SCX-tip-aided LC-MS detection of active ricin via oligonucleotide substrates for depurination kinetics

The accurate and sensitive detection of active biotoxin proteins and the determination of their kinetics are vital for the upsurge of chemical attacks but still limited. Herein, we report a liquid chromatography-tunable ultraviolet spectroscopic-quadrupole mass spectrometric detection (LC-TUV-QDa) method of active ricin. This method has the advantage of the accurate quantification of active ricin in decreased oligonucleotide (oligo) substrates as well as the produced adenine, in which the QDa detection offers the confirmative evidence of oligo and adenine products. We invented a strong cation exchange (SCX)-tip sample pretreatment way to facilitate the requirement of clean product injection without any fouling proteins. After full-method validation, a wide linear range of 1-5000 ng mL^-1 was obtained with a high sensitivity of 1 ng mL^-1 active ricin based on the most preferable deoxynucleobase-hybrid RNA (Rd) substrate, Rd12, and without any enrichment. We also fully depicted the kinetic parameters of ricin and its six Rd or RNA substrates and evaluated 11 nucleobase-modified oligos as substrates based on Rd12. Further, we fulfilled an improved molecular docking analysis and revealed that the binding of Rd12 to ricin was more likely to occur at pH 7.4 (typical in vitro and in vivo conditions) than at pH 4.0 (typical ex vitro conditions). With the aid of SCX-tip as a microenzymatic reactor, we can exert the catalytic activity of ricin as N-glycosidase in pH 7.4 toward its Rd12 substrate, with a comparable catalytic efficiency at pH 4.0. This is the first successful implementation of an ex vitro experiment toward oligo substrates at neutral pH, standing on the shoulder of plenty of previously reported efforts all performed under acidic conditions. This method will provide a new and powerful way to detect active ricin when tackling relevant problems in public safety and security.