The ternary complex of Mn²⁺, synthetic decapeptide DP1 (DEHGTAVMLK), and orthophosphate is a superb antioxidant

Mn²⁺ coordinated by orthophosphate (Pi), metabolites, or peptides acts as a superoxide dismutase (SOD), and these Mn antioxidant complexes are universally accumulated in extremely radiation-resistant cell types across the tree of life. This behavior prompted design of decapeptide DP1 (DEHGTAVMLK) as a Mn²⁺ ligand, and development of a highly potent Mn²⁺-antioxidant (MDP) containing [Pi] = 25 mM, and [DP1] = 3 mM, the ratio found in the radioresistant bacterium Deinococcus radiodurans, with [Mn²⁺] = 1 mM. MDP is an exceptional antioxidant, both in vitro and in vivo, and has reinvigorated the development of radiation-inactivated whole-cell vaccines. This study investigates the nature of the active Mn²⁺ complex in MDP. We measure the affinity of DP1 for the substitutionally labile Mn²⁺ ion using isothermal-titration calorimetry (ITC) and use changes in the Mn²⁺ solution EPR spectrum to determine affinities of Mn²⁺ for DP1 and for Pi, and to monitor Mn²⁺ ligation while titrated with the fixed Pi/DP1 ratio of MDP, 25/3, using ENDOR/ESEEM to characterize DP1 ligation to Mn²⁺. In parallel, ¹H NMR of DP1 was used to monitor binding interactions between Pi and DP1, and DP1 binding to the diamagnetic Ca²⁺. We report: i) DP1 forms an extremely weak, dynamic Mn²⁺ complex (K_a ≈ 40 M^-1) ii) Mn²⁺ binds Pi much more strongly (K_a ≈ 390 M^-1) as shown previously, but iii) DP1 and Pi jointly bind to Mn²⁺ in MDP to form a ternary Mn²⁺ (Pi) (DP1) complex with greater formation-constant than Pi alone (K_a^app ≈ 670 M^-1). It is this ternary complex that is the superb antioxidant in MDP.