Dos Alexandra, Schimming Volkmar, Huot Monique Chan, Limbach Hans-Heinrich
Institut für Chemie und Biochemie, Takustrasse 3, Freie Universität Berlin, D-14195 Berlin, Germany.
J Am Chem Soc. 2009 Jun 10;131(22):7641-53. doi: 10.1021/ja901082a.
The acid-base and base-base interactions of the (15)N-labeled side-chain amino groups of dry solid poly-L-lysine (PLL) and the consequences for the secondary structure have been studied using high-resolution solid state (15)N and (13)C CPMAS NMR spectroscopy. In a previous study we had shown that at acid/base ratios of 1 per amino group the halogen acids HI, HCl and HBr form PLL salts in the beta-pleated sheet but not in the alpha-helical structure, whereas HF and various oxygen acids form 1:1 acid-base hydrogen-bonded complexes in both secondary structures. In the present study we performed NMR experiments at reduced acid/base ratios in order to elucidate whether also 1:2 and 1:3 acid-base complexes are formed under these conditions. Generally, the PLL samples containing HF, HBr, HCl, HI, CH(3)COOH, H(3)PO(4), H(2)SO(4), or HNO(3) were obtained by lyophilization at different pH. For comparison, samples were also obtained by letting dry acid-free PLL interact with gaseous HCl. In a theoretical section we first study the probability of the different acid-base complexes as a function of the acid/base ratio and the equilibrium constants of the complex formation. Using this information, the (15)N NMR spectra of acid doped PLL obtained were analyzed and assigned. Indeed, evidence for the formation of 1:2 and 1:3 acid-base complexes at lower acid/base ratios could be obtained. Moreover, the salt structures of the halides of PLL are already destroyed at acid/base ratios of about 0.8. By contrast, when acid-free poly-L-lysine is exposed to HCl gas, a biexponential conversion of amino groups into ammonium groups is observed without formation of 1:2 and 1:3 complexes. (13)C NMR reveals that the beta-pleated sheet environments of acid-free PLL react rapidly with HCl, whereas the alpha-helices first have to be converted in a slow reaction to beta-pleated sheets before they can react. Interestingly, after partial doping with HCl, exposure to gaseous H(2)O catalyzes the interconversion of the ammonium and amino groups into a mixture of 1:1, 1:2 and 1:3 complexes. Finally, the (15)N NMR assignments were assisted by DFT calculations on methylamine-acid model complexes.
利用高分辨率固态(^{15}N)和(^{13}C)交叉极化魔角旋转（CPMAS）核磁共振光谱，研究了干燥固体聚-L-赖氨酸（PLL）中(^{15}N)标记侧链氨基的酸碱和碱碱相互作用及其对二级结构的影响。在之前的一项研究中，我们已经表明，在每个氨基的酸碱比为1时，氢卤酸HI、HCl和HBr在β-折叠结构中形成PLL盐，但在α-螺旋结构中不形成，而HF和各种含氧酸在两种二级结构中都形成1:1酸碱氢键复合物。在本研究中，我们在降低的酸碱比下进行了核磁共振实验，以阐明在这些条件下是否也会形成1:2和1:3酸碱复合物。一般来说，含有HF、HBr、HCl、HI、CH₃COOH、H₃PO₄、H₂SO₄或HNO₃的PLL样品是通过在不同pH值下冻干获得的。为了进行比较，还通过让干燥的无酸PLL与气态HCl相互作用获得了样品。在理论部分，我们首先研究了不同酸碱复合物形成的概率与酸碱比和复合物形成平衡常数的函数关系。利用这些信息，对获得的酸掺杂PLL的(^{15}N)核磁共振光谱进行了分析和归属。确实，在较低的酸碱比下可以获得形成1:2和1:3酸碱复合物的证据。此外，PLL卤化物的盐结构在酸碱比约为0.8时就已经被破坏。相比之下，当无酸聚-L-赖氨酸暴露于HCl气体时，观察到氨基向铵基的双指数转化，而没有形成1:2和1:3复合物。(^{13}C)核磁共振显示，无酸PLL的β-折叠结构环境与HCl反应迅速，而α-螺旋首先必须在一个缓慢的反应中转化为β-折叠结构，然后才能反应。有趣的是，在用HCl部分掺杂后，暴露于气态H₂O会催化铵基和氨基相互转化为1:1、1:2和1:3复合物的混合物。最后，通过对甲胺-酸模型复合物的密度泛函理论（DFT）计算辅助进行了(^{15}N)核磁共振归属。
