Adhesion of protein crystals: Measurement of the detachment force

The degree of adhesion of protein crystals, heterogeneously nucleated and grown on different supports (e.g. glass plates and plates coated with poly-L-lysine, hexamethyl-disilazane and silicon) is measured directly with a purposely-developed technique. The sticking force crystal/support is determined by means of a flexible glass fibre, which bending is calibrated by means of series of weights. In this way an elastic constant, specific for each glass fiber is determined individually. Appropriate glass fibres with relative bending less than 10% (Hook's law) are used. The force which is necessary to be exerted, by means of a micro-manipulator, in order to detach the crystal from the support is taken as a quantitative measure for the adhesion strength. Forces between 10 N cm(-2) and 1 N cm(-2) for differently oriented tetragonal hen-egg-white lysozyme and cubic ferritin crystals, and 0.1 N cm(-2) for rhombohedral (porcine) insulin and orthorhombic trypsin crystals are measured. The tetragonal HEWL and rhombohedral insulin crystals show anisotropy of the adhesion strength. In contrast, the cubic ferritin crystals are isotropic also in this respect. For comparison purposes adhesion measurements are performed with NaCl and sugar crystals. An attempt is made to evaluate also the adhesion energy of the protein crystals.