsulatus cytochrome c' a homodimeric paramagnetic protein of 2×14 kDa. A resonance assignment strategy, based on a low-concentration (13)C/(15)N-labelled sample and a more concentrated (15)N-labelled sample, is proposed for proteins where the expression system shows a limited efficiency.

The temperature-composition phase diagram and mesophase structure characterization of monopentadecenoin in water

The temperature-composition phase diagram of monopentadecenoin, a monoacylglycerol with a cis monounsaturated fatty acid 15 carbon atoms long (C15:1c10) in water was constructed using x-ray diffraction. Low- and wide-angle diffraction patterns were collected from samples of fixed hydration as a function of temperature in the heating direction on x-ray-sensitive film. The temperature and hydration ranges investigated were 0-104 degrees C and 0-60% (w/w) water, respectively. The phases identified in the system include the lamellar crystalline phase, the lamellar liquid crystalline phase, the fluid isotropic phase, and two inverted cubic phases belonging to space groups la3d (Q230) and Pn3m (Q244). Particular attention has been devoted to the issues of phase equilibrium, phase boundary verification, and structure characterization. The phase diagrams of monopentadecenoin, monomyristolein (C14:1c9), and monoolein (C18:1c9) are compared, and the impact of molecular structure on mesophase stability and structure is discussed.

Kinetics and mechanism of the barotropic lamellar gel/lamellar liquid crystal phase transition in fully hydrated dihexadecylphosphatidylethanolamine: a time-resolved x-ray diffraction study using pressure jump

The kinetics and mechanism of the barotropic lamellar gel (L beta')/lamellar liquid crystal (L alpha) phase transition in fully hydrated 1,2-dihexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) has been studied using time-resolved x-ray diffraction (TRXRD). The phase transition was induced by pressure jumps of varying amplitudes in both the pressurization and depressurization directions at controlled temperature (78 degrees C). Both low- and wide-angle diffracted x rays were recorded simultaneously in live time using an x-ray-sensitive image intensifier coupled to a CCD camera and Super-VHS videotape recorder. Such an arrangement allowed for the direct and quantitative characterization of the long- (lamellar repeat spacing) and short-range order (chain packing) during a kinetic experiment. The image-processed live-time x-ray diffraction data were fitted using a nonlinear least-squares model, and the parameters of the fits were monitored continuously throughout the transition. The pressure-induced transitions from the L alpha to the L beta' phase and from the L beta' to the L alpha phase was two-state (no formation of intermediates apparent during the transition) to within the sensitivity limits of the method. The corresponding transit time (the time during which both phases coexist) associated with the long- and short-range order of the pressurization-induced L alpha-to-L beta' phase transition decreased to a limiting value of approximately 50 ms with increasing pressure jump amplitude. This limiting value was close to the response time of the detector/recording system. Thus, the intrinsic transit time of this transition in fully hydrated DHPE at 78 degrees C was less than or equal to 50 ms. In contrast, the depressurization-induced L beta'-to-L alpha phase transition was slower, taking approximately 1 s to complete, and occurred with no obvious dependence of the transit time on pressure jump amplitude. In the depressurization jump experiment, the lipid responded rapidly to the pressure jump in the L beta' phase up to the rate-determining L beta'-to-L alpha transition. Such behavior was examined carefully, as it could complicate the interpretation of phase transition kinetic measurements.

Assignment of the 13C and 13CO resonances for Rhodobacter capsulatus ferrocytochrome c2 using double-resonance and triple-resonance NMR spectroscopy

Rhodobacter capsulatus cytochrome c2 uniformly labelled with 13C/15N has been prepared. The 13C resonances of the reduced state, including those of the carbonyl and heme 13C, have been assigned using a combination of various two- and three-dimensional correlated NMR experiments. Assignment of the sidechain 13C resonances facilitated correction of a small number of previously misassigned sidechain 1H and led to the additional assignment of 32 1H. It was found that 13C alpha and 13CO secondary shifts were better indicators of secondary structure than 1H alpha and 13C beta secondary shifts. Moreover, it was demonstrated that, despite the significant ring current effects present in heme proteins, 13C alpha and 13CO secondary shifts can be employed to accurately identify secondary structure in heme proteins, independently of NOE experiments.

Structure characterization of membrane bound and surface adsorbed protein

X-ray standing waves (XSW) have been used to study the topology of the protein cytochrome c, bound to a negatively charged model membrane and adsorbed at a metal surface. At the metal surface, cytochrome c forms an hexagonally close-packed monolayer. A similar packing arrangement is observed at the surface of a self-assembled lipid film on silver. The data suggest that cytochrome c maintains its native globular structure upon surface binding and subsequent storage for an extended period. Further, the data are consistent with a protein docking mechanism wherein the heme plane is oriented perpendicular to and with its exposed edge facing the surface. This study demonstrates the utility of XSW as a new and powerful structural tool for investigating membrane- and surface-protein interactions.

The curvature elastic-energy function of the lipid-water cubic mesophase

Cell and lipid membranes are able to bend, as manifested during membrane fusion and the formation of non-lamellar lyotropic mesophases in water. But there is an energy cost to bending of lipid layers, called the curvature elastic energy. Although the functional form of this energy is known, a complete quantitative knowledge of the curvature elastic energy, which is central to predicting the relative stability of the large number of phases that lipid membranes can adopt, has been lacking. Here we use X-ray synchrotron diffraction measurements of the variation of lattice parameter with pressure and temperature for the periodic Ia3d (Q230) cubic phase of hydrated monoolein to calculate the complete curvature elastic-energy function for the lipid cubic mesophase. This allows us to predict the stabilities of different cubic and lamellar phases for this system as a function of composition.

Phases and phase transitions of the glycoglycerolipids

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. The database is considered comprehensive for glycerophospholipids, glycoglycerolipids, sphingolipids and natural membrane extracts. Here, a review of the LIPIDAT data subset referring to glycoglycerolipids is presented together with an analysis of these data. The glycoglycerolipids subset represents 4% of all LIPIDAT records. It includes data collected over a 20-year period and consists of 419 records obtained from 37 articles in 13 journals. An analysis of the data in the subset has allowed us to identify trends in hydrated glycoglycerolipids phase behavior reflecting differences in hydrocarbon chain length, chain branching, chain-glycerol linkage type (ether vs. ester), sugar headgroup-glycerol linkage type (alpha vs. beta) and sugar headgroup identity. Included is a summary of the data concerning the effect of pH and of stereochemical purity on glycoglycerolipid phase behavior. Information on the mesomorphism of biologically derived and dry glycoglycerolipids is also presented. This review includes 92 references.

The temperature-composition phase diagram of monomyristolein in water: equilibrium and metastability aspects

The temperature-composition phase diagram of monomyristolein in water was constructed using x-ray diffraction. Low- and wide-angle diffraction patterns were collected from samples of fixed hydration as a function of temperature in the heating direction on x-ray-sensitive film and/or image plates. The phases identified in the system include the lamellar crystalline phase, the lamellar liquid crystalline phase, the fluid isotropic phase, and two inverted cubic phases. Particular attention has been devoted to the issues of phase equilibrium and phase boundary verification. Cubic phase undercooling was examined by adjusting the temperature of several samples in the cubic phase to a value where the lamellar liquid crystalline phase represents equilibrium behavior. Cooling-induced structure and phase changes were monitored continuously over a 30-min period by recording low-angle diffraction patterns from the samples using a streak camera. The cubic-to-lamellar transition rate decreased with increasing sample hydration. Additionally, the transition proceeded more rapidly at an incubation temperature of 25 degrees C compared to that at 0 degrees C. A mechanism is proposed that accounts for the hydration and temperature sensitivity of the phase transition under nonequilibrium conditions.

The neutral area surface of the cubic mesophase: location and properties

A neutral area surface can be defined as one whose area remains fixed upon bending. It is assumed that such a surface exists within the amphiphilic monolayers that constitute the bicontinuous cubic mesophases and that it parallels approximately the highly convoluted polar/apolar interface in such systems. Here, we report on how the neutral area surface in the cubic phase (space group Ia3d) of the hydrated monoacylglycerol, monoolein, was determined. It is located at a distance of approximately 8.8 A from the methyl terminus of the acyl chain. At 25 degrees C, the surface area per lipid molecule at the neutral area surface is 35.1 +/- 0.2 A2, which is remarkably similar to the mean cross-sectional area of hydrated monoolein in the lamellar liquid crystalline phase at this same temperature.

Phases and phase transitions of the hydrated phosphatidylethanolamines

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Here, a review of the LIPIDAT data subset referring to hydrated phosphatidylethanolamines (PE) is presented together with an analysis of these data. The PE subset represents 14% of all LIPIDAT records. It includes data collected over a 38-year period and consists of 1511 records obtained from 203 articles in 35 different journals. An analysis of the data in the subset has allowed us to identify trends in synthetic PE phase behavior reflecting changes in lipid chain length, chain unsaturation (number, isomeric type and position of double bonds), chain asymmetry and branching, type of chain-glycerol linkage (ether vs. ester) and headgroup modification. Also included is a summary of the data concerning the effect of pH, stereochemical purity, and different additives such as salts, saccharides, alcohols, amino adds and alkanes on PE phase behavior. Information on the phase behavior of biologically derived PE is also presented. This review includes 236 references.

Observation of inter-subunit nuclear Overhauser effects in a dimeric protein. Application to the Arc repressor

For the structure determination of symmetric protein dimers it is necessary to distinguish between intra- and inter-subunit NOEs. A method is presented to measure selectively the inter-subunit NOEs using uniform 15N and 13C isotope labelling. This is accomplished by double filtered 2D NOE experiments on mixtures of native protein with isotope-labeled protein. The method has been applied to the Arc repressor and allows the characterization of virtually all proton-proton NOEs in terms of their intra- or inter-subunit nature.

Thermodynamic, thermomechanical, and structural properties of a hydrated asymmetric phosphatidylcholine

1-Behenyl-2-lauryl-sn-glycero-3-phosphocholine (22/12 PC) belongs to a unique group of phospholipids in which the molecule has one acyl chain almost twice as long as the other. The temperature-composition phase diagram for this lipid in the range of 25-65 degrees C, and 0 to 84.3% (w/w) water has been constructed by using the isoplethal method in the heating direction and x-ray diffraction for phase identification and structure characterization. At water contents between 10.3 and 34% (w/w) and at temperatures below 43 degrees C, a single mixed interdigitated lamellar gel phase (Lm beta, [symbol: see text]) of the type described by Hui et al. (1984. Biochemistry. 23:5570-5577) and McIntosh et al. (1984. Biochemistry. 23:4038-4044) was found. A second phase consisting of bulk aqueous solution coexists with the Lm beta phase at hydration levels above 34% (w/w) water in the temperature range between 25 and 43 degrees C. Above 43 degrees C, a partially interdigitated lamellar liquid crystalline (Lp alpha) phase ([symbol: see text]) is seen in the water concentration range extending from 0 to 84.3% (w/w). The pure Lp alpha phase is found below 43% (w/w) water, while coexistence of the Lp alpha phase and the bulk aqueous solution is observed above this water concentration which marks the hydration boundary. Interestingly, the latter boundary for both Lm beta and Lp alpha phases is nearly vertical in the temperature range studied. Furthermore, the lamellar chain-melting transition temperature appears to be relatively insensitive to hydration in the range 0-85% (w/w) water. We have confirmed the identify of the Lm beta phase by constructing a 5.7-A resolution electron density profile on oriented samples by the swelling method. Temperature-induced chain melting effects an increase in lipid bilayer thickness suggesting that the Lp alpha phase has chains packed in the partially as opposed to the mixed interdigitated configuration. Unlike the symmetric phosphatidylcholines a ripple (P beta') phase was not found as an intermediate between the low and high temperature lamellar phases of 22/12 PC. The specific volume of 22/12 PC is 940 (+/- 1) microliter/g and 946 (+/- 1) microliter/g in the hydrated lamellar gel state at 28 (+/- 2) and 40 (+/- 2) degrees C, respectively, from neutral buoyancy experiments. Based on measurements of the temperature dependence of the various lattice parameters of the different phases encountered in this study the corresponding lattice thermal expansion coefficients have been measured. These are discussed and their dependence on lipid hydration is reported.

X-rays destroy the lamellar structure of model membranes

Lipid membranes undergo a dramatic lamellar-to-non-lamellar phase transformation upon being irradiated with X-rays. The structure change was evidenced by chemical breakdown of the lipid and by a change in the X-ray diffraction properties of the model membrane indicating complete disruption of lamellar stacking. Considering the importance of X-ray diffraction as a structure probe the problem of radiation damage to biological materials must be duly recognized. Because X-ray damage has been shown to be free radical-mediated these results suggest a means by which free radicals accumulating in cells during aging might compromise membrane integrity and contribute to cell death.