Many of the pure solutions of globular and fibrous proteins investigated hitherto have shown only monolayer adsorption. An exception is laminin allowed to adsorb in the presence of calcium ions (Ramsden 1993b), in which case adsorption switches from monolayer in the absence of Ca2+ to multilayer. The canonical behaviour, which is supposed to also describe the formation of amyloid plaques in higher organisms, is the sequence (starting from a dilute solution of A suddenly placed in contact with a surface): (i) adsorption of A at the solid/liquid interface; either (ii) polymerization of A (facilitated by the juxtaposition of the individual A on the surface) and conformational change of A (^ A' due to polymerization) or (iii) conformational change of A (^ A' due to adsorption) and polymerization of A'; (iv) adhesion of A to A'.
Multilayers have also been observed in the case of adsorption of complex mixtures of proteins, e.g. blood serum (Kurrat et al. 1998). The behaviour is extremely complex and not well understood. An approach starting with a mixture of the two most abundant proteins, followed by the three most abundant, etc., would doubtless help to elucidate the process, but given that there are about 1000 different proteins in blood serum, the development of high throughput screening methods capable of yielding the level of physico-chemical information currently obtainable from OWLS is required for seriously advancing towards understanding the mechanisms involved.
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