Differences between non-specific and bio-specific, and between equilibrium and non-equilibrium, interactions in biological systems.

The interaction forces between biological molecules and surfaces are much more complex than those between non-biological molecules or surfaces, such as colloidal particle surfaces. This complexity is due to a number of factors: (i) the simultaneous involvement of many different molecules and different non-covalent forces - van der Waals, electrostatic, solvation (hydration, hydrophobic), steric, entropic and 'specific', and (ii) the flexibility of biological macromolecules and fluidity of membranes. Biological interactions are better thought of as 'processes' that evolve in space and time and, under physiological conditions, involve a continuous input of energy. Such systems are, therefore, not at thermodynamic equilibrium, or even tending towards equilibrium. Recent surface forces apparatus (SFA) and atomic force microscopy (AFM) measurements on supported model membrane systems (protein-containing lipid bilayers) illustrate these effects. It is suggested that the major theoretical challenge is to establish manageable theories or models that can describe the spatial and time evolution of systems consisting of different molecules subject to certain starting conditions or energy inputs.