Table 1. Comparison of near-infrared iRFPs engineered from bacterial phytochromes with several far-red FPs of the GFP-like family as probes for deep-tissue imaging.

Fluorescent protein	Excitation maximum (nm)	Emission maximum (nm)	Molecular brightness a vs. iRFP713 (%)	Signal-to-background ratio at 7 mm depth in tissue vs. iRFP713 b (%)	Signal-to-background ratio at 18 mm depth in tissue vs. iRFP713 b (%)
Far-red GFP-like FPs
eqFP650	592	650	253	9	5
mNeptune	600	650	168	4	2
E2-Crimson	605	646	114	13	6
eqFP670	605	670	68	2	1
NIR FPs from bacterial phytochromes
iRFP670	643	670	205	72	74
iRFP682	663	682	165	58	59
iRFP702	673	702	124	110	133
iRFP713	690	713	100	100	100
iRFP720	702	720	93	122	119

^aDefined as a product of extinction coefficient and quantum yield. The molecular brightness values are from [19] for five iRFPs, [10] for mNeptune, [11] for E2-Crimson and [13] for eqFP650 and eqFP670.

^bData are from [19]. The signal-to-background ratios were measured for equal amounts of purified FPs imaged in a planar epifluorescence mode on an IVIS Spectrum instrument (PerkinElmer, Waltham, MA) inside a XFM-2 fluorescent phantom mouse (PerkinElmer) at the indicated depths from the mouse surface using the optimal filter channels for each FP.