Arnold successfully evolved subtilisin E, an enzyme used in detergents, to be active in organic solvents, a significant achievement that demonstrated the potential of directed evolution for creating enzymes with improved stability and activity in non-natural environments.

Arnold's lab developed methods for evolving cytochrome P450 enzymes, versatile catalysts that can perform a wide range of chemical transformations. These engineered P450s have been used in various applications, including the synthesis of pharmaceuticals and biofuels.

Arnold's team made significant advances in expanding the genetic code to incorporate non-canonical amino acids into proteins. This allowed them to create enzymes with new functionalities and catalytic capabilities.

Arnold's lab engineered enzymes capable of catalyzing the formation of carbon-silicon bonds, a reaction not known to occur in nature. This groundbreaking achievement opened up new possibilities for creating novel materials and pharmaceuticals.

Arnold's directed evolution technology continues to be applied in a wide range of areas, including sustainable chemistry, biocatalysis, and the development of biofuels, pharmaceuticals, and other valuable products.