A study on synthesis and gelation capability of Fmoc and Boc disubstituted cyclo(L-Lys-L-Lys)s

Fmoc and Boc disubstituted cyclo(L-Lys-L-Lys)s were synthesized using Boc-Lys(Cbz)-OH as starting material to prepare lysine cyclic dipeptide followed by the reaction with Fmoc-OSu and (Boc)₂O, respectively. Using test tube inversion method, two derivatives were found to form stable organogels in common alcoholic, methylbenzene and chlorinated solvents holding a minimum gelation concentration (MGC) in a range of 1%~5% (w). All the formed organogels exhibited thermal reversibility, which can change into transparent solutions under heating. The TEM and SEM observations of their xerogels showed that these organogelators were self-assembled into 3D network structures, such as nanofibres, nanoribbons or nanosheets. The thinner the fiber diameter was, the more transparent the organogel became. The rheological measurements revealed that the storage modulus of two kinds of gels was higher than the loss modulus, and the complex viscosity was reduced linearly with the increase of scanning frequency. Fluorescence spectroscopy of Fmoc disubstituted cyclo(L-Lys-L-Lys) (6) in methanol and 1,2-dichloroethane showed that the main emission peak of Fmoc red-shifted to 342 and 328 nm respectively, and more multiple emission peaks appeared between 350 nm and 600 nm. FT-IR results of Boc disubstituted cyclo(L-Lys-L-Lys) (7) organogels and its solid sample indicated that not only lysine cyclic dipeptide skeleton itself participated in the self-assembly, but also the hydrogen bondings between the urethane on the side-chain contributed to the organogel formation. The hydrogen bondings played mainly a role in driving the self-assembly process of gelators in methylbenzene and chlorinated solvents, whereas π-π stacking generally acted as a driving force for the self-assembly process in alcoholic solvents. A X-Ray diffraction pattern was obtained from the sample prepared with 7 in methylbenzene, showing reflections at 4.6 and 24.4 Å, attributed to the molecular simulation size and the interstrand distances between β-sheet structures, respectively.