Acid-Induced Amino Side-Chain Interactions and Secondary Structure of Solid Poly-L-lysine Probed by 15N and 13C Solid State NMR and ab Initio Model Calculations

The acid-base and base-base interactions of the N-15-tabeled 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 N-15 and C-13 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, CH3COOH, H3PO4, H2SO4, or HNO3 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 N-15 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. C-13 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 H2O catalyzes the interconversion of the ammonium and amino groups into a mixture of 1:1, 1:2 and 1:3 complexes. Finally, the N-15 NMR assignments were assisted by DFT calculations on methylamine-acid model complexes.