Section 12.2.5.1. Lack of isomorphism

This problem is by far the most common in protein crystallography. An isomorphous derivative is one in which the crystalline arrangement has not been disturbed by derivatization. An early study of Crick & Magdoff (1956) proposed a rule of thumb that a change in any of the cell dimensions by more than around 5% would result in a lack of isomorphism that would defeat any attempt to locate the heavy-atom positions or extract useful phase information. Lack of isomorphism can, however, be more subtle; sometimes a natural variation in the native crystal form may occur, resulting in poor merging statistics of data obtained from different crystals. Coupling this variation with commonly observed structural changes upon heavy-atom binding can provide a considerable barrier to obtaining satisfactory phases. Dumas (1994a) has provided a theoretical consideration of this problem.

One practical approach is to collect native and derivative data sets from the same crystal, a technique that has been successful in the structure determination of cyclohydrolase (Nar et al., 1995), proteosome (Löwe et al., 1995) and a number of other proteins. Nonisomorphism can be used, however. In the structure solution of carbamoyl sarcosine hydrolase (Romao et al., 1992), derivatives fell into two (related) crystalline classes. By judicious use of two `native' crystal forms, heavy-atom positions could be obtained in each of the two classes. Phasing and resultant averaging between the two classes provided an interpretable electron density. In the case of ascorbate oxidase (Messerschmidt et al., 1989), multiple isomorphous replacement failed to provide an interpretable density. It was possible, however, to place the initial density into a second crystal form, which in turn provided phases of sufficient quality to determine heavy-atom sites in derivatives of the second form. Phase-combination and density-modification techniques in the two crystal forms allowed the solution of the structure.