Synthesis and conformation of deuterated spermidine for investigating weak interaction with polyanionic biomolecules

Polyamines such as spermidine and spermine are known to interact with polyanionic biomolecules under physiological conditions. Conformation analysis of these highly flexible acycles is virtually unprecedented due to the lack of appropriate methodologies. Spin–spin coupling constants, which are often utilized for acyclic systems, are rather uninformative for polyamines due to unresolved 1H NMR signals of their tri- and tetra-methylene moieties. We attempted to solve this problem by synthesizing diastereospecifically deuterated spermidine, (1S∗,2S∗)-1,8-diamino-4-azaoctane-1,2,3,3,5,5,6,6,7,7,8,8-d12. To evaluate its utility, 1H–1H spin coupling constants were measured as trihydrochloride or tripolyphosphate salt. The relevant coupling constant markedly decreased upon complexation to tripolyphosphate. Under neutral pH, where the net charge of tripolyphosphate became tetravalent, the coupling constant was small; as pH was lowered to 1.4, where the charge was divalent, the constant increased significantly. These data clearly indicate that, when interacting with polyanions, spermidine undergoes conformational changes to increase bent conformation, which could be effectively monitored by the deuterated spermidine.