Dos Alexandra, Schimming Volkmar, Tosoni Sergio, Limbach Hans-Heinrich
Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany.
J Phys Chem B. 2008 Dec 11;112(49):15604-15. doi: 10.1021/jp806551u.
The interactions of the 15N-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 15N and 13C CPMAS NMR spectroscopy (CP = cross polarization and MAS = magic angle spinning). For comparison, 15N 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 15N chemical shifts of the amino groups, DFT and chemical shielding calculations were performed on isolated methylamine-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)2POOH, 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***H-X)n are formed in the absence of water. 15N 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 15N 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.
利用固态(^{15}N)和(^{13}C)交叉极化魔角旋转核磁共振波谱（CP = 交叉极化，MAS = 魔角旋转）研究了干燥固体聚-L-赖氨酸（PLL）中(^{15}N)标记氨基与各种卤化氢和含氧酸(HX)的相互作用及其与二级结构的关系。作为对比，测量了PLL水溶液的(^{15}N)核磁共振波谱随pH的变化。为了理解质子化和水合作用对氨基(^{15}N)化学位移的影响，对孤立的甲胺-酸配合物以及((CH_3NH_3(+)X(-))_n)型周期性卤化物簇进行了密度泛函理论（DFT）和化学屏蔽计算。实验和计算结果相结合表明，氨基氮与含氧酸(HX = HF)、(H_2SO_4)、(CH_3COOH)、((CH_3)_2POOH)、(H_3PO_4)、(HNO_3)以及氨基与气态(CO_2)反应形成的内源性氨基甲酸相互作用时会发生低场位移。有证据表明，在无水情况下仅形成((Lys-NH_2***H-X)_n)型氢键物种。当(H)位于氢键中心时，(^{15}N)化学位移最大，然后在完全质子化时再次降低，如在低pH水溶液中所发现的那样。相比之下，卤化氢的相互作用方式不同。它们通过许多酸碱对的相互作用形成((Lys-NH_3(+)X(-))_n)型内盐。这种盐的形成仅在β-折叠构象中才有可能。相比之下，氢键配合物的形成在β-折叠结构域以及α-螺旋结构域中都可能发生。当相互作用的卤离子尺寸从氯离子增加到碘离子以及相互作用的阴离子数量增加时，质子化铵基团的(^{15}N)化学位移会增加。因此，水合作用下铵基团观察到的高场(^{15}N)位移是相互作用的卤原子被氧原子取代的结果。
