Modification of a specific tyrosine enables tracing of the end‐to‐end distance during apomyoglobin folding

In order to follow the overall geometry of the apomyoglobin molecule during folding, we have converted a specific tyrosine residue into 3-nitro-tyrosine. The specificity of the modification was verified by proteolytic cleavage of the modified protein and mass spectroscopy of the resulting fragments. By measuring the energy transfer from the tryptophanyl side-chains to the modified residue the average end-to-end distance can be followed. The experiment shows that after initiation of folding the N- and C-termini are rapidly brought into proximity, possibly to a near-native distance.