Excited State Proton Transfer ESPT from the Neutral Chromophore

3 Excited State Proton Transfer (ESPT) from the Neutral Chromophore 

Ultrafast ESPT from the neutral form readily explains why excitation into the A and B bands of AvGFP leads to a similar green anionic fluorescence emission [84], Simplistic thermodynamic analysis, by way of the Forster cycle, indicates that the excited state protonation pK.J of the chromophore is lowered by about 9 units as compared to its ground state. However, because the green anionic emission is slightly different when it arises from excitation into band A or band B (Fig. 5) and because these differences are even more pronounced at low temperatures [81, 118], fluorescence after excitation of the neutral A state must occur from an intermediate anionic form I not exactly equivalent to B. State I is usually viewed as an excited anionic chromophore surrounded by an unrelaxed, neutral-like protein conformation. The kinetic and thermodynamic system formed by the respective ground and excited states of A, B, and I is sometimes called the three state model (Fig. 7). 

I decays to the ground state by fluorescence emission with a lifetime of 3.3 ns (see above), then I returns to A in less than a nanosecond, a process that is slightly thermally activated, strongly sensitive to deuteration, and apparently involves several intermediates [118, 132, 144]. Because no production of the B state is observed during I deexcitation and return to A, states I and B must exchange on timescales at least slower than the nanosecond. 

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