Table 1.

Isotropic and anisotropic parts of HF tensors for strongly coupled histidine ¹⁵N_δ ligands detected in the (+−) quadrant, and HF couplings of weakly coupled ¹⁵N nuclei currently resolved in the (++) quadrant of ¹⁵N HYSCORE spectra of the selected Rieske-type proteins

Parameters	ARF		SDX		Rhodobacter sphaeroides Rieske protein fragment
(+−) quadrant	N_δ1^a (His44)^b	N_δ2^a (His64)^b	N_δ1^a (His44)^b	N_δ2^a (His64)^b	N_δ1^a (His131)^b	N_δ2^a (His152)^b

Q	2.44	2.07	2.64	2.11	2.40	2.13
G, MHz²	13.7	16.2	13.3	16.3	13.8	15.8
a, MHz	6.5	7.9	6.0	7.8	6.6	7.6
T, MHz	1.5	1.6	1.2	1.3	1.6	1.5

(++) quadrant^c,^d

g_z, MHz	0.43; 1.03^e		0.3; 1.03^e		0.36; 1.13^e
g_x, MHz	0.49; 1.1		0.42; 1.04		0.43; 1.22
g_y, MHz^d	0.25; 1.22		0.31; n.r.^f		n.r.^f; 1.01

references	this work		[18]		[21]

The terminology for ¹⁵N_δ1,2 is based on Ref. [18].

In R. sphaeroides cytochrome bc₁ complex, the isotropic HF constant of one of two histidine ¹⁴N_δ ligands (¹⁴a_iso ~5MHz; equivalent to ¹⁵N_δ2 in Table 1) in the presence of the Q_o-site occupant, stigmatellin, is different from the configurations in the presence of myxothiazol, suggesting that the N_δ2, at which the changes identified occur, likely belongs to His152 involved in the interaction with the Q_o-site occupants [28]. Tentative assignments of N_δ1,2 in Table 1 are made based on this previous observation in conjunction with the amino acid sequence homology, and should not be taken as definitive.

The positions of the peak maxima in this quadrant were determined with the accuracy ~0.03 MHz.

HYSCORE spectra recorded at the low- and high-field edges near the maximal and minimal g values give “single-crystal-like” patterns from the reduced cluster, whose g_z and g_x axes are directed along the external magnetic field. In contrast, the resonance condition at the intermediate g_y value is fulfilled by many different, yet well-defined orientations.

The relative ESEEM intensity of the largest coupling ~1.03 MHz in ¹⁵N-ARF is only ~70% of that of the equivalent couplings in the high-potential protein homologs including SDX (see Fig. 2B).

Not resolved.