A double focusing X-ray camera for use with synchrotron radiation

New ordered metastable phases between the gel and subgel phases in hydrated phospholipids

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 microl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC(15)PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L(beta) phase transforms directly into subgel L(c) phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L(R1) and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L(R1) phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L(R1) and SGII phases are metastable phases, which may only be formed by cooling the respective gel L(beta) and L(beta') phases, and not by heating the subgel L(c) phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase.

An ordered metastable phase in hydrated phosphatidylethanolamine: the Y-transition

By using time-resolved X-ray diffraction, differential scanning calorimetry and scanning densitometry, we observed rapid formation at low temperature of a metastable ordered phase, termed LR1 phase, in fully hydrated dihexadecylphosphatidylethanolamine (DHPE). The LR1 phase has the same lamellar repeat period as the gel Lbeta phase but differs from the latter in its more ordered, orthorhombic hydrocarbon chain arrangement. It forms at about 12 degrees C upon cooling and manifests itself as splitting of the sharp, symmetric wide-angle X-ray peak of the DHPE gel phase into two reflections. This transition, designated the 'Y-transition', is readily reversible and proceeds with almost no hysteresis between cooling and heating scans. Calorimetrically, the LR1-->Lbeta transition is recorded as a low-enthalpy (0.2 kcal/mol) endothermic event. The formation of the LR1 phase from the gel phase is associated with a small, about 2 microl/g, decrease of the lipid partial specific volume recorded by scanning densitometry, in agreement with a volume calculation based on the X-ray data. The formation of the equilibrium Lc phase was found to take place from within the LR1 phase. This appears to be the only observable pathway for crystallisation of DHPE upon low-temperature incubation. Once formed, the Lc phase of this lipid converts directly into Lbeta phase at 50 degrees C, skipping the LR1 phase. Thus, the LR1 phase of DHPE can only be entered by cooling of the gel Lbeta phase. The data disclose certain similarities between the low-temperature polymorphism of DHPE and that of long-chain normal alkanes.

Sample environments and techniques combined with small angle X-ray scattering

The number of synchrotron radiation-based Small Angle X-ray Scattering beamlines has increased considerably over the last decade. With the high X-ray flux and collimation of these beamlines it not only has become possible to perform time-resolved experiments on time scales down to the millisecond/frame range, but also it allows experimenters to utilise new sample environments and use simultaneous several experimental techniques on one sample. An overview of recent developments in this field is given.

Low amounts of PEG-lipid induce cubic phase in phosphatidylethanolamine dispersions

By using time-resolved X-ray diffraction we demonstrate that low amounts (5-10 mol%) of a phospholipid with two saturated hydrocarbon acyl chains 14 carbon atoms long and PEG550 chain covalently attached to its phosphoethanolamine polar head group, DMPE(PEG550), induce spontaneous formation of a cubic phase with lattice constant 20.5 nm (cubic aspect #8, space group Im3m) in aqueous dispersions of dielaidoylphosphatidylethanolamine (DEPE). This phase displays a highly resolved X-ray diffraction pattern with 17 low-angle reflections. The cubic phase was found to intrude in the temperature range between the lamellar liquid crystalline (L(alpha)) phase and the inverted hexagonal phase (H(II)) known to form in pure DEPE/water dispersions. A higher DMPE(PEG550) amount of 20 mol% was found to eliminate the non-lamellar phases in the temperature scale up to 100 degrees C. DMPE grafted with PEG5000 only shifts the L(alpha)-H(II) transition of DEPE to higher temperatures but does not promote formation of cubic phase. These findings indicate that, consistent with their bulky head groups, the PEG-lipids decrease the tendency for negative interfacial mean curvature of the DEPE bilayers.

X-ray diffraction studies on thermally induced tension generation in rigor muscle

Muscle fibres in the rigor state and free of nucleotide contract if heated above their physiological working temperature. Kinetic studies on the mechanism of this process, termed rigor contraction, indicate that it has a number of features in common with the contraction of maximally Ca2+ activated fibres. De novo tension generation appears to be associated with a single, tension sensitive, endothermic step in both systems. Rigor contraction differs in that steps associated with crossbridge attachment and detachment are absent. We investigated structural changes associated with rigor contraction using X-ray diffraction. Overall changes in the low angle X-ray diffraction pattern were surveyed using a two-dimensional image plate. Reversible changes in the diffraction pattern included a 28% decrease in intensity of the 14.5 nm meridional reflection, a 12% increase in intensity of 5.9 nm actin layer-line and a somewhat variable 34% increase in intensity of 5.1 nm actin layer-line in laser temperature-jump experiments. When fibres were heated with a temperature ramp, we found that a 70% decrease in intensity of the myosin-related meridional reflection at (14.5 nm)-1 correlated with tension generation. A similar decrease in intensity of the 14.5 nm reflection is seen during tension recovery following a step change in the length of maximally Ca2+ activated fibres. Signals both from actin and actin-bound myosin heads contribute to the 5.1 and 5.9 nm actin layer-lines. Our observed changes in intensity are interpreted as contraction-associated changes in crossbridge shape and/or position on actin.

New phases induced by sucrose in saturated phosphatidylethanolamines: an expanded lamellar gel phase and a cubic phase

A new lamellar gel phase (L beta *) with expanded lamellar period was found at low temperatures in dihexadecylphosphatidylethanolamine (DHPE) and dipalmitoylphosphatidylethanolamine (DPPE) dispersions in concentrated sucrose solutions (1-2.4 M). It forms via a cooperative, relatively broad transition upon cooling of the L beta gel phase of these lipids. According to the X-ray data, the transformation between L beta and L beta * is reversible, with a temperature hysteresis of 6-10 degrees C and a transition width of about 10 degrees C. No specific volume changes and a very small heat absorption of about 0.05 kcal/mol accompany this transition. The L beta *-L beta transition temperature strongly depends on the disaccharide concentration. From a value of about 10 degrees C below the melting transition of DHPE, it drops by 25 degrees C with decrease of sucrose concentration from 2.4 M to 1 M. The low-temperature gel phase L beta * has a repeat spacing by 8-10 A larger than that of the L beta gel phase and a single symmetric 4.2 A wide-angle peak. It has been observed in 1, 1.25, 1.5 and 2.4 M solutions of sucrose, but not in 0.5 M of sucrose. The data clearly indicate that the expanded lamellar period of the L beta * phase results from a cooperative, reversible with the temperature, increase of the interlamellar space of the L beta gel phase. Other sugars (trehalose, maltose, fructose, glucose) induce similar expanded low-temperature gel phases in DHPE and DPPE. The L beta * phase is osmotically insensitive. Its lamellar period does not depend on the sucrose concentration, while the lattice spacings of the L alpha, L beta and HII phases decrease linearly with increase of sucrose concentration. Another notable sugar effect is the induction of a cubic phase in these lipids. It forms during the reverse HII-L alpha transition and coexists with the L alpha phase in the whole temperature range between the HII and L beta phases. The cubic phase has only been observed at sucrose concentrations of I M and above. In accordance with previous data, sucrose suppresses the L alpha phase in both lipids and brings about a direct L beta-HII phase transition in DHPE. A raid, reversible gel-subgel transformation takes place at 17 degrees C in both DPPE and DHPE. Its properties do not depend on the sucrose concentration. The observed new effects of disaccharides on the properties of lipid dispersions might be relevant to their action as natural protectants.

X-ray diffraction studies of the structural state of crossbridges in skinned frog sartorius muscle at low ionic strength

Low-angle X-ray diffraction diagrams were obtained from chemically skinned frog sartorius muscles under low ionic strength relaxing conditions. Experiments on single muscle fibres from rabbit muscle and on muscle proteins in solution have suggested the presence of a 'low ionic strength attached state' of the myosin crossbridges to actin, in which the overall ATP splitting and force-generating cycle is still blocked. This opened up the possibility that structural information about one of the intermediate states in the crossbridge cycle might be obtained under these conditions. Using synchrotron radiation as a high intensity X-ray source we were able to record the appropriate diffraction diagrams with short exposure times and were able to compare the same muscles at normal and at low ionic strength. Changes in the intensities of the equatorial reflections an increase in the 143 A meridional intensity can be interpreted in a similar way. However, these attached bridges do not give rise to changes in the actin-based layer line reflections, nor is their presence associated with a weakening of the myosin layer line pattern. These results provide further evidence for the existence of bound states of crossbridges, in which their orientation relative to actin is not sharply defined.

The superstructure of chromatin and its condensation mechanism. I. Synchrotron radiation X-ray scattering results

Synchroton radiation X-ray scattering experiments have been performed on chicken erythrocyte chromatin fibres over a wide range of ionic conditions and on various states of the fibres (i.e. "native" in solution, in gels and in whole nuclei; chromatin depleted of the H1 (H5) histones and chromatin with bound ethidium bromide). A correlation between the results obtained with the various chromatin preparations provides evidence for a model according to which at low ionic strength the chromatin fibre already possesses a helical superstructure, with a diameter comparable to that of condensed chromatin, held together by the H1(H5) histone. The most significant structural modification undergone upon an increase of the ionic strength is a reduction of the helix pitch, this leads to condensation in a manner similar to the folding of an accordion. The details of this process depend on whether monovalent or divalent cations are used to raise the ionic strength, the latter producing a much higher degree of condensation. Measurements of the relative increase of the mass per unit length indicate that the most condensed state is a helical structure with a pitch around 3.0-4.0 nm. In this paper we give a detailed presentation of the experimental evidence obtained from static and time-resolved scattering experiments, which led to this model.

A low-angle X-ray diffraction analysis of osteonic inorganic phase using synchrotron radiation

Using synchrotron radiation the low-angle X-ray diffraction method has been applied to single osteon samples to yield new data on the texture of the inorganic bone fraction. Two sample types--cylindrically shaped osteonic samples and osteonic radial hemisections--were prepared from longitudinal and alternate osteons at both the initial and final stages of calcification. The results indicate that the diffraction pattern is due to the inorganic phase, which reveals the same axial periodicity as native collagen fibrils and fits into the main band. No change is appreciable as osteons pass from the initial to the final stage of calcification. This means that when crystallites covering much more than a collagen axial period are observed under the electron microscope, they do not appreciably affect the calcified banding of collagen fibrils. The osteonic axis corresponds to the main direction of collagen orientation both in longitudinal and alternate osteons. The degree of orientation, however, is lower in alternate osteons than in longitudinal ones, where only few thin, incomplete transversal lamellae are found.

Aggregation of bovine serum albumin upon cleavage of its disulfide bonds, studied by the time-resolved small-angle X-ray scattering technique with synchrotron radiation

A rapid mixing system of the stopped-flow type, used with small-angle X-ray scattering equipment using synchrotron radiation, is described. The process of aggregation of bovine serum albumin was traced with a time interval of 50 s, initiated upon cleavage of its disulfide bonds by reduction with dithiothreitol. The results indicate that a 218-fold molar excess of dithiothreitol over the number of moles of disulfide bonds in bovine serum albumin is sufficient to initiate the reaction immediately after mixing, which reaches equilibrium in about 15 min. On the other hand, half this amount is not sufficient to initiate the reaction, so that the reaction is delayed by about 150 s. Such a single-shot time-resolved experiment showed that experiments with a time interval of 100 ms are possible with repeated multi-shot runs.

Stress-induced molecular rearrangement in tendon collagen

Tension-induced molecular rearrangements in wet native fibres of rat-tail tendons and human finger flexor tendons are registered with the help of time-resolved diffraction spectra using synchrotron radiation. The tension-induced increase of the 67 nm D period is combined with changes in the intensities of some orders of the meridional small angle reflection. Both effects are reversible when unloading the fibre, but are preserved when the load is held constant until the fibre tears. The increase in the D period is partly due to a sliding of the triple helices relative to each other and partly due to a stretching of the triple helices themselves. The sliding of the triple helices results in an alteration of the D stagger, leading to a change in the length of the gap and overlap regions, and to a stretching of the cross-linked telopeptides. This interpretation is supported by comparison with the relative intensities derived from a model with varying length of gap and overlap regions, as well as by comparison with model calculations that include the telopeptides.

Changes in the X-ray reflections from contracting muscle during rapid mechanical transients and their structural implications

During normal contractions of vertebrate striated muscle, it is believed that the cross-bridges which produce the sliding force undergo asynchronous cyclical changes in their structure. Thus, an X-ray diffraction diagram from a muscle under these conditions will give structural information averaged over the whole range of cross-bridge states. Such diagrams show characteristic and informative differences from those given by relaxed muscle, but can give little information about changes in the configuration of the cross-bridges at different stages of their working stroke. However, it is possible to effect a partial synchronization of these changes by applying very rapid changes in length, completed in less than one millisecond to an otherwise isometrically contracting muscle. If the amplitude of these length changes is comparable to the length of the cross-bridge stroke (say 100 A per half-sarcomere), then it should bring about a transient but significant redistribution of cross-bridge states, which would show up in the X-ray diagram. We have made use of synchrotron radiation as a high intensity X-ray source in order to record such patterns with the necessary time resolution (1 ms or less) and have found major changes in the intensity of the 143 A meridional reflection accompanying the rapid length changes of the muscle. These changes appear to arise from specific configurational changes in the cross-bridges during the working stroke. A model is suggested in which the 143 A meridional intensity in a contracting muscle arises mainly from attached cross-bridges and is generated by the part of the myosin head near the S1-S2 junction. During normal contraction, cross-bridges go through their structural cycle asynchronously with each other, since they start at different times, but if the S2 changes in length rather little, then the configurational changes in the myosin heads are synchronized with the actin filament movement in such a way that the S1-S2 junction remains relatively fixed in its axial position. In a quick release, it is suggested that bringing many S1 heads simultaneously to the end of their working strokes on actin disrupts the 143 A axial repeat of their distal ends near S2, and brings about the large decrease of the 143 A meridional reflection. This model therefore involves a large change in the position of part of the myosin head structure relative to actin during the working stroke of the cross-bridge.

Stopped-flow solution scattering using synchrotron radiation: apparatus, data collection and data analysis

We have constructed an experimental system, under remote control, for stopped-flow X-ray scattering using synchrotron radiation. It has been used, in conjunction with an annular detector and its associated electronics, to obtain good scattering curves, with time-slices as short as 200 ms, in a new study of the dissociation of the enzyme complex aspartate transcarbamylase. The data have been analysed by new statistical methods, and they agree well with the results from parallel chemical quench experiments. For studying dissociation reactions, stopped-flow X-ray scattering is a quite practical method, which need not use very much more material than conventional stopped-flow experiments.

Transformation of the structure of collagen. A time-resolved analysis of mechanochemical processes using synchrotron radiation

Mechanochemically induced molecular transformations of collagen fibres were analysed using time-resolved small-angle diffraction spectra and histomechanical measurements. In particular, the influence of aqueous and methanolic perchlorate solutions was examined. According to a transformation continuing from the periphery towards the centre, the macroscopic contraction that is completed less than five minutes after incubation with perchlorate is caused by peripherally transformed fibrils only, whereas the centrally situated fibrils first undergo an accordion-like folding, but after more than 20 minutes are transformed similarly. The triple-helical transformation is preceded by a structure-breaking effect on structural water that can be monitored in time-resolved diffraction spectra. The combined loss of meridional low-angle reflections and cross-striated fibrils in micrographs is irreversible. By dialysis of colloidally dissolved collagen against a solution of ATP, however, segment-long spacing aggregates are obtained. Under isometric conditions, an instantaneous transformation of intermittent regions leads to an increase in the long period of adjacent, still structured regions of the same fibril that is correlated with a delayed increase in tension in the fibre. Increase of tension under isometric conditions as well as the flow-properties of a fibre relaxed in perchlorate are interpreted in terms of the parallel sliding of subunits of varying lengths, which has been demonstrated by diffraction analysis.

Structure of tubulin rings studied by X-ray scattering using synchrotron radiation

X-ray patterns have been obtained from solutions of microtubules and tubulin rings using synchrotron radiation. They are interpreted on the basis of the oriented fiber X-ray diagram of microtubules, and by comparing them with the calculated scattering traces of model structures. The data are consistent with models in which rings consist of coiled protofilaments. This is in agreement with some published ring models while it excludes others.