Table 1.

Experimental parameters obtained from triplet quenching and FRET measurements

Solvent	Peptide	Triplet quenching		FRET
		kR (106 s−1)	ηkD+ (106 s−1)	〈E〉	n	ϕD	J(10−14 M−1 cm−1 nm4)	R0 (Å)	〈r2〉1/2 (Å)
Water	agq	5.3	20	0.82	1.33	0.09	2.32	27.0	18.0
	age	3.7	28	0.81	1.33	0.09	2.33	27.0	18.3
	aqe	1.3	6.0	0.68	1.33	0.10	2.34	27.5	22.8
Urea	agq	2.3	14	0.74	1.40	0.14	2.36	28.2	21.4
	age	2.1	15	0.70	1.40	0.14	2.33	28.2	22.7
	aqe	0.7	4.0	0.64	1.40	0.15	2.33	28.5	24.9
GdmCl	agq	2.9	7.6	0.69	1.44	0.12	2.35	27.0	22.1
	age	1.2	6.1	0.68	1.44	0.12	2.33	27.0	22.4
	aqe	1.1	1.6	0.62	1.44	0.13	2.33	27.4	24.7

The reaction-limited rate k_R and the diffusion-limited rate ηk_D+ for tryptophan-cysteine contact formation are extracted fitting the viscosity dependence of the measured triplet quenching lifetime τ_obs with Eq. 1. The average transfer efficiency 〈E〉 is extracted from the tryptophan emission intensities for DNP-labeled and unlabeled peptides (see Materials and Methods). Independently measured values of refractive index n, tryptophan quantum yield ϕ_D, and overlap integral J (see the Supporting Material) are used to compute the characteristic transfer distance R₀ according to Eq. 4. Through Eq. 3, each set of 〈E〉 and R₀ are used to select a Gaussian donor-acceptor distance distribution, from which the rms value of the tryptophan-cysteine distance, 〈r²〉^1/2, is obtained after correcting for the effect of the DNP-cysteine linker (see Materials and Methods).