Lonely Arginine Seeks Friendly Environment

The recent perspectives on membrane protein insertion, protein–bilayer interactions, and amino acid side hydrophobicity (J. Gen. Physiol., 129:351–377) provide an excellent opportunity to return to the fundamental issue of the solvation of amino acids and their thermodynamic stability in different environments (Andersen, 2007). Though seemingly “solved” long ago, in recent years this issue has resurfaced in discussions of the voltagegating mechanism of K+ channels, and the likely confi guration of charged arginine side chain of the voltage sensor with respect to the membrane lipids (Mackinnon, 2005). The issues have, at some level, been wrapped in controversy. The perspectives point to ways to dissipate some of the controversies, upon which I wish to elaborate further in this letter. First, let us recall the obvious. Water is a good “high dielectric” solvent for polar and charged species and a poor solvent for nonpolar substances (Friedman and Krishnan, 1973; Wolfenden, 2007), which gives rise to the hydrophobic effect. Indeed, electrostatics and hydrophobicity are two fundamentally opposing solvation forces. Charged molecules interact strongly with water molecules and are diffi cult to dehydrate, whereas nonpolar molecules are diffi cult to dissolve in water. To compare the relative magnitude of those two forces, it is useful to consider fi rst a simple situation corresponding to the partition of molecules between aqueous and hydrocarbon phases. Such basic two-phase partitioning experiments can be converted into a thermodynamic scale of transfer free energy. The transfer free energy of a charged side chain from water to oil is unfavorable and on the order of tens of kcal/mol. On the other hand, the transfer of a nonpolar side chain from water to oil is favorable and on the order of a few kcal/mol. Electrostatic solvation forces are much larger than the hydrophobic effect. For example, electrostatic solvation free energy of a single arginine side chain is 10–20 larger than the hydrophobic solvation for a single leucine side chain (Deng and Roux, 2004; Shirts and Pande, 2005; MacCallum et al., 2007). The guanidinium group of the arginine side chain is by no means a hydrophobic cation. Let us now move on to a more complex (and ambiguous) situation, the partition of amino acids between water and octanol (Wimley et al., 1996). The observed trend in the water/octanol scale is somewhat similar to that in the water/cyclohexane scale, namely, the transfer of nonpolar species from water to octanol is favorable whereas that of charged and polar species is unfavorable. Importantly, transfer free energy for the charged species is much smaller than for the water/cyclohexane scale (by one order of magnitude) (Wolfenden, 2007). The reason can be traced back to the experimental conditions used for measuring the equilibrium partition of amino acids between water and octanol, which implies that there is free exchange of water between the two phases. Therefore, the octanol is saturated with water (up to 5–10%). The real “hydrophobic” phase in this case is therefore wet-octanol, a complex environment in which there are droplets of water surrounded by hydrocarbon chains (see Fig. 1). The transfer of a nonpolar compound from water to octanol is favorable because it is surrounded by hydrocarbon chains. But the transfer of a charged side chain such as arginine, though slightly unfavorable, is not prohibitive because the guanidinium group can be solvated by a small droplet of water molecules in octanol (Fig. 1). As a result, the free energy cost of transfer for arginine from water to octanol is much smaller than the free energy transfer to a true bulk alkane phase. Let us now consider the transfer of amino acids from water to a bilayer membrane. According to the calculations of MacCallum et al. (2007), transfer from bulk water to the membrane interior correlates well with the water/cyclohexane scale. In contrast, the transfer of free energy from bulk water to the membrane interface correlates very well with the water/octanol scale. This is the result one would expect a priori. Indeed, it was noted previously that the partitioning of short peptide, designed to occupy the membrane–solution interface, correlates well with the water/octanol scale (Wimley and White, 1996; Wimley et al., 1996). In summary, the water/octanol thermodynamic scale does not refl ect the partitioning of amino acids in the interior of membranes but to the membrane–solution interface. Finally, let us consider an even more complex situation, the