Table of contents

The high brilliance of third-generation synchrotron radiation sources allows new applications in x-ray microdiffraction and microsmall-angle scattering. Beam sizes down to about one µm are routinely used and sub-µm beam sizes are becoming available. Scanning diffractometry can be used to examine samples like single fibres without the necessity for sectioning, as is required for transmission electron scattering experiments. Examples are taken principally from weakly scattering polymers and biopolymers. In single-crystal diffraction, sub-µm³crystal volumes have been reached for inorganic crystals. Protein crystallography has been demonstrated for a few tenths of µm linear crystal size, which reduces the crystallization time for many proteins.

Mössbauer spectroscopy not only offers information about the structural and electronic properties of iron centres in biomolecules but also about their dynamic behaviour. In order to apply this nuclear method to biologically relevant iron centres knowledge of nuclear and molecular physics is required. This review introduces the basic physical concepts of ⁵⁷ Fe-Mössbauer spectroscopy. The various oxidation and spin states of iron are discussed in a simple orbital frame. Aspects of the ligand field theory of paramagnetic molecules are introduced, and a review covering the iron centres in proteins which have been presently characterized is given. The review covers Mössbauer studies starting from heme proteins, continuing with the above-mentioned proteins containing di-nuclear iron clusters, iron storage proteins and iron-sulfur proteins, and concludes with the complex iron clusters of nitrogenase. The physical properties of the different forms of iron centres as well as their biological relevance are elaborated upon. The effects of electronic spin dynamics on the Mössbauer spectra are reviewed. In addition, the dynamics of the iron ion itself and how it can be studied by Mössbauer spectroscopy is discussed. An introduction into the recently developed technique using synchroton radiation which is also called Mössbauer spectroscopy in the time domain shall give an impression about the future developments in this field of spectroscopy.

This review describes advances which have occurred during the past decade in chemical reaction dynamics using crossed molecular beams. After a brief historical introduction, advances in the generation and state selection of beams of reactants, and in the schemes for product detection, which are at the basis of our increased ability to measure state-averaged and state-resolved reactive differential cross sections in crossed beam experiments, are described. The novel couplings of laser and synchrotron radiation to these experiments are noted. A selection of case studies are considered in some detail, to exemplify recent improvements in our understanding of gas-phase neutral reaction dynamics. The examples include prototype reactions involving three-atom and four-atom systems, and the results are discussed in the light of the most recent, synergistic, theoretical developments for treating both the potential energy surfaces and the reaction dynamics. Progress made in studies of reactions of molecular radicals and of chemically important atoms, such as carbon, nitrogen, and oxygen, with polyatomic molecules, as made possible by recent developments in the classic crossed beam technique with mass spectrometric detection, is emphasized. Some complementary techniques that recently have contributed to our understanding of chemical reactivity are also described briefly.

Many biological processes are affected by electromagnetic fields. Hence incremental internal fields generated by external magnetic fields can be expected to affect that biology. With some exceptions, most of the present interest in the effects of exogenous static and low-frequency magnetic fields centres on three intensity regions. (1) There are concerns that 50-60 Hz power distribution fields as small as 0.2 µT, may affect the health of populations. (2) ac fields that are larger than 1 mT (and usually smaller than 100 mT) with frequencies of a few kHz or less may have therapeutic value with respect to the healing of bone fractures and soft-tissue injuries. (3) The very large slowly varying fields of the order of 2 T used in magnetic resonance imaging might affect the physiology of the patients.

Magnetic fields interact with biological systems through forces on the electrical currents associated with physiological functions and through the torques exerted on the magnetic moments of biologically important molecules and the electrons that play a role in the binding of geminate radicals. The torques that the Earth's magnetic field applies to the ferrimagnetic domains of biologically formed magnetite affects the biology of species in several different phyla and may have consequences in humans. Low-frequency magnetic fields also induce electric fields through the Faraday effect that may have biological consequences. For either the direct magnetic fields or the magnetically induced electric fields to affect the biology of living systems, the interactions with such systems must generally be larger then the interactions with endogenous physiological and thermal noise. This constraint seems to exclude the possibility that the environmental fields less than 1 µT from the electric power distribution system affect health and places important constraints on the minimal fields that can be expected to have therapeutic value.