Reaction pathways of OH and alanine are different under condition with Br^-${\text{Br}}^{-}$ or not.

Br^-${\text{Br}}^{-}$ had a shielding effect on α-carbon of alanine, when it reacted with OH.

Abstraction of H from α-carbon in alanine by OH was the dominant reaction under condition without Br^-${\text{Br}}^{-}$.

Abstraction of H from amino group in alanine by OH was the dominant reaction under condition with Br^-${\text{Br}}^{-}$.

All rate constants of relevant reactions were determined by laser flash photolysis.