Reader proteins play significant roles in cell signaling from site-specific lysine methylation, but progress of new reader investigations is rather slow. Photoreactive crosslinkers enable the discovery of reader proteins, but the chemical synthesis required to optimize the placement of the photoreactive group is laborious. In addition, active intermediates such as nitrene and carbene may cause significant non-specific crosslinking. Here we report dimethylsulfonium as a methyllysine mimic that binds to specific readers and can subsequently crosslink to a conserved tryptophan inside the binding pocket through single electron transfer under UV irradiation. The crosslinking requires σ-π electron-donor-acceptor interaction between sulfonium and indole as a key step, facilitating excellent tryptophan site-selectivity and orthogonality to distinct methyllysine readers. Moreover, excited tryptophan without proximate sulfonium leads to minimal non-specific crosslinking from relaxation. This method could escalate the discovery of methyllysine readers from complex cell samples. Furthermore, this photo crosslinking concept could also be expanded to develop more types of microenvironment-dependent conjugations to site-specific tryptophan.