The molecular origin of the nonideal behavior for concentrated binary solutions of biochemical compounds is examined. The difference between activities expressed in the molar and molal conventions can be large. Considering the range from dilute to concentrated, we show that molar activity coefficients can be represented by simple but rigorous equations involving between one and three parameters only. We derive a universal relationship interconverting the scales of molarity and molality without requiring the density of the solution. The equations are developed from first principles using a statistical thermodynamic theory of molar activity coefficients. It is shown how to express activity coefficients in different concentration scales, and the advantages and disadvantages of using certain scales are discussed and compared with the experimental data. Several classes of biochemically relevant compounds, many of which are naturally occurring osmolytes, are discussed: six saccharides (glucose, xylose, maltose, mannose, raffinose, and sucrose), four polyols (glycerol, mannitol, erythritol, and sorbitol), five amino acids (glycine, alanine, sarcosine, glycine betaine, and proline), and urea. Of the 16 solutes, 10 could be described in terms of a single parameter that is due to pure first-order effects (packing, hydration, or space limitation). The remaining six exhibit significant second-order effects (solute-solute interactions) and require two additional parameters, one typically identified with the volume occupied per solute molecule in the pure solute (crystal or liquid) and the other with a self-association constant. The activity coefficients of the osmolytes roughly display the rank order found with respect to their ability to stabilize proteins. These findings are discussed in terms of the physical principles that give rise to the activity coefficients.
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