Solid-state NMR studies of aminocarboxylic salt bridges in L-lysine modified cellulose

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 (15)N and (13)C CPMAS NMR experiments on LysCel samples labeled with (15)N in the alpha-position or epsilon-position. Furthermore, (13)C-(15)N REDOR experiments were performed on LysCel where half of the aminocarboxyl groups were labeled in 1-position with 13C and the other half in alpha-position with (15)N. 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 A 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 (13)C...(15)N 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.