1 research outputs found
Effect of Metals in Biomimetic Dimetal Complexes on Affinity and Gas-Phase Protection of Phosphate Esters
Although the biomimetic dimetal complex
[LGa<sub>2</sub>(OH)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>3+</sup> [L = 2,6-bisÂ((<i>N</i>,<i>N</i>′-bisÂ(2-picolyl)Âamino)Âmethyl)-4-tertbutylphenolate]
provides efficient protection against phosphate loss in phosphopeptides
upon collision-induced dissociation tandem mass spectrometry (CID
MS/MS), the underlying mechanism remains unknown. Here, we explored
the mechanism in detail and investigated the selective binding to
phosphate groups in solution. Dimetal complexes containing combinations
of Ga<sup>3+</sup>, In<sup>3+</sup>, Fe<sup>3+</sup>, Co<sup>3+</sup>, Zn<sup>2+</sup>, Cu<sup>2+</sup>, and V<sup>2+</sup> were reacted
with HPO<sub>4</sub><sup>2–</sup>, phosphoserine, and a phosphopeptide
(FQÂpSÂEEQÂQQTÂEDEÂLQÂDK, abbreviated
“βcas”) and studied with isothermal titration
calorimetry (ITC), CID MS/MS, and density functional theory (DFT). <i>K</i><sub>a</sub> for HPO<sub>4</sub><sup>2–</sup> binding
scaled with the metal charge and was 35-fold larger for [LGa<sub>2</sub>(OH)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>3+</sup> (3.08
± 0.31 × 10<sup>6</sup> M<sup>–1</sup>) than for
[LZn<sub>2</sub>(HCOO)<sub>2</sub>]<sup>+</sup>. CID MS/MS of [LGa<sub>2</sub>(βcas)]<sup><i>n</i>+</sup> revealed protection
against phosphate detachment (<3% of the total ion intensity).
Phosphate detachment from βcas was 22–40% and increased
to 42–71% when bound to dimetal complexes of lower charge than
{LGa<sub>2</sub>}<sup>5+</sup>. CID data suggests that facile metal–phosphate
dissociation is associated with proton transfer from the intermediate
oxazoline ring formed in the phosphopeptide to the metal–phosphate
complex. The observed phosphate stabilization was attributed to a
significant reduction in the gas-phase basicity (GB) of the phosphate
group when bound to {LGa<sub>2</sub>}<sup>5+</sup>/{LIn<sub>2</sub>}<sup>5+</sup> complex cores. Absence of proton transfer results
in formation of an ion–zwitterion intermediate with a greater
dissociation threshold. This hypothesis is supported by DFT calculations
for [LGa<sub>2</sub>(PO<sub>4</sub>)]<sup>2+</sup>, [LGaZnÂ(PO<sub>4</sub>)]<sup>+</sup>, [LZn<sub>2</sub>(PO<sub>4</sub>)], and 2,4-dimethyl-3-oxazoline
showing that [LGa<sub>2</sub>(PO<sub>4</sub>)]<sup>2+</sup> is the
only compound with a substantial lower GB (321 kJ/mol less) than 2,4-dimethyl-3-oxazoline