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Notes: A water-cooled double-crystal monochromator for x-ray absorption spectroscopy is described. It has a wide aperture (3 mrad) and is attached to a linear slide through a twin axis goniometer to a large Huber rotary table for accurate alignment of the crystal planes to the incident beam. The center of rotation of the goniometers has been chosen as the geometric center at the surface of the first crystal. The linear slide adjusts the monochromator vertically so that the axis of rotation intercepts the synchrotron radiation. The two goniometers provide the monochromator with a transverse and longitudinal tilt with respect to the x-ray beam. Water cooling is incorporated in the first crystal to dissipate the expected heat loading of 120 W from the SRS wiggler. The second crystal is supported by three Inchworms which control the yaw and tilt. Then adjustments are required to optimize spectral purity across the beam alignment and harmonic rejection, respectively, with respect to the first crystal. The performance of the monochromator is discussed.

Notes: [Auszug] EXAFS provides information concerning the number and type of atoms and their bond distances from a metal atom in a metalloprotein5'6. The limitations of the technique include (1) atoms of similar atomic weight (for example oxygen and nitrogen) cannot be distinguished from one another7, (2) outer ...

Notes: Summary Results were obtained from contracting frog muscles by collecting high quality time-resolved, two-dimensional, X-ray diffraction patterns at the British Synchrotron Radiation Source (SERC, Daresbury, Laboratory). The structural transitions associated with isometric tension generation were recorded under conditions in which the three-dimensional order characteristic of the rest state is either present or absent. In both cases, new layer lines appear during tension generation, subsequent to changes from activation events in the filaments. Compared with the ‘decorated’ actin layer lines of the rigor state, the spacings of the new layer lines are similar whereas their intensities differ substantially. We conclude that in contracting muscle an actomyosin complex is formed whose structure is not like that in rigor, although it is possible that the interacting sites are the same. Transition from rest to plateau of tension is accompanied by approximately 1.6% increase in the axial spacing of the myosin layer lines. This is explained as arising from the axial disposition of the interacting myosin heads in the actomyosin complex. Model calculations are presented which support this view. We argue that in a situation where an actomyosin complex is formed during contraction, one cannot describe the diffraction features as being either thick or thin filament based. Accordingly, the layer lines seen during tension generation are referred to as actomyosin layer lines. It is shown that these layer lines can be indexed as submultiples of a minimum axial repeat of approximately 218.7 nm. After lattice disorder effects are taken into account, the intensity increases on the 15th and 21st AM layer lines at spacings of approximately 14.58 and 10.4 nm respectively, show the same time course as tension rise. However, the time course of the intensity increase of the other actomyosin layer lines and of the spacing change (which is the same for both phenomena) shows a substantial lead over tension rise. These findings suggest that the actomyosin complex formed prior to tension rise is a non-tension-generating state and that this is followed by a transition of the complex to a tension-generating state. The intensity increase in the 15th actomyosin layer line, which parallels tension rise, can be accounted for assuming that in the tension-generating state the attached heads adopt (axially) a more perpendicular orientation with respect to the muscle axis than is seen at rest or in the non-tension-generating state. This suggests the existence of at least two structurally distinct interacting myosin head conformations. The results of comparing the meridional intensities between the myosin layer lines at rest and the actomyosin layer lines at the plateau of tension (measured to a resolution of approximately 2.6 nm) are interpreted to indicate that the majority of the myosin heads in the actomyosin complex do not perform random axial rotations with a mean value greater than approximately 3.0 nm. From this we conclude that the extent of axial order in the interacting heads must be at least as high as is that of resting heads.