The Surface Forces Apparatus (SFA) allows one to measure various interaction forces between surfaces as a function of their separation in aqueous solutions. In addition, the optical technique used allows one to directly visualize various interfacial phenomena at the molecular level, such as slow structural rearrangements, that may be occurring during an interaction. In this way, complex biological interactions may be studied at the molecular level both in space and time. Recent results on a variety of systems show that these have much more complex and time-dependent interaction potentials than normally occur between simpler (colloidal) surfaces, i.e., their interactions are not simply described by van der Waals attraction and electrostatic repulsion (the two DLVO forces) but can also involve - in the same interaction - discrete ion, structural, hydration, hydrophobic, polymer-mediated, thermal fluctuation and bio-specific forces. Specific examples will be given of how these different forces arise in different types of charged and uncharged systems: polyelectrolytes, proteins, PEG, lipid bilayers, cell membranes, and bio-specific lock-and-key type binding. These recent results show that even though all forces have a common origin biocolloidal interactions can differ from normal, non-specific "colloidal" interactions in three important ways: (1) biological, especially bio-specific, interactions are qualitatively different in that many molecular groups are often involved "sequentially" (in different regions of space and time) in such a way that "the whole is greater than the sum of the parts," (2) interacting bio-colloidal surfaces are usually `asymmetric' which, as will be shown, gives rise to very different interactions than those that arise between similar (symmetric) surfaces, and (3) non-equilibrium and time effects often play a crucial role in regulating biological interactions. It is unlikely that a single, generic interaction potential can be written that covers all possible situations, but careful consideration of the surfaces, the molecules and solution conditions should allow reasonable predictions to be made in many cases. An understanding of these forces and their mode of operation can also have important implications for understanding various dynamic processes that involve sequential interactions, such as locomotion and cell fusion.
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