To clarify the extensibility of thin actin and thick myosin filaments in muscle, we examined the spacings of actin and myosin filament-based reflections in x-ray diffraction patterns at high resolution during isometric contraction of frog skeletal muscles and steady lengthening of the active muscles using synchrotron radiation as an intense x-ray source and a storage phosphor plate as a high sensitivity, high resolution area detector. Spacing of the actin meridional reflection at approximately 1/2.7 nm-1, which corresponds to the axial rise per actin subunit in the thin filament, increased about 0.25% during isometric contraction of muscles at full overlap length of thick and thin filaments. The changes in muscles stretched to approximately half overlap of the filaments, when they were scaled linearly up to the full isometric tension, gave an increase of approximately 0.3%. Conversely, the spacing decreased by approximately 0.1% upon activation of muscles at nonoverlap length. Slow stretching of a contracting muscle increased tension and increased this spacing over the isometric contraction value. Scaled up to a 100% tension increase, this corresponds to a approximately 0.26% additional change, consistent with that of the initial isometric contraction. Taken together, the extensibility of the actin filament amounts to 3-4 nm of elongation when a muscle switches from relaxation to maximum isometric contraction. Axial spacings of the layer-line reflections at approximately 1/5.1 nm-1 and approximately 1/5.9 nm-1 corresponding to the pitches of the right- and left-handed genetic helices of the actin filament, showed similar changes to that of the meridional reflection during isometric contraction of muscles at full overlap. The spacing changes of these reflections, which also depend on the mechanical load on the muscle, indicate that elongation is accompanied by slight changes of the actin helical structure possibly because of the axial force exerted by the actomyosin cross-bridges. Additional small spacing changes of the myosin meridional reflections during length changes applied to contracting muscles represented an increase of approximately 0.26% (scaled up to a 100% tension increase) in the myosin periodicity, suggesting that such spacing changes correspond to a tension-related extension of the myosin filaments. Elongation of the myosin filament backbone amounts to approximately 2.1 nm per half sarcomere. The results indicate that a large part (approximately 70%) of the sarcomere compliance of an active muscle is caused by the extensibility of the actin and myosin filaments; 42% of the compliance resides in the actin filaments, and 27% of it is in the myosin filaments.

The erasable phosphor imaging plate developed for medical radiography has found new uses in the laboratory. X-ray diffraction, protein crystallography and autoradiography have all benefited from this technology transfer from the clinic.

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Thermodynamic and polymorphic behavior of POP (sn-1,3-dipalmitoyl-2-oleoylglycerol) and OPO (sn-1,3-dioleoyl-2-palmitoylglycerol) binary mixtures was examined using differential scanning calorimetry and conventional and synchrotron radiation X-ray diffraction. A molecular compound, β C , was formed at the 1:1 (w/w) concentration ratio of POP and OPO, giving rise to two monotectic phases of POP/compound and compound/OPO in juxtaposition. β C has a long-spacing value of 4.2 nm with a double chainlength structure and the melting point of 31.9°C. A structural model of the POP-OPO compound is proposed, involving the separation of palmitoyl and oleoyl chain leaflets in the double chainlength structure. In the polymorphic occurrence of the POP-OPO mixtures, the POP fraction transformed from α to β′ with no passage through γ, then transformed to β. The presence of OPO in POP promoted the β′-β transformation of POP during the melt-mediated crystallization. JAOCS 74, 1213-1220 (1997).KEY WORDS: Binary mixture, differential scanning calorimetry, phase behavior, polymorphism, synchrotron radiation X-ray diffraction, triacylglycerol.Triacylglycerols (TAG) are employed in oils, margarine, cream, chocolate, and other edible fats products, and the fat structures are the major factors to determine their physical properties, such as texture, plasticity, morphology, and so on (1-4). Polymorphism is of the highest importance in the physical properties of TAG. Polymorphism of TAG has been elucidated at a molecular level for pure systems (5-7). In the practical systems, however, the fats are present in multicomponent states consisting of different types of TAG species. The first step in approaching the multicomponent systems is to study the binary systems of major TAG components, which reveal the nature of molecular interactions between the component materials at macroscopic and microscopic dimensions.Various types of phase diagrams of the binary mixtures of TAG were reviewed by Small (8), Knoester et al. (9), Timms (10), and Rossell (11). As a typical mixture system, tristearin-tripalmitin (SSS-PPP) mixtures have been studied by many researchers (11-13), where metastable α and β′ forms exhibited continuous solid solutions, yet eutectic nature was revealed in the most stable β form. The formation of the molecular compound was suggested in the mixture of sn-1,3-dipalmitoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-palmitoylglycerol (POP-OPO) (P, palmitoyl; O, oleoyl) by Moran (14) who indicated two binary systems, in juxtaposition, of POP/compound and compound/OPO. In this mixture, the compound was estimated to form at an equal concentration ratio. Most recently, Engstrom (15), Koyano et al. (16), and Minato et al. (17) reported precise analyses of the mixture systems of sn-1,3-distearoyl-2-oleoylglycerol and 1,2-distearoyl-3-oleoylglycerol (SOS/SSO) (S, stearoyl), sn-1,3-distearoyl-2-oleoylglycerol and sn-1,3-dioleoyl-2-stearoylglycerol (SOS/OSO), and sn-1,3-dipalmitoyl-2-oleoylglycerol and 1,2-dipalmitoyl-3-oleoylglycerol (POP/PPO), r...

The phase behavior of PPO (sn-1,2-dipalmitoyl-sn-3-oleoylglycerol) and POP (sn-1,3-dipalmitoyl-sn-2-oleoylglycerol) binary mixture was examined using differential scanning calorimetry, conventional and synchrotron radiation X-ray diffraction (XRD), and highly pure samples. A molecular compound, βC, was formed at the 1:1 concentration ratio of PPO and POP, giving rise to two monotectic phases of PPO/compound and compound/POP in a juxtapositional way. βC has a long spacing value of 4.1 nm of double chain length structure. In the PPO/compound region, the DSC melting peak increased with increasing PPO concentration, whereas the DSC melting peak increased with increasing POP concentration in the compound/POP region. The melting point of βC was lowest at 31.2 °C. Time-resolved XRD study unveiled the formation of molecular compounds in metastable forms, αC and β‘C, having the same PPO/POP concentration ratio as βC. αC and β‘C exhibited monotectic phases with corresponding metastable forms of the pure components. In αC, two hexagonal packing XRD short spacing peaks were obtained for αC, corresponding to a differently packed hexagonal subcell of two leaflets. A structure model of the PPO/POP molecular compound is proposed, involving separation of palmitoyl chain leaflet and palmitoyl−oleoyl mixed-acid chain leaflet in the double chain length structure. This model justifies the structure of βC of SOS/SSO proposed by Engstrom (Engstrom, L. J. Fat Sci. Technol. 1992 94, 173−181), in which the saturated and unsaturated chains are packed in the same leaflet of the double chain length. The present work additionally proved that the formation of the molecular compound was present in the αC and β‘C metastable forms, as directly proved by dynamic X-ray diffraction study using synchrotron radiation.

Small-angle X-ray scattering experiments for supercritical CO2 have been carried out. In the previous study, it was pointed out that correlation lengths and density fluctuations form a ridge along the extension of the coexistence curve to the supercritical region and that singular points of various quantities are also on this curve. The present study, carried out in better experimental conditions, confirms that the preliminary results are correct and shows that the dependence of the density fluctuation on the correlation length differs across the boundary of the extension curve, which we argue to correspond to the boundary of a higher-order phase transition.

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