Solid-State NMR Studies of Aminocarboxylic Salt Bridges in l-Lysine Modified Cellulose
Ricardo Manríquez†‡, Fernando A. López-Dellamary‡, Jaroslaw Frydel†, Thomas Emmler†, Hergen Breitzke§, Gerd Buntkowsky§, Hans-Heinrich Limbach† and Ilja G. Shenderovich*†#
J. Phys. Chem. B, 2009, 113 (4), pp 934–940
Abstract: LysCel is a cellulose-based material in which l-lysine molecules are grafted with their amino side chains to the cellulose hydroxyl groups. This modification increases considerably the mechanical strength and resistance of cellulosic structures toward water. It has been attributed to the formation of double salt bridges between lysine aminocarboxyl groups in the zwitterionic state. In order to characterize this unusual structure, we have performed high-resolution solid-state 15N and 13C CPMAS NMR experiments on LysCel samples labeled with 15N in the α-position or ε-position. Furthermore, 13C−15N REDOR experiments were performed on LysCel where half of the aminocarboxyl groups were labeled in 1-position with 13C and the other half in α-position with 15N. The comparison with the 13C and 15N chemical shifts of l-leucine lyophilized at different pH shows that the aminocarboxyl groups of LysCel are indeed zwitterionic. The REDOR experiments indicate distances of about 3.5 Å between the carboxyl carbon and the nitrogen atoms of different aminocarboxyl groups, indicating that the latter are in close contact with each other. However, the data are not compatible with isolated aminocarboxyl dimers but indicate the assembly of zwitterionic aminocarboxyl dimers either in a flat ribbon or as tetramers, exhibiting similar intra- and interdimer 13C···15N distances. This interaction of several aminocarboxyl groups is responsible for the zwitterionic state, in contrast to the gas phase, where amino acid dimers exhibiting two OHN hydrogen bonds are neutral.