See answer in Lehninger on page AS–2.

The solubility experiment is performed using a volume of solvent to which anhydrous crystalline amino acid is added. A small amount of solute is added and allowed to dissolve. This process is repeated until crystalline solute remains. At this point, the solubility limit (the saturation point) has been reached and no more solute will dissolve in this volume. The soluble concentration of the amino acid is then measured to determine the solubility limit.

The key difference in solubility between these two free amino acids is not the hydrophobicity difference, but it is the difference in hydrogen bonding in the crystal (note that this is a discussion of the free amino acid, not in the context of a polypeptide). In order for a molecule to dissolve in water, the interactions stabilizing the crystal form must be broken. It turns out that in the crystal structure of the amino acid tyrosine, almost all possible hydrogen bonds are formed at nearly ideal angles and distances. These hydrogen bonds stabilize the crystal such that the solubility of tyrosine in water is very small. Phenylalanine, without the hydroxyl group in the side chain, can not form as many, nor as ideal, a set of hydrogen bonds in the crystal. Thus, phenylalanine dissolves more readily in water because the molecular interactions are more favorable when dissovled in water.

The amino acid most soluble in water is proline at 1622 g kg-1. While the structure of proline is rather nonpolar (all methylene groups in the side chain), the crystal structure shows no good hydrogen bonds—it is more stable for proline to dissolve in water than to remain in the crystalline lattice. The solubility of proline in water at 25 oC is ≈3500 times greater than that of the least soluble amino acid tyrosine!