Sensitivity improvement of amino acids by N‐terminal diisopropyloxyphosphorylation in electrospray ionization mass spectrometry

Abstract

With the introduction of a diisopropyloxyphosphoryl (DIPP) group at the N-terminus of amino acids, the signal response in electrospray ionization mass spectrometry can be enhanced by 10-100-fold in positive mode and even more in negative mode. An enhancement of proton affinity due to the N-terminal DIPP derivatization was considered to be responsible for the sensitivity improvement in positive mode, which was supported by kinetic method studies. Two possible mechanisms, i.e., an increase in gas-phase acidity due to the introduction of the electron-withdrawing DIPP group, and the effect of pH on the formation of anions of amino acids and their phosphorylated derivatives, were considered for the signal response enhancement in negative mode.

An Isotope (18O, 15N, and 2H) Technique to Investigate the Metal Ion Interactions Between the Phosphoryl Group and Amino Acid Side Chains by Electrospray Ionization Mass Spectrometry

“…This demonstrates that the metal ion having multidentate binding ability can preferentially coordinate with the carbonyl and phosphoryl oxygens and this might be the energetically preferred reaction intermediate to make the carbon atom and phosphorus atom more susceptible to nucleophilic attack. These results are consisted with literature reports [32,51,52]. Furthermore, it is worth noting that the relatively more intense rearrangement ion II is observed for Li + and Na + adducts of DIPP-dipeptides, which might be due to the metal ion binding affinity of DIPPdipeptides following the order Li + 9 Na + 9 K + [53,54].…”

A novel rearrangement in electrospray ionization multistage tandem mass spectrometry of amino acid ester cyclohexyl phosphoramidates of AZT

“…The metal ion could coordinate with both the carbonyl oxygen and phosphoryl oxygen to form a seven‐membered ring, which might help the attack of the methoxy group on phosphorus. In contrast, a proton can only attach either to the phosphoryl oxygen or to the carbonyl oxygen,15 but not both. We believe that the ring formation associated with the metal ion coordination can facilitate the migration7 of the methoxy group from the ester moiety to the phosphoryl moiety.…”

Formation of cyclic acylphosphoramidates in mass spectra of N‐monoalkyloxyphosphoryl amino acids using electrospray ionization tandem mass spectrometry

“…In our previous work, N-phosphoryl amino acids and peptides have been extensively explored as small molecule model for the study of the intrinsic relationships between phosphoryl group and biological molecules by ESI-MS. For example, N-terminal phosphorylation of amino acids could enhance the signal response of the amino acids up to 100-fold in ESI-MS. [21] Several unusual phosphoryl group (P O) participated rearrangement reactions have also been observed in gas phase, such as the phosphoryl rearrangement of P-N to P-O bonds, [22] methoxyl group migration [23] and metal ion-catalyzed carbonyl oxygen migration, [24] which imply that the phosphoryl groups have a strong affinity to the oxygen atoms. Interestingly, the gas-phase reactions through a fivemembered cyclic pentacoordinated phosphorus transition state are complimentary to those observed in liquid-phase reactions.…”