Ronald C. Gamble

Preparation of lipid vesicles containing high levels of entrapped radioactive cations

Abstract The ionophore A23187 was incorporated into the lipid bilayer of unilamellar vesicles and used to earry externally added indium ions to a chelator, nitrilotriacetic acid, which was previously entrapped in the vesicles. The extent of vesicle loading was assayed by Sephadex chromatography and γ-ray perturbed angular correlation spectroscopy. Incubation conditions were developed which allow rapid loading of more than 90% of added 121 In 3+ with specific activities of 200–300 μCi/mg lipid.

A focusing monochromator for small-angle diffraction studies with synchrotron radiation

An X-ray focusing monochromator for small-angle diffraction studies was designed for use with the synchrotron radiation from the storage ring, SPEAR, at the Stanford Linear Accelerator Center. It incorporates a 7 cm long silicon crystal, cut at 8°30′ to the (111) planes and bent to a logarithmic spiral curvature for horizontal focusing and monochromatization. A 120 cm long elliptically curved float-glass mirror is used for vertical focusing, and provides means of eliminating higher-order harmonics of smaller wavelength. With SPEAR operating at 3.7 GeV, 20 mA, the two elements produce a 0.5 × 0.5 mm focused beam with an intensity of 6 × 108 photons s−1. The diffraction pattern of frog sciatic nerve myelin obtained with this system was compared with that obtained with a 300 W conventional microfocus X-ray source and a toroidal camera. The new system shows a 190-fold gain in the integrated intensity on photographic film. Synchrotron radiation provides a broad X-ray spectrum. The monochromator is tunable to any wavelength between 0.5 and 3 A, with a total wavelength spread in the focused beam of about 0.01 at 1.74 A. The broad spectrum allows wavelength selection for anomalous-scattering experiments.

Linear position‐sensitive x‐ray detector incorporating a self‐scanning photodiode array

A linear position-sensitive x-ray detector for x-ray spectroscopy and diffraction applications has been tested which can provide excellent spatial resolution, wide dynamic range and good sensitivity. The heart of the system is a self-scanning, photosensitive silicon diode array. It is interfaced via fiber optics to a thin layer of ZnS which fluoresces visible light upon absorption of x-radiation. The conversion to visible light and optical coupling provide several-fold gain in the efficiency of detection as compared to the direct detection of x-ray by the diode array. Equally important is that the array is protected from irreversible damage by high energy radiation, a limitation which previously hindered this application of silicon diode technology.

Application of synchrotron radiation to the structural studies of metalloproteins

Abstract With the advent of synchrotron radiation, there has been renewed interest in X-ray absorption spectroscopy, particularly for studies of biological systems. The applications of this newly revived methodology to metalloproteins will be reviewed. The lecture will focus on a number of electron transport proteins, and will discuss the information which can be obtained from both the absorption edge fine structure (AEFS) and the extended absorption edge fine structure (EXAFS).

Remotely controlled mirror of variable geometry for small angle x-ray diffraction with synchrotron radiation

A total‐reflecting mirror of 120‐cm length was designed and built to focus synchrotron radiation emanating from the electron–positron storage ring at the Stanford Linear Accelerator Center (SPEAR). The reflecting surface is of unpolished float glass. The bending and tilt mechanism allows very fine control of the curvature and selectability of the critical angle for wavelengths ranging from 0.5 to 3.0 A. Elliptical curvature is used to minimize aberrations. The mirror is placed asymmetrically onto the ellipse so as to achieve a tenfold demagnification of the source. The bending mechanism reduces nonelastic deformation (flow) and minimizes strains and stresses in the glass despite its length. Special design features assure stability of the focused image. The mirror reduces the intensity of shorter wavelength harmonics by a factor of approximately 100.

Evidence for the absence of photoreduction of the metal centers of cytochrome c oxidase by x-irradiation

Samples of X-irradiated cytochrome c oxidase were examined by electron paramagnetic resonance and optical spectroscopy. Both radiation from the Stanford Synchrotron Radiation Laboratory and a conventional X-ray source (W target) were utilized. The X-ray flux from these sources ranges from 10(9) to 10(13) photons/s. No evidence was found for photoreduction of the metal centers in the enzyme by X-ray photons. These results demonstrate that the integrity of cytochrome c oxidase is maintained using the conditions under which X-ray absorption measurements are presently being made.