In
the course of an earlier investigation into the crystallization
of proteins based on the addition of intermolecular ligands, the Kunitz
type trypsin inhibitor from soybean (SBTI) was crystallized as a complex
with 1,5-disulfonylnaphthalene (ligand library 21D). The two molecules
within the asymmetric unit of the monoclinic crystals are related
by a near-exact NCS 2-fold axis and have essentially the same conformation
as was found for them in previous analyses. The protein dimer is maintained
through electrostatic interactions with the sulfonyl groups of four
1,5-disulfonylnaphthalene molecules, which are bound in pairs at the
dimer interface and involve symmetrical pairs of arg 30 and arg 47
side chains of each SBTI molecule. The binding of the 1,5-disulfonylnaphthalene
molecules at the interface is disordered in that only a coordinated
single pair of 1,5-disulfonylnaphthalene molecules can be bound at
any one time. There are also molecules of the MES buffer present in
the crystal structure that are responsible for additional lattice
interactions. This crystal structure provides a remarkable demonstration
of how unusual, nonphysiologic, small molecules can create symmetry
and promote specific lattice interactions within protein crystals.