Koenig S H, Brown R D, Lindstrom T R
Biophys J. 1981 Jun;34(3):397-408. doi: 10.1016/S0006-3495(81)84858-8.
It is now more than 20 years since Davidson and collaborators (1957, Biochim. Biophys, Acta. 26:370-373; J. Mol. Biol. 1:190-191) applied the theoretical ideas of Bloembergen et al. (1948. Phys. Rev. 73:679-712) on outer sphere magnetic relaxation of solvent protons to studies of solutions of methemoglobin. From then on, there has been debate regarding the relative contributions to paramagnetic solvent proton relaxation by inner sphere (ligand-exchange) effects and by outer sphere (diffusional) effects in methemoglobin solutions. Gupta and Mildvan (1975. J. Biol. Chem 250:146-253) extended the early measurements, attributed the relatively small paramagnetic effects to exchange with solvent of the water ligand of the heme-Fe3+ ion, and interpreted their data to indicate cooperativity and an alkaline Bohr effect in the presence of inositol hexaphosphate. They neglected the earlier discussions entirely, and made no reference to outer sphere effects. We have measured the relaxation rate of solvent protons as a function of magnetic field for solutions of methemoglobin, under a variety of conditions of pH and temperature, and have given careful consideration to the relatively large diamagnetic corrections that are necessary by making analogous measurements on oxyhemoglobin, carbonmonoxyhemoglobin, and cyano- and azide-methemoglobin. (The latter two, because of their short electronic relaxation times, behave as though diamagnetic). We show that the paramagnetic contribution to solvent relaxation can be dominated by outer sphere effects, a result implying that many conclusions, including those of Gupta and Mildvan, require reexamination. Finally, we present data for fluoro-methemoglobin, which relaxes solvent protons an order of magnitude better than does methemoglobin. Here one has a startling breakdown of the dogma that has been the basis for interpreting many ligand-replacement studies; in contrast to the prevailing view that replacement of a water ligand of a protein-bound paramagnetic ion by another ligand should decrease relaxation rates, replacement of H2O by F- increases the relaxation rate drastically. The data can all be reconciled, however, with what is anticipated from knowledge of ligand interactions in the heme region.
自戴维森及其合作者（1957年，《生物化学与生物物理学报》26:370 - 373；《分子生物学杂志》1:190 - 191）将布洛姆贝根等人（1948年，《物理评论》73:679 - 712）关于溶剂质子外球磁弛豫的理论观点应用于高铁血红蛋白溶液的研究以来，已经过去了20多年。从那时起，关于在高铁血红蛋白溶液中，内球（配体交换）效应和外球（扩散）效应在顺磁溶剂质子弛豫中相对贡献的争论就一直存在。古普塔和米尔德万（1975年，《生物化学杂志》250:146 - 253）扩展了早期的测量，将相对较小的顺磁效应归因于血红素 - Fe3 + 离子的水配体与溶剂的交换，并解释他们的数据表明在存在肌醇六磷酸时存在协同性和碱性玻尔效应。他们完全忽略了早期的讨论，并且没有提及外球效应。我们在各种pH和温度条件下，测量了高铁血红蛋白溶液中溶剂质子的弛豫速率随磁场的变化，并通过对氧合血红蛋白、碳氧血红蛋白以及氰化物和叠氮化物高铁血红蛋白进行类似测量，仔细考虑了所需的相对较大的抗磁校正。（后两者由于其短的电子弛豫时间，表现得如同抗磁性物质）。我们表明，对溶剂弛豫的顺磁贡献可能由外球效应主导，这一结果意味着许多结论，包括古普塔和米尔德万的结论，都需要重新审视。最后，我们给出了氟高铁血红蛋白的数据，它使溶剂质子弛豫的程度比高铁血红蛋白好一个数量级。在这里，一直作为解释许多配体置换研究基础的教条出现了惊人的瓦解；与普遍观点相反，即蛋白质结合的顺磁离子的水配体被另一种配体取代应该会降低弛豫速率，H2O被F - 取代却极大地增加了弛豫速率。然而，这些数据都可以与从血红素区域配体相互作用的知识中预期的情况相协调。
