Systematic high-resolution measurements of the collective excitations in normal and superfluid 4 He have been made as a function of both pressure (density) and temperature using neutron inelastic scattering. The IN6 time-of-flight spectrometer at the Institut Laue-Langevin has been used to obtain accurate and consistent data over a wavevector range 0.3 < Q < 2.35 Å −1 . We present and discuss the experimental data. An analysis, based on the simple-subtraction model, is made; single-excitation energies, widths, and magnitudes are extracted. Many changes in the excitations are shown to occur at the λ-transition. The single excitations are discussed in the light of current theory. The structure of the multiphonon continuum and its dependence on pressure is also discussed.

The frequencies of extended X-ray absorption fine-structure (EXAFS) measurements, which are oscillations occurring on the high-energy side of an X-ray absorption edge, can be used to identify interatomic distances in materials. We have used a dispersive X-ray spectrometer, which has no moving components, to make rapid measurements with minimal energy drift of the difference in EXAFS from the Fe K edge in an iron-cobalt thin film undergoing periodic strain through magnetostriction. We show that magnetostriction can be detected by differential X-ray absorption. The magnitude of the recorded signal relative to the noise shows a sensitivity to mean differential atomic motion of one femtometre: a factor of 100 times more sensitive than that normally available.

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The imidazole N epsilon 2 of His-195 plays an essential part in the proposed general base mechanism of chloramphenicol acetyltransferase (CAT), hydrogen bonding to and a abstracting a proton from the primary hydroxyl group of chloramphenicol. Replacement of His-195 by alanine or glutamine results in apparent decreases in kcat of (9 x 10(5)- and (3 x 10(5))-fold, respectively, whereas Km values for both substrates (chloramphenicol and acetyl-CoA) are similar to those of wild-type CAT. The structure of Gln-195 CAT has been solved at 2.5-A resolution and is largely isosteric with that of wild-type CAT. Substitution of His-195 by glutamate resulted in a (5 x 10(4))-fold decrease in kcat together with a 3-fold increase in the Km for chloramphenicol. Direct determination of binding constants for both substrates demonstrated that these substitutions result in only small decreases in the affinity of CAT for acetyl-CoA (Kd values increased 2- to 3-fold), whereas chloramphenicol Kd values are elevated 26-, 20-, and 53-fold for Ala-195 CAT, Gln-195 CAT and Glu-195 CAT, respectively. The pH dependence of kcat/Km yields apparent pKa values of 6.5 and 6.7 for Ala-195 CAT and Gln-195 CAT, respectively, which are very similar to that (6.6) determined for the ionization of His-195 in wild-type CAT. In contrast, the pH dependence of kcat/Km for Glu-195 CAT (pKa = 8.3) is very different from that of wild-type CAT.(ABSTRACT TRUNCATED AT 250 WORDS)