Ultrafast dynamics in photoreceptors have focussed on the dynamics of the light absorbing chromophore and its property changes in the photoproduct. In rhodopsins the retinal chromphore isomerizes on a sub-picosecond time-scale introducing a conformational change. It is assumed that steric interaction betweeen the isomerized chromophore and the protein governs the downstream photocycle in an efficient way. Therefore, processes during the electronic excited state and before isomerization of the chromophore are mainly considered unimportant.

In this publication, we discuss protein dynamics, such as protonation changes, prior to and after the isomerization process demonstrating protein changes in the electronic excited state and after its decay. We present a new perspective on protein-cofactor interactions via an impulsive electric field change in the electronic excited state. This fast electric field change acts like a ‘Coulomb hammer’ interacting with polar groups in the protein on a sub 100 fs time-scale. The impact of the impulsive electric field change can alter protonation status, orientations, and interactions of hydrogen-bonded groups in the protein paving the way for a reactive protein conformation. The isomerized chromophore represents a non-reversible structural change inhibiting relaxation to the initial ground state.