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Do you know where your tryptophan electrons are going?12 May 2015
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The native fluorescence of tryptophan has been used extensively to assist in studies of protein conformation. These studies typically involve using a fluorescent acceptor and monitoring the ability of tryptophan to transmit energy to this acceptor in a process called FRET. Since this transfer of energy is reliant on the tryptophan and acceptor being in proximity a change in the FRET signal signifies a change in the conformation of the folded protein.
However, what if some of the energy was not going to the acceptor but was instead being transferred to another part of the protein or another molecule associated with the protein? This is what is happening in myoglobin as reported in the recent PNAS article entitled: ‘Tryptophan-to-heme electron transfer in ferrous myoglobin’. In this paper scientists at EPFL in Switzerland used a unique spectroscopic technique to carefully study the tryptophan residues that are proximal to heme in myoglobin and convincingly show that electrons are indeed being transferred. Since the transfer of electrons from tryptophan to heme decreases the amount of FRET the authors suggest that this calls into question the use of tryptophan fluorescence in FRET studies of protein dynamics. Although, this has only been definitively shown for tryptophan to heme it is possible that other protein / molecular structures could act similarly.

It is certainly something to be taken into consideration when analyzing tryptophan fluorescence FRET results.

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