Such thermodynamicstudies have been performed for the transfer of the naturally occurring aminoacids from a number of nonpolar solvent to water. To make these measurements more representative of the hydrophobicities of amino acids withinproteins, workers use analogues of the amino acids in which the amino andcarboxylate charges are neutralized (e.g., using N-acetyl ethyl esters of theamino acids). Combining this type of thermodynamic information for thedifferent solvent systems, one can develop a rank order of hydrophobicities forthe 20 amino acids. A popular rank order used in this regard is that developedby Kyte and Doolittle (1982); the Kyte and Doolittle hydrophobicity indicesfor the amino acid are listed in Table 3.1. In general, hydrophobic amino acidsare found on the interior of folded proteins, where they are shielded from therepulsive forces of the polar solvent, and polar amino acids tend to be foundon the solvent-exposed surfaces of folded proteins.