The excited state dynamics of an anionic model photoactive yellow protein (PYP) chromophore have been investigated using the ab initio multiple spawning ( AIMS) method. We observe two isomerization products, differing in the bond about which isomerization takes place. We also carried out AIMS simulations including a single point charge to mimic the Arg52 residue which is present in the protein environment. We show that a single point charge is sufficient to change the photoproduct distribution such that isomerization is restricted to only one of the two bonds where isomerization is observed in the isolated chromophore. These results corroborate recent speculations concerning the role of Arg52 in the photocycle of PYP. (C) 2008 Elsevier B.V. All rights reserved.