Toyota Central R&D Labs., Inc., 41-1, Nagakute, Aichi, 480-1192, Japan.
Advanced NMR Application and Platform Team, NMR Research and Collaboration Group, NMR Science and Development Division, RIKEN SPring-8 Center, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa, 230-0045, Japan.
Sci Rep. 2020 Nov 10;10(1):19468. doi: 10.1038/s41598-020-76527-y.
Lanthanide ions (Ln) show similar physicochemical properties in aqueous solutions, wherein they exist as + 3 cations and exhibit ionic radii differences of less than 0.26 Å. A flexible linear peptide lanthanide binding tag (LBT), which recognizes a series of 15 Ln, shows an interesting characteristic in binding specificity, i.e., binding affinity biphasically changes with an increase in the atomic number, and shows a greater than 60-fold affinity difference between the highest and lowest values. Herein, by combining experimental and computational investigations, we gain deep insight into the reaction mechanism underlying the specificity of LBT3, an LBT mutant, toward Ln. Our results clearly show that LBT3-Ln binding can be divided into three, and the large affinity difference is based on the ability of Ln in a complex to be directly coordinated with a water molecule. When the LBT3 recognizes a Ln with a larger ionic radius (La to Sm), a water molecule can interact with Ln directly. This extra water molecule infiltrates the complex and induces dissociation of the Asn5 sidechain (one of the coordinates) from Ln, resulting in a destabilizing complex and low affinity. Conversely, with recognition of smaller Ln (Sm to Yb), the LBT3 completely surrounds the ions and constructs a stable high affinity complex. Moreover, when the LBT3 recognizes the smallest Ln, namely Lu, although it completely surrounds Lu, an entropically unfavorable phenomenon specifically occurs, resulting in lower affinity than that of Yb. Our findings will be useful for the design of molecules that enable the distinction of sub-angstrom size differences.
镧系元素离子（Ln）在水溶液中表现出相似的物理化学性质，它们以+3 价阳离子形式存在，并且离子半径差异小于 0.26Å。一种灵活的线性肽镧系元素结合标签（LBT），可以识别一系列 15 种 Ln，具有有趣的结合特异性特征，即结合亲和力随原子序数的增加呈双相变化，并且在最高值和最低值之间表现出大于 60 倍的亲和力差异。在此，通过实验和计算研究的结合，我们深入了解了 LBT3（一种 LBT 突变体）对 Ln 的特异性的反应机制。我们的结果清楚地表明，LBT3-Ln 结合可以分为三种，并且大的亲和力差异基于在配合物中 Ln 直接与水分子配位的能力。当 LBT3 识别具有较大离子半径的 Ln（La 到 Sm）时，水分子可以直接与 Ln 相互作用。这个额外的水分子渗透到配合物中，并导致 Asn5 侧链（配位之一）从 Ln 上解离，导致不稳定的配合物和低亲和力。相反，当识别较小的 Ln（Sm 到 Yb）时，LBT3 完全包围离子并构建稳定的高亲和力配合物。此外，当 LBT3 识别最小的 Ln，即 Lu 时，尽管它完全包围 Lu，但会发生特定的熵不利现象，导致亲和力低于 Yb。我们的发现将有助于设计能够区分亚埃大小差异的分子。
