24 J. Appl. Cryst. (1981). 14, 24-27 Neutron Photographic Measurements of Protein Single-Crystal Reflections BY D. HOHLWEIN Institut fiir Kristallographie der Universitdt Tdbingen, Charlottenstrasse 33, D-7400 Tiibingen, Federal Republic of Germany AND S. A. MASON Institut Max von Laue-Paul Langevin, 156X Centre de Tri, F-38042 Grenoble CEDE)(, France (Received 9 June 1980; accepted 9 September 1980) Abstract Photographic neutron intensity measurements on a 2 mm 3 single crystal of triclinic lysozyme are compared with conventional neutron diffractometer data. The reflections were recorded with the oscillation technique. The structure factors were derived from the optical densities scanned by an automatic microdensitometer and processed by computer programs. A statistical analysis shows that the photographic data are of about the same accuracy as recently collected diffractometer data. For the same flux and crystal volume the data collection time is reduced by up to two orders of magnitude. Introduction The collection of protein single-crystal intensity data on a conventional four-circle neutron diffractometer is very time consuming. The reasons are the relatively small diffracted intensities even on a high-flux reactor, the large number of reflections and the intense incoherent background. Until recently, extensive neu- tron diffraction data had been measured on two proteins only: myoglobin at the Brookhaven high-flux reactor; and triclinic lysozyme at the Grenoble HFR (Bentley, Du6e, Mason & Nunes, 1979). In each case a single-detector diffractometer was used. The data collection time can be reduced dramatically using a multidetector system, as has now been shown in the extension of the measurements on CO myoglobin to 1-5 A, resolution (Schoenborn & Hansen, 1979); and using a segmented linear detector system for successful measurements on a 1.6 mm 3 crystal of an inhibited trypsin (Kossiakoff & Spencer, 1979). A linear position- sensitive detector has been used at a medium-flux reactor to record reflections to 2 A, resolution on ribonuclease-A (Wlodawer, 1980). A curved 4 x 64 ° multiwire detector is under development at the ILL, Grenoble. In spite of these developments, the ideal two- dimensional electronic neutron detector for protein measurements does not yet exist. 0021-8898/81/010024-04$01.00 The high speed and the quality of neutron film methods have already been shown (Hohlwein, 1978). With photographic data from a 0.3 mm 3 histidine crystal collected by the modified Laue method in only 30 h a full structure refinement was achieved (Hohlwein, 1977). The photographic measurement of neutron data on protein crystals is difficult because of the high background, which is due mostly to incoherent scattering by the hydrogen atoms. Therefore, a preliminary study of lysozyme was performed to get information about the accuracy and time gain possible with the photographic method. Lysozyme was chosen because diffractometer measurements are available for comparison. Scintillator-film system Some characteristics of the detection system are presented in Table 1. A light-sensitive film is pressed -2 between two converter foils. 2000 neutrons mm produce an optical density of 1.0 which corresponds to a medium blackening (Hohlwein, 1975). The optical density is not only a function of exposure (neutrons mm-2) but depends also on the time or flux (neutrons mm- 2 s- 1), the well known Schwarzschild effect. If all diffraction spots are produced in equal times, a universal calibration curve can be determined (Hohlwein, 1977). This curve was also used for the evaluation of the oscillation photographs described below. As the exposure time for one reflection in this method is proportional to the reflection width Am (Am~At = constant), the calibration curve will be correct only if the widths Am are not too different. Photographic measurements The photographic measurements were done with the instrument D12A (Hohlwein, 1975) of the ILL high-flux reactor. It is situated at the end of a thermal-neutron guide. A first series of exposures was made with a wavelength of 1.53/k selected by a graphite monochro- me) 1981 International Union of Crystallography