The phenylalanine residues 300 and 309 in the enzyme tyrosine hydroxylase are known to aid in the positioning and binding of tetrahydrobiopterin (BH4) to the enzyme active site. The residues phenylalanine 254 and tyrosine 325 similarly aid in binding BH4 in phenylalanine hydroxylase. BH4 is a cofactor necessary for enzyme function, and mutations in these residues have been shown to cause a decrease in enzyme function. We examine the pairwise interactions between each aromatic residue and BH4 using second-order Moller Plesset theory and density functional theory to determine the amount of binding due to these aromatic residues. Further, we perform in silico point mutations of these residues to determine if several likely mutations can cause a decrease in protein function. Our results show that dispersion dominates these interactions, and electrostatics alone is not enough to bind the BH4. © 2007 American Chemical Society.