Lifson S, Felder C E, Shanzer A
Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel.
J Biomol Struct Dyn. 1984 Dec;2(3):641-61. doi: 10.1080/07391102.1984.10507598.
The alkali-ion binding properties of two natural depsipeptide ion carriers, enniatin B (EnB) and valinomycin (VM), are examined and compared by the empirical force field method. While VM has been shown to bind preferentially K+, Rb+, and Cs+ over Na+ in most solvents, EnB is considerably less specific. We find that EnB forms two kinds of complexes, internal and external. In internal complexes, the ion binds to all six carbonyl oxygens, while in external ones, only three oxygens, preferentially those of the D-hydroxy-isovaleryl residues, are bound. The size of the internal cavity is best suited for Na+, while K+ and Rb+ squeeze in asymmetrically by distorting the molecule, and Cs+ not at all. External binding is much less specific. Since internal complexes possess much higher strain energies than external ones, the latter may be at least as stable as the former, even in fairly non-polar solvents. VM is calculated to bind only internally, and with much less strain energy than EnB. The size of its internal cavity is well suited for binding the ions K+, Rb+, and Cs+, but is too big for Na+. The difference between the binding energies of Na+ and K+ is much smaller than that between the corresponding hydration enthalpies, thus explaining the binding preference for the latter ion.
通过经验力场方法研究并比较了两种天然缩肽离子载体恩镰孢菌素B（EnB）和缬氨霉素（VM）的碱金属离子结合特性。虽然在大多数溶剂中，VM已被证明对K⁺、Rb⁺和Cs⁺的结合优先于Na⁺，但EnB的特异性要低得多。我们发现EnB形成两种配合物，即内部配合物和外部配合物。在内部配合物中，离子与所有六个羰基氧结合，而在外部配合物中，只有三个氧原子，优先是D-羟基异戊酰基残基的氧原子与离子结合。内部空腔的大小最适合Na⁺，而K⁺和Rb⁺通过扭曲分子不对称地挤入，Cs⁺则根本无法进入。外部结合的特异性要低得多。由于内部配合物的应变能比外部配合物高得多，因此即使在相当非极性的溶剂中，后者可能至少与前者一样稳定。计算得出VM仅形成内部配合物，且其应变能比EnB小得多。其内部空腔的大小非常适合结合K⁺、Rb⁺和Cs⁺离子，但对Na⁺来说太大了。Na⁺和K⁺结合能之间的差异远小于相应水合焓之间的差异，从而解释了对后一种离子的结合偏好。
