Acid-Base Interactions and Secondary Structures of Poly-L-Lysine Probed by 15N and 13C Solid State NMR and Ab initio Model Calculations
Acid-Base Interactions and Secondary Structures of Poly-L-Lysine Probed by 15N and 13C Solid State NMR and Ab initio Model Calculations
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DOI：
10.1021/jp806551u
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发表时间：
2008-12-11
影响因子：
3.3
通讯作者：
Limbach, Hans-Heinrich
中科院分区：
文献类型：
--
作者：
Dos, Alexandra;Schimming, Volkmar;Limbach, Hans-Heinrich
本文用固体N-15和C-13 CPMAS NMR谱（CP =交叉极化，MAS =魔角自旋）研究了N-15-标记的固体聚-L-赖氨酸（PLL）的氨基与各种卤酸和含氧酸HX的相互作用及其与二级结构的关系。为了进行比较，N-15 NMR谱的PLL的水溶液的测量作为pH值的函数。为了了解质子化和水合作用的氨基的N-15化学位移的影响，DFT和化学屏蔽计算进行隔离的甲基胺-酸复合物和周期性的卤化物簇的类型（CH 3 NH 3 +X-）（n）。结合实验和计算结果揭示了低场位移的氨基氮与含氧酸HX = HF，H2SO 4，CH 3COOH，（CH 3）（2）POOH，H3 PO 4，HNO 3，和内部氨基甲酸反应形成的氨基与气态CO2相互作用后。得到的证据是，在不存在水的情况下，仅形成（Lys-NH 2中心点中心点H-X）（n）类型的氢键物种。N-15化学位移是最大的H位于氢键中心，然后再次减少后，充分质子化，发现在低pH值的水溶液。相比之下，卤酸以不同的方式相互作用。它们通过许多酸碱对的相互作用形成（Lys-NH3+X-）（n）型内盐。这种盐的形成只可能在β-折叠构象中。相比之下，氢键复合物的形成可以发生在β-折叠结构域以及α-螺旋结构域中。质子化铵基团的N-15化学位移增加时，相互作用的卤素阴离子的大小从氯离子到碘离子增加，当相互作用的阴离子的数量增加。因此，所观察到的高场15 N位移的铵基团水合作用的结果取代相互作用的卤素原子的氧原子。
The interactions of the N-15-labeled amino groups of dry solid poly-L-lysine (PLL) with various halogen and oxygen acids HX and the relation to the secondary structure have been studied using solid-state N-15 and C-13 CPMAS NMR spectroscopy (CP = cross polarization and MAS = magic angle spinning). For comparison, N-15 NMR spectra of an aqueous solution of PLL were measured as a function of pH. In order to understand the effects of protonation and hydration on the N-15 chemical shifts of the amino groups, DFT and chemical shielding calculations were performed on isolated methyl amine-acid complexes and on periodic halide clusters of the type (CH3NH3+X-)(n). The combined experimental and computational results reveal low-field shifts of the amino nitrogens upon interaction with the oxygen acids HX = HF, H2SO4, CH3COOH, (CH3)(2)POOH, H3PO4, HNO3, and internal carbamic acid formed by reaction of the amino groups with gaseous CO2. Evidence is obtained that only hydrogen-bonded species of the type (Lys-NH2 center dot center dot center dot H-X)(n) are formed in the absence of water. N-15 chemical shifts are maximum when H is located in the hydrogen bond center and then decrease again upon full protonation, as found for aqueous solution at low pH. By contrast, halogen acids interact in a different way. They form internal salts of the type (Lys-NH3+X-)(n) via the interaction of many acid-base pairs. This salt formation is possible only in the beta-sheet conformation. By contrast, the formation of hydrogen-bonded complexes can occur both in beta-sheet domains as well as in alpha-helical domains. The N-15 chemical shifts of the protonated ammonium groups increase when the size of the interacting halogen anions is increased from chloride to iodide and when the number of the interacting anions is increased. Thus, the observed high-field 15N shift of ammonium groups upon hydration is the consequence of replacing interacting halogen atoms by oxygen atoms.