(6R)-5,6,7,8-Tetrahydrobiopterin (BH4) is a key cofactor involved in the electron transfer to the P-450 heme of nitric oxide synthase. We calculated the electronic structure of the neutral, cationic, and anionic forms of BH4 in the gas phase, in solution ( both dielectric and explicit water), and in the protein environment using density functional theory (B3LYP/6-31+G(d,p)). Subsequently, we derived the ionization potential (IP) and electron affinity (EA) of the cofactor in these chemical environments. We found that the electronic structure of BH4 is susceptible to the presence of an external electric field and that conformational changes in the structure of BH4 alone do not affect its electronic structure significantly. In the gas phase, water, and protein environments neutral BH4 is the most stable species, while in the dielectric environment the anion becomes the most stable species. The IP of BH4 in the protein environment is about half of that in the gas phase, and its EA is about 5 times smaller than that in the gas phase. Our results indicate that changes in the external electric field created by moving charged amino acid residues around BH4 may lead to configurations that have the BH4 ion as stable as or more stable than the neutral form, thus facilitating the electron transfer.