Table 2.

Comparison of Fe^II, Co^II, and Ni^II paraCEST Agents.

Complex	N[a]	m[b]	Chemical Shift (ppm)	kex[c] (s−1)	T1 relaxivity[d] (mM−1·s−1)	% CEST[e]
[Fe(TCMT)]2+	6	3	69	240	0.210	19
[Fe(AMPT)]2+	6	6	6.5	--	0.021	18
[Fe(BZT)]2+	3	3	53	-	-	13
[Fe(STHP)]2+	4	4	54	3000	0.2321	17
[Fe(TCMC)]2+	8f	4f	50	400	0.2631	23
[Ni(TCMT)]2+	6	3	76	364	0.210	13
[Ni(CCRM)]2+	8g	1g	76	328	0.097	14
[Ni(NOPE)]2+	4	2	72	241	0.012	39
[Co(TCMT)]2+	6	3	32	890	0.125	33
[Co(TCMC)]2+	8	4	45	300	0.096	21
[Co(NOPE)]2+	4	2	59	240	0.038	38
[Co(CCRM)]2+	8g	1g	112	510	0.008	29
[Co(TPT)]2+	3	3	135	9200	0.093	21

^[a] Total number of exchangeable protons on the paraCEST agent.

^[b] Number of magnetically equivalent protons that give rise to CEST peak.

^[c] Rate constant (k_ex) for proton exchange at 37 ºC.

^[d] T₁ relaxivity on a 4.7 T MR scanner, at 37 ºC.

^[e]% CEST on an 11.7 T NMR spectrometer on 10 mM complex, 20 mM HEPES pH 7.4, 100 mM NaCl, B₁=24 μT [77]presaturation 2s at 37 ºC, except for [Fe(AMPT)]²⁺ (4 mM, pH 7.0, B₁ = 11 μT, 4s), [Fe(BZT)]²⁺ (3 mM, pH 6.3, B₁ = 23 μT, 3s ), [Co(TPT)]²⁺ (8 mM, pH 7.0, B₁ = 23 μT, 3s).

^ffor coordination of all four amide pendent groups.

^gfor coordination of two pendent groups with each amide proton magnetically inequivalent.