The development of multidetectors for use in X‐ray and neutron scattering experiments has created an interest in methods for integrating Bragg peaks in three‐dimensional data arrays representing intensity scattered from single crystals. A method of using a priori information, extracted from the strongest peaks, to obtain statistically optimum results has been developed at the Institut Laue–Langevin (ILL), Grenoble. The method is outlined in this paper and results of its application to neutron diffraction are discussed.

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Crystals of hen egg white lysozyme soaked in 15% (v/v) dimethyl sulfoxide have been studied with single-crystal neutron diffraction to determine the effect of the solvent molecules on the protein configuration. A total of 9423 statistically significant Bragg reflections to a resolution of approximately 1.8 A were used to locate 6 dimethyl sulfoxide molecules, and structure refinements including a model for the flat solvent lead to a final crystallographic agreement factor of 0.130. The mode of location of the dimethyl sulfoxide molecules was compared with that in previous studies employing ethanol. This showed that hydrophobic interactions can be an essential factor in fixing the probe molecules on the protein surface. There was, however, no sign of any significant change in the protein configuration; so although possibly at higher concentrations of dimethyl sulfoxide the protein will unfold, there was no indication of any precursor effect.

An electron density map of horse spleen apoferritin at 0.28-nm (2.8 A) resolution and its preliminary interpretation have been described previously. Rigorous examination of this and newer maps at the same nominal resolution but calculated from more extensive data sets, including model building in a Richards' comparator, now allows us to report on structural features in more detail. We list inter-helical angles within and between neighbouring subunits, and describe a new short region of inter-subunit anti-parallel pleated sheet. A short section of electron density not properly accounted for in the first model has also been found. We also report on two alternative ways of connecting helical regions which account almost equally well for the observed electron density, and we assess these two alternative conformations and compare them with the conformations of other known proteins.

Reaction of [Mo,(CO)~(~-HC,H) (?-C,H,),] with Me0,CC-CC0,Me affords several products, one of which, of molecular formula [Mo,((HC,H) (Me02CC2C0,Me),)(q-C5H5)2] ( l ) , is here the subject of an X-ray diffraction study. Crystals of (1) are triclinic, space group P7 (no. 2) with Z = 2 in a unit cell of dimensions a = 11.519(7), b = 18.602(9), c = 8.709(3) A, ct = 86.65(4), p = 100.99(4), y = 97.54(5)', and they encapsulate two molecules of solvent (CH,CI,) per molecule of complex. The structure has been refined to R 0.054 for 3 450 reflections. The acetylenic moieties have joined to form a c 8 chain ; within this chain the terminal atoms and the two central atoms are attached to one of the metal atoms, while the first three and the last three form x-ally1 attachments to the other metal atom. The (C,H,)Mo=Mo(C,H,) unit and the C8 chain together form a framework of mirror symmetry, but the molecule as a whole does not possess C, symmetry because of the methoxycarbonyl ligands which are bonded to C(11), C(12), C(13), C(14), C(17), and C(18). When a similar reaction is carried out with [MO,(CO)~(~-MeO,CC,CO,Me)(q-C5H5),] as the starting material, one of the products is again a complex containing a c8 chain with methoxycarbonyl ligands on all eight carbon atoms, [Mo,{p-( Me0,CC,C02Me),}(q-C5H5)2] (2). An X-ray diffraction study of (2) reveals a remarkable difference in the bonding mode. Crystals of (2) are monoclinic, space group P2,lc (no. 14) with Z = 4 in a unit cell of dimensions a = 14.737(2), b = 11.224(2), c = 23.882(5) A, p = 11 5.92(1)', and the structure has been refined to R 0.088 for 4 897 reflections. The molecule (2) has idealised C, symmetry (not crystallographically required), and the C, chain forms a ' flyover ' from one metal atom to the other. The two groups of four carbon atoms are each planar (and co-planar with the o-bonded metal atom) and form a diene-type interaction with the other metal atom. A further marked difference from ( 1) is that the central bond of the C8 chain, which in (1) is orthogonal to the Mo=Mo link, is in ( 2 ) parallel to that direction. The metalmetal distances, 2.618(1) A in (1) and 2.635(1) A in ( 2 ) , are indicative of Mo-Mo double bonding.

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