Guo Feng, Friedman Joel M
Department of Biophysics and Physiology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461, USA.
J Am Chem Soc. 2009 Aug 12;131(31):11010-8. doi: 10.1021/ja902240j.
Gadolinium (Gd(3+)) vibronic sideband luminescence spectroscopy (GVSBLS) is used to probe, as a function of added Hofmeister series salts, changes in the OH stretching frequency derived from first-shell waters of aqueous Gd(3+) and of Gd(3+) coordinated to three different types of molecules: (i) a chelate (EDTA), (ii) structured peptides (mSE3/SE2) of the lanthanide-binding tags (LBTs) family with a single high-affinity binding site, and (iii) a calcium-binding protein (calmodulin) with four binding sites. The vibronic sideband (VSB) corresponding to the OH stretching mode of waters coordinated to Gd(3+), whose frequency is inversely correlated with the strength of the hydrogen bonding to neighboring waters, exhibits an increase in frequency when Gd(3+) becomes coordinated to either EDTA, calmodulin, or mSE3 peptide. In all of these cases, the addition of cation chloride or acetate salts to the solution increases the frequency of the vibronic band originating from the OH stretching mode of the coordinated waters in a cation- and concentration-dependent fashion. The cation dependence of the frequency increase scales with charge density of the cations, giving rise to an ordering consistent with the Hofmeister ordering. On the other hand, water Raman spectroscopy shows no significant change upon addition of these salts. Additionally, it is shown that the cation effect is modulated by the specific anion used. The results indicate a mechanism of action for Hofmeister series ions in which hydrogen bonding among hydration shell waters is modulated by several factors. High charge density cations sequester waters in a configuration that precludes strong hydrogen bonding to neighboring waters. Under such conditions, anion effects emerge as anions compete for hydrogen-bonding sites with the remaining free waters on the surface of the hydration shell. The magnitude of the anion effect is both cation and Gd(3+)-binding site specific.
钆（Gd(3+)）振动边带发光光谱法（GVSBLS）用于探究，作为添加的霍夫迈斯特系列盐的函数，源自Gd(3+)水溶液中第一壳层水以及与三种不同类型分子配位的Gd(3+)的OH伸缩频率的变化：（i）一种螯合剂（EDTA），（ii）具有单个高亲和力结合位点的镧系元素结合标签（LBTs）家族的结构化肽（mSE3/SE2），以及（iii）具有四个结合位点的钙结合蛋白（钙调蛋白）。与与Gd(3+)配位的水的OH伸缩模式相对应的振动边带（VSB），其频率与与相邻水的氢键强度呈负相关，当Gd(3+)与EDTA、钙调蛋白或mSE3肽配位时，频率会增加。在所有这些情况下，向溶液中添加阳离子氯化物或醋酸盐会以阳离子和浓度依赖的方式增加源自配位水的OH伸缩模式的振动带的频率。频率增加的阳离子依赖性与阳离子的电荷密度成比例，产生与霍夫迈斯特排序一致的顺序。另一方面，水拉曼光谱在添加这些盐后没有显示出显著变化。此外，还表明阳离子效应受到所用特定阴离子的调节。结果表明了霍夫迈斯特系列离子的作用机制，其中水合壳层水之间的氢键受到多种因素的调节。高电荷密度阳离子以一种排除与相邻水形成强氢键的构型隔离水。在这种情况下，阴离子效应出现是因为阴离子与水合壳层表面剩余的自由水竞争氢键位点。阴离子效应的大小是阳离子和Gd(3+)结合位点特异性的。
