Foldit classifies all amino acids as either hydrophilic or hydrophobic. Hydrophilic amino acids tend to be found on the surface of the protein. When the "Hydro" coloring option is selected, hydrophilic amino acids appear in blue.
The term "hydrophilic" means these amino acids aren't repelled by water. The hiding score component for hydrophilics tends to penalize them for not being hidden in the protein's core. For example, a histidine on the outside of a protein might have a hiding score of -20.9 points, while nearby histidine pointing toward the core might have a hiding score of -16.2 points.
In Foldit, the hydrophilic amino acids are:
- Aspartate (aspartic acid)
- Glutamate (glutamic acid)
Not all amino acids are equally hydrophilic, there's a hydropathy index that estimates how hydrophilic each acid is in typical conditions. The hydropathy index can be found in the table on the amino acids page.