Acid−Base Interactions and Secondary Structures of Poly-l-Lysine Probed by ¹⁵N and ¹³C Solid State NMR and Ab initio Model Calculations

Abstract

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.

NMR Crystallography of a Carbanionic Intermediate in Tryptophan Synthase: Chemical Structure, Tautomerization, and Reaction Specificity

“…We also note that the ε-nitrogen chemical shift of βLys87 appears to track the expected correlation with protonation state established in aqueous solution. 97 In contrast, the chemical shift of poly- l -lysine (PLL) salts formed with various acids shows a much wider range of chemical shifts. 97 A priori, there is no reason to expect that lysine residues buried in the interior of a protein would show the same chemical shift dependence with ionization state as those exposed to aqueous solution, yet this appears to be the case experimentally for the TS active site and is reproduced by our first principles computational predictions.…”

Multi-responsive cellulose nanocrystal–rhodamine conjugates: an advanced structure study by solid-state dynamic nuclear polarization (DNP) NMR

“…5a 0 ). 65,66 The 15 N NMR spectrum of CNC-RhB-130 1C shows the disappearance of the spirolactam signal at À223 ppm (I) and the appearance of a new signal at À205 ppm (VI) corresponding to amide groups. In addition, CNC-RhB-130 1C displays a second new signal at À355 ppm (VII).…”

The structure and IR signatures of the arginine-glutamate salt bridge. Insights from the classical MD simulations

“…6 In particular, salt bridges stabilize α-helical structures [16][17][18][19][20] and β-sheets. [21][22][23] About 40% of ion-pairs within proteins involve arginine-carboxylate interactions, 24,25 e.g., the arginine-glutamate (Arg-Glu) and arginine-aspartate side-chain interactions. By extracting orientation information a) Author to whom correspondence should be addressed.…”