Thalassospiramides A and B, immunosuppressive peptides from the marine bacterium Thalassospira sp.

Two new cyclic peptides, thalassospiramides A and B (1 and 2), were isolated from a new member of the marine α-proteobacterium Thalassospira. The thalassospiramides, the structures of which were assigned by combined spectral and chemical methods, bear unusual γ-amino acids and show immunosuppressive activity in an interleukin-5 production inhibition assay (IC50 = 5 μM for thalassospiramide B).