Rubipodanin A, the first natural ndesmonomethyl rubiaceae-type cyclopeptide from rubia podantha, indicating an important role of the n9- methyl group in the conformation and bioactivity

Abstract

One new cyclic hexapeptide named rubipodanin A (1), which is the first identified natural Ndesmonomethyl Rubiaceae-type cyclopeptide, together with six known Rubiaceae-type cyclopeptides (2-7) were obtained using the TLC cyclopeptide protosite detection method with ninhydrin from the roots and rhizomes of Rubia podantha. The cyclopeptide structures were elucidated by extensive spectroscopic analysis, including 1D-NMR, 2D-NMR, IR, UV and MS. The solution conformation and biological activities of 1 and RA-V (4) were evaluated, and the results demonstrated that the N9-methyl group plays a vital role in the maintenance of the conformation and bioactivity.