How x-ray diffraction with synchrotron radiation got started

The need to record low angle scattering x-ray fibre diagrams from muscle with milli-second time resolution drove the use of synchrotron radiation as an x-ray light source. The first smudgy diffraction patterns were obtained from a slice of insect flight muscle. Out of this grew the EMBL Outstation at DESY.

Muscle - the Problem

Muscle is an isothermal engine which works by hydrolysing ATP (adenosine triphosphate) with an efficiency near 50%. When muscle contracts, two sets of protein filaments, the actin filaments and the myosin filaments glide over each other. The gliding is driven by the cyclical interactions of the myosin "cross-bridges" with actin so as to "row" one set of filaments past the other: a cross-bridge binds to actin in a initial position and "swings" into a final position. This movement is driven by the binding to actin which enables release of the products of ATP hydrolysis (ADP and phosphate). At the end of the stroke ATP rebinds to the myosin cross bridge causing rapid release from actin. Subequently ATP is hydrolyzed to ADP and phosphate and the cycle repeats (Fig 1) (Lymn & Taylor, 1971). One major aim of muscle research has been to demonstrate and understand what actually happens when a cross bridge "swings".

The cross bridges in muscle fibres repeat along the fibre axis with a repeat distance of ca. 14.5nm. Thus they give rise to an x-ray diffraction pattern with a series of strong meridional reflexions. Alterations in shape of the cross bridges lead to changes in the intensities of these reflexions. The sartorius muscle from frog can be dissected out intact and made to contract by electrical stimulation. X-ray diffraction patterns can then be recorded from an actively contracting muscle. Pioneering work with conventional sources was carried out in the 1960's by H.E. Huxley (Fig 2) (Huxley & Brown, 1967). However, the scattering is weak and muscles quickly become fatigued. We in Heidelberg hoped that insect flight muscle might provide an alternative system to frog muscle for studying the cross bridges by x-ray diffraction. Insect flight muscle changes its structure on adding ATP which leads to dramatic changes in the low angle meridional reflexions (Reedy et al ., 1965). Moreover, it is highly crystalline. However, we met all the intensity problems encountered by H. E. Huxley for frog muscle but in a more acute form since the specimens are much smaller. To get further we needed much stronger x-ray sources.

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