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Crystals were prepared from samples of noncovalent and covalently linked dimers previously isolated as intermediates and end products in the interconversion of conformational isomers of immunoglobulin light chains from the patient Mcg. In ammonium sulfate these dimers crystallized in the trigonal form characteristic of the native Bence-Jones protein and in an abnormal needle form associated with conformational changes in the vicinity of the interchain disulfide bond. Trigonal forms were compared with the native covalent dimer by difference Fourier analysis at 3.5-A resolution. Criteria were established for recognizing and cutting twinned trigonal crystals into fragments useful for diffraction experiments. The packing of molecules in the trigonal crystal lattice was examined in detail to determine the steric limitations governing chemical modifications, chemical modifications within crystals or in solutions slated for crystallization. When the modifications involved only the cleavage and alkylation of the interchain disulfide bond, the difference Fourier maps I n the interconversions of conformational isomers of the Mcg immunoglobulin light chains (see Firca et al., 1978), we initially considered crystallization of the products to be just a routine step in the correlation of chemical and diffraction results. Instead, the unexpected appearance of different crystal forms and variations in the properties of the expected crystal forms directed us into a more enlightened study of conformational changes in the constituent molecules. By using the crystal properties in conjunction with chemical modifications and CD spectroscopy (see Firca et al., 1978), we were able to identify regions involved in conformational changes during the interconversion experiments. The morphology and pertinent properties of these crystals will be described in the present indicated only local conformational changes mainly in the COOH-terminal pentapeptide segment of monomer 2 of the dimer. When light chains were dissociated from heavy chains and reassembled into a dimer, there were changes in segments which interact to stabilize the V domain dimer (i.e., in the lining of the deep binding pocket in the V interface). These changes, as well as the more extensive changes in the needle forms, could be reversed by dissolving the crystals. cleaving the interchain disulfide bond, and allowing it to reoxidize. The resulting proteins crystallized as trigonal forms indistinguishable from those of the native dimer. After the binding of two molecules of bis(dinitropheny1)lysine and subsequent removal of the ligands by dialysis, the dimer crystallized only as needles. The needles could be converted into trigonal forms as before. These results suggest that the binding of ligands by the V domains can lead to conformational changes in the most distal regions of the C domain dimer.article. In cases in which the crystal morphology was similar to that of the trigonal form of the native Bence-Jones dimer (Edmundson et al., 1971). subtle conformational changes were s...

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