The Brazilian National Synchrotron Light Laboratory (LNLS) has a dedicated protein crystallography beamline. The optical elements of the beamline include an elastically bent cylindrical mirror and a triangular bent-crystal monochromator, which focus synchrotron radiation at the position of the sample in the vertical and horizontal planes, respectively. The monochromatic radiation is tunable between 2.0 and 1.2 A with the optimum wavelengths from 1.3 to 1.6 A, chosen to maximize the photon flux from the bending magnets of the storage ring (1.37 GeV). Diffraction images are recorded on a 345 mm-diameter MarResearch image-plate detector system with on-line readout. The experimental parameters of the beamline, such as the integral monochromatic flux and focus size, have been measured. The size of the beam at the position of the focal point is 0.5 x 0.5 mm(2). The flux density is between 4.4 x 10(10) and 8 x 10(10) photons s(-1) mm(-2) for wavelengths from 1.28 to 1.6 A. The energy resolution is sufficient to measure absorption edges of elements between 1.28 and 2 A. The facility, intended to serve the national and international community, has been commissioned and is available for users.

The functional L-asparaginase from Escherichia coli is a homotetramer with a molecular weight of about 142 kDa. The X-ray structure of the enzyme, crystallized in a new form (space group C2) and refined to 1.95 A resolution, is compared with that of the previously determined crystal form (space group P2(1)). The asymmetric unit of the new crystal form contains an L-asparaginase dimer instead of the tetramer found in the previous crystal form. It is found that crystal contacts practically do not affect the conformation of the protein. It is shown that subunit C of the tetrameric form is in a conformation which is systematically different from that of all other subunits in both crystal forms. Major conformational differences are confined to the lid loop (residues 14-27). In addition, the stability of this globular protein is analyzed in terms of the interactions between hydrophobic parts of the subunits.

The efficiency of time-modulation of synchrotron radiation by a double-crystal monochromator periodically curved by pulsed surface acoustic waves has been studied both experimentally and theoretically. It has been shown that the efficiency of such a system is significantly higher as compared to x-ray modulation by surface acoustic waves applied to x-ray mirrors—that is under the conditions of total external reflection. Possible modifications and improvement of a synchrotron radiation double-crystal diffractive modulator are discussed. The experiments were done at the diffraction beamline at the Brazilian National Synchrotron Light Laboratory (LNLS).

First experiments with the acoustically controlled LiNbO3 double-crystal monochromator for synchrotron radiation have been performed on the diffraction beamline of the Brazilian National Synchrotron Light Laboratory. In order to modify x-ray optical properties of LiNbO3 crystals, 290 MHz surface acoustic waves were generated with the aid of interdigital transducers, fabricated on the polished crystal surfaces by using microelectronics technology. It was shown that by the appropriate choice of diffraction planes, the LiNbO3 double-crystal monochromator can be used for both high-flux and high-resolution applications. Surface acoustic waves allow adjustment of an angular width of diffraction profile to a divergence of the incident synchrotron radiation, optimizing matching conditions for x rays of different wavelengths. Diffraction measurements under surface acoustic wave excitation demonstrated that the parameters of the x-ray beam issuing from the monochromatization system are indeed acoustically controlled, providing a 2–2.5-fold broadening of the diffraction profile and a 1.5-fold gain in the maximum diffraction intensity.

l-Asparaginase is an enzyme which hydrolyzes asparagine to produce aspartic acid and ammonia. It is an effective chemotherapeutic drug, especially in the treatment of acute lymphoblastic leukaemia in children. The enzyme from Escherichia coli was crystallized in a new crystal form with space group C2, unit-cell parameters a = 76.3 (0), b = 134.6 (2), c = 64.8 (7) A Ê , = 110.5 (1) and a dimer in the asymmetric unit. Synchrotron-radiation diffraction data have been collected to 1.95 A Ê resolution.

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