Single-molecule X-ray diffraction

Free-electron lasers could provide femtosecond X-ray flashes with a peak brilliance 10-11 orders of magnitude higher than that which is currently available from synchrotrons. Such pulses may allow structural studies of single biomolecules before radiation damage destroys them and may permit the imaging of complex structures without the need to amplify scattered radiation through Bragg reflections.     

Electron diffraction of membranes

Summary Electron diffraction conducted on myelin membranes, photosynthetic and photoreceptor membranes yielded spot diffraction patterns indicating an ordered state of membranes; the interplanar spacings being of the order of Å units. It was observed, too, that a membrane specimen accommodates different space structures. Based on these findings it is suggested that membrane functions-like active transport-are exercised through phase transitions; the lattice acts as a coupling agent.     

Neutron diffraction of interphase nuclei

The neutron small-angle diffraction patterns obtained with native interphase nuclei are quite featureless and difficult to interpret. However, the difference between two patterns which have been obtained before and after having induced a change in the packing of the more stable structural entities, e.g. by a reduction of the salt concentration, shows significant changes at different scattering angles. These changes allow us to estimate the overall size of buffer-filled spaces which were formed by the separation of more stable structural entities and to establish accurate and non-destructive conditions at which changes of structure occur.Rat liver interphase nuclei have been prepared in a polyamine-containing buffer. The buffer was diluted from one-half down to one-fifth and one-tenth of its original salt concentration. In the first dilution step ($\text{1}\text{2}$ to $\text{1}\text{5}$), the difference spectrum was indicative of the formation of buffer-filled spaces with a most probable distance between a pair of scattering buffer molecules of 18 nm and with an overall size of the order of 45 to 50 nm. The second dilution step ($\text{1}\text{5}$ to $\text{1}\text{10}$) resulted in a strong increase of the intensity of the difference spectrum which had been observed in the first dilution step.     

Developments in fiber diffraction.

Improved specimen preparation methods, third generation synchrotron sources, new data processing algorithms and molecular dynamics refinement techniques are, together, allowing the high-resolution structure determination of larger and larger macromolecular complexes by fiber diffraction. New synchrotron sources are also making possible both time-resolved studies and studies of ordered fibers only a few microns in diameter.     

Muscle diffraction theory. Relationship between diffraction subpeaks and discrete sarcomere length distributions.

A theoretical discussion is presented that describes the diffraction on monochromatic light by a three-dimensional sarcomere array having the following properties. The basic repetitive diffracting unit is the sarcomere. The contiguous arrangement of physically attached serial sarcomeres in the myofibril is contained within the model so that relative position of sarcomeres depend upon the lengths of intervening ones. Sarcomere length is described by a distribution function. This function may be discrete or continuous and contain one or more subpopulations. Two arrangements of sarcomeres are considered: (a) when sarcomeres of different lengths are arranged randomly in myofibrils the amplitude and width of mth order (m greater than or equal to 1) peaks and associated secondary diffraction maxima decrease and increase monotonically, respectively, as the standard deviation of the length distribution increases. No subpeaks are present regardless of the number of subpopulations within the distribution function. This behavior is shown to follow from the dependence of sarcomere position on the length of intervening sarcomeres. (b) When sarcomeres belonging to the same length subpopulation are arranged in serial contiguous fashion to form domains and more than one length subpopulation is present, then mth order diffraction peaks split to form subpeaks. The theoretical basis for this behavior is developed for the first time and may explain the subpeaks evident in diffraction patterns from cardiac and skeletal muscle.     

Neutron diffraction studies of digalactosyldiacylglycerol

The structure of the digalactosyldiacylglycerol bilayer is calculated using neutron diffraction data. The polar head group of this lipid is oriented parallel to the plane of the bilayer such that the galactose moieties are tightly packed at the bilayer surface into a 0.8 nm thick polar layer. The thickness of this layer is independent of water activity over a wide range (15-100% relative humidity). The constant thickness of both the galactose layer and the hydrocarbon layer constrain the structure factor amplitudes to lie on a single continuous transform for repeat periods between 4 and 5 nm.     

Coherent convergent-beam time-resolved X-ray diffraction

The use of coherent X-ray lasers for structural biology allows the use of nanometre diameter X-ray beams with large beam divergence. Their application to the structure analysis of protein nanocrystals and single particles raises new challenges and opportunities. We discuss the form of these coherent convergent-beam (CCB) hard X-ray diffraction patterns and their potential use for time-resolved crystallography, normally achieved by Laue (polychromatic) diffraction, for which the monochromatic laser radiation of a free-electron X-ray laser is unsuitable. We discuss the possibility of obtaining single-shot, angle-integrated rocking curves from CCB patterns, and the dependence of the resulting patterns on the focused beam coordinate when the beam diameter is larger or smaller than a nanocrystal, or smaller than one unit cell. We show how structure factor phase information is provided at overlapping interfering orders and how a common phase origin between different shots may be obtained. Their use in refinement of the phase-sensitive intensity between overlapping orders is suggested.