The protonated Tryptophan has his more basic site on the Nitrogen of the amino group as shown in the figure.

This ion has similar electronic property than the neutral molecule, it is a ground singlet state.

Is the ps* state important in this molecule?

Why is the fluorescence lifetime so short when the tryptophan is protonated??

Photofragmentation of the protonated Tryptophan

set up

When excited at 266nm the Protonated tryptophan losing an H atom. this H atom lost is not observed in collision experiment. The dissociation channel is specific of the optical excitation.

In fact, as shown by the figure below, in the case of protonated tryptamine, he first excited state of the molecule is dissociative along the NH coordinate.

potential curves of the Protonated Tryptamine along the NH coordinate.

When the NH distance is stretched the excited state ps* state become repulsive.

This is seen in orbital picture.

From this picture, one expect that a very fast dissociation. This explain why tryptophan in water at very low PH has a very small fluorescence quantum yield!

Femtosecond dissociation of Protonated Tryptamine

In the middle

difference mass spectrum obtained with and without pump laser (266nm)

the parent ion (mass 161) is depopulated by the photon:

mass 160 appears indicating the H loss process

In a pump/probe experiment some fragments exhibit some femtosecond dynamics, and some not.

the 400fs dynamics observed on mass 131 reflect the excited state lifetime : the H loss process is fast!

After the H loss the system is still fragmenting leading to mass 130 and 131

There is a competition between the H loss and the internal conversion : fragment 132 and 144 are coming from the internal conversion and do not present any dynamics.

The excited dissociative state is crossing the ground state. There is a competition between the H loss and the Internal conversion

What is measured is the variation of the fragmentation upon the energy of the system.

The probe photon can be absorbed in the S1 to Sn transition

2 pathways

IC or H loss

After the H loss : formation of a radical cation which can absorb a photon in the 800nm region

femtosecond control of protonated tryptophan

	why 2 lifetime
	two isomers