Direct Measurement of the Mn(II) Hydration State in Metal Complexes and Metalloproteins through ¹⁷O NMR Line Widths

Abstract

Here we describe a simple method to estimate the inner-sphere hydration state of the Mn(II) ion in coordination complexes and metalloproteins. The linewidth of bulk H217O is measured in the presence and absence of Mn(II) as a function of temperature, and transverse 17O relaxivities are calculated. It is demonstrated that the maximum 17O relaxivity is directly proportional to the number of inner-sphere water ligands (q). Using a combination of literature data and experimental data for twelve Mn(II) complexes, we show that this method provides accurate estimates of q with an uncertainty of ±0.2 water molecules. The method can be implemented on commercial NMR spectrometers working at fields of 7T and higher. The hydration number can be obtained for micromolar Mn(II) concentrations. We show that the technique can be extended to metalloproteins or complex:protein interactions. For example, Mn(II) binds to the multi-metal binding site A on human serum albumin with two inner-sphere water ligands that undergo rapid exchange (1.06 × 108 s−1 at 37 °C). The possibility of extending this technique to other metal ions such as Gd(III) is discussed.

A Bisamide Derivative of [Mn(1,4‐DO2A)] – Solution Thermodynamic, Kinetic, and NMR Relaxometric Studies

“…Caravan recently reported a method to estimate q (±0.2) on the basis of the experimental maximum 17 O transverse relaxivity (r 0 2max ) and a stand- ard value of the hyperfine coupling constant. [24] By using this approach, we estimated q = 0.8 ± 0.2, in perfect agreement with our hypothesis.…”

“…[6,25] The crystallographic data revealed the formation of a dimeric [Mn(1,4-DO2A)] 2 complex in which the coordination geometry around each Mn II ion can be described as a distorted monocapped trigonal prism in which the two triangular faces formed by two ring N atoms and the carboxylate O atoms are nearly parallel (the distortion angle is 1.8°) and the capping positions are occupied by two bridging carboxylate O donor atoms. [24] The optimized geometries of the complexes [Mn(1,4-DO2A)] and [Mn(1,4-DO2A)(H 2 O)] obtained from DFT calculations highly resemble that found in the solid state, that is, a trigonal prism and a monocapped trigonal prism constructed from two parallel (the distortion angle is 5.4°) triangular faces defined by two ring N atoms and the carboxylate O atoms, respectively. [6] We assume that the predominant species of [Zn(1,4- 2+ obtained at 273 K, the signals associated with the protons of the cyclen ring give rise to unresolved multiplets, whereas an AB multiplet (-CH 2 -) and two singlets (-CH 3 ) are attributed to the protons of the acetamide pendant arms.…”

A Manganese Alternative to Gadolinium for MRI Contrast

“…46,47 The water exchange rate at 37 °C for [Mn(PyC3A)(H 2 O)] - is k ex = 1.0 × 10 8 s -1 ; associated kinetic parameters are listed in Table 2.…”

Amplifying the Sensitivity of Zinc(II) Responsive MRI Contrast Agents by Altering Water Exchange Rates

“…31 Interestingly, it has also been shown that the rate of water exchange in various Ln(III) derivatives of MS-325 is quite sensitive to relatively minor structural differences between albumin from different mammalian sources 35 and when various Mn(II) complexes bind to albumin. 36 $$R_1^{\text{pobs}}={10}^3\times \left\{false(r_1^f·c_{1}false)+\frac{1}{2}false(r_1^c-r_1^{f}false)\times false(n·c_{\text{HSA}}+c_1+K_A^{-1}false)-\sqrt{{false(n.c_{\text{HSA}}+c_1+K_A^{-1}false)}^2-4·n·c_{\text{HSA}}·C_{1}false)}\right\}$$For the Zn(II) sensors presented here, HSA binding takes place mostly by the electrostatic interaction between the Zn(II)-DPA subunits and the tyrosine residue Y411 in the pocket site 2. 37 The binding affinity of each Zn(II) sensor to HSA was estimated by proton relaxation enhancement (PRE) titrations, a relaxometric technique commonly used to determine the dissociation constants ( K D = 1/ K A ) for binding of Gd(III) complexes to HSA.…”