Valine

An essential amino acid (a building block of skin proteins like collagen) that our body cannot produce itself but has to take from external sources, like diet. 

It's a branched chain amino acid that is claimed to enhance energy, increase endurance and aid in muscle tissue recovery and repair when taken as a supplement. It's not clear what valine does when you put it on the skin, but as all amino acids, it must be at least a great skinhydrator.