Tryptophan is quenched by:
Excited tryptophan is powerful electron donor subject to quenching by electron accepting or proton donating groups nearby in sequence or three-dimensional fold. Protonated aspartate, glutamate, histidine, cysteine, disulfide bridges, and backbone amide can accept electron or proton via photoinduced electron transfer, providing non-radiative routes that diminish fluorescence intensity and lifetime significantly. Such quenching is strongly distance dependent, informing on spatial proximity and conformational dynamics. Water mainly induces spectral shift not quenching, guanidine relieves quenching by unfolding, tyrosine acts as energy donor to tryptophan rather than effective quencher.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.