In situ atomic force microscopy studies of surface morphology, growth kinetics, defect structure and dissolution in macromolecular crystallization

Surface morphologies of thaumatin, catalase, lysozyme and xylanase crystals were investigated using in situ atomic force microscopy. For thaumatin, ly sozyme and xylanase crystals, growth steps having a height equal to the uni t cell parameter were produced both by screw dislocations and two-dimension al nuclei. Growth of catalase crystals proceeded in alternating patterns ex clusively by two-dimensional nucleation and the successive deposition of di stinctive growth layers, each having a step height equal to half the unit c ell parameter. The shapes of islands on successive layers were related by 2 -fold rotation axes along the [0 0 1] direction. Experiments revealed that step bunching on crystalline surfaces occurred either due to two- or three- dimensional nucleation on the terraces of vicinal slopes or as a result of uneven step generation by complex dislocation sources. Growth kinetics for thaumatin and catalase crystals were investigated over wide supersaturation ranges. Strong directional kinetic anisotropy in the tangential step growt h rates in different directions was seen. From the supersaturation dependen cies of tangential step rates and the rates of two-dimensional nucleation, the kinetic coefficients of the steps and the surface free energy of the st ep edge were calculated. Adsorption of impurities which formed filaments on the surfaces of catalase and thaumatin crystals was recorded. Cessation of growth of xylanase and lysozyme crystals was also observed and appeared to be a consequence of the formation of dense impurity adsorption layers. Cry stal growth resumed upon scarring of the impurity adsorption layer and clea ring of the crystal surface with the AFM tip. Adsorption of three-dimension al clusters, which consequently developed into either properly aligned mult ilayer stacks or misaligned microcrystals was recorded. For catalase crysta ls, incorporation of misoriented microcrystals as large as 50 x 3 x 0.1 mu m(3) produced elastic deformations in growth layers of approximate to 0.6%, but did not result in the defect formation. Etching experiments on catalas e crystals revealed high defect densities. (C) 1999 Elsevier Science B.V. A ll rights reserved.