Table 1. Photophysical Properties of Fluorescent Probes^a.

probe	Iad	E	FF	R0 (nm)	Φf	ε (M–1cm–1)
12	3.3	0.40	19.2	3.0	0.74	24 200
13	0.1	0.13	6.8	2.1	0.068	3500
14	2.2	0.21	4.8	2.7	0.476	7800
15	0.6	0.35	8.0	2.8	0.066	11 600

^aThe intensity (I_ad) of FRET acceptors (100 nM) was measured at Em. λ_max when bound to SA (400 nM, Ex. 295 nm) and normalized to I_d. The efficiency (E) of FRET with tryptophan when bound to SA was calculated as E = 1 – I_da/I_d. FF was calculated as FF = I_ad/I_a. R₀ is the theoretical Förster distance calculated for each Trp-acceptor pair as described in the Experimental Section. I_da = intensity of Em. of tryptophan (340 nm) in the presence of the acceptor. I_d = intensity of Em. of tryptophan (340 nm) in the absence of the acceptor. I_ad and I_a are the intensities of Em. of the acceptor [457 (12), 532 (13), 467 (14), and 475 nm (15)] in the presence and absence of the donor (SA), respectively. Values for Φ_f and molar extinction coefficients (ε) were measured as described in the Experimental Section and Supporting Information.