Epitaxial protein inclusion in sinapic acid crystals

The authors report on the inclusion of protein molecules in growing crystals of trans-3,5-dimethoxy-4-hydroxy-cinnamic acid (trans-sinapic acid). This material is important as the 'matrix' compound in matrix-assisted lasar desorption, a promising new ion source for heavy molecule mass spectrometry. The crystal structure of trans-sinapic acid was determined. It belongs to the space group P2 1 /n and has a structure in which the planar trans-sinapic acid molecules are hydrogen bonded into extended sheets. Dyes were used to follow protein incorporation into growing trans-sinapic crystals. The staining pattern obtained demonstrated that protein molecules were included only on crystal faces parallel to these extended sheets. This result is the first demonstration that proteins can be incorporated into a growing crystal by a selective interaction with one of the crystal faces. The structure of the crystal plane that interacts with the protein was consistent with a 'hydrophobic' bonding of the protein to the crystal face.