Figure 2.

Scheme for triplet quenching and FRET measurements. (a) The excited triplet state of tryptophan becomes populated after UV pulsed excitation. Tryptophan can then contact cysteine in a diffusion-limited process with rate k_D+. During the lifetime of the contact pair, the quenching process occurring with a rate q competes with the rate k_D− of diffusive dissociation of the complex. (b) A generic p(r) (solid line) is displayed together with the distance dependence of the tryptophan-cysteine quenching (thin solid line delimiting the gray area) as reported by Lapidus et al. (39) and with the distance dependence of the FRET efficiency (dashed line) for R₀ = 27 Å. The region below the estimated tryptophan-cysteine contact distance of 4 Å is removed from the plot.