Ron, Reverett,
it takes me a while to prepare an answer. The difference between l-dopa and dopamine is a carboxyl-group (COOH). Because of this group l-dopa can easily be methylated and transported through the BBB. I'm discussing this issue with a Dutch chemistry researcher. I'm still not sure how exactly this works. What I do know is that some Metals can be methylated (e.g., mercury, arsenic, lead, selenium, and tin) and that methylation enables them to cross the blood-brain barrier.
Joop