First Direct Evidence of Interpartner Hydride/Deuteride
Exchanges for Stored Sodiated Arginine/Fructose-6-phosphate Complex
Anions within Salt-Solvated Structures
Mass spectrometric
investigations of noncovalent binding between
low molecular weight compounds revealed the existence of gas-phase
(GP) noncovalent complex (NCC) ions involving zwitterionic structures.
ESI MS is used to prove the formation of stable sodiated NCC anions
between fructose (F6P) and arginine (R) moieties. Theoretical calculations
indicate a folded solvated salt (i.e., sodiated carboxylate interacting
with phosphate) rather than a charge-solvated form. Under standard
CID conditions, [(F6P+R-H+Na)-H]− competitively
forms two major product ions (PIs) through partner splitting [(R-H+Na)
loss] and charge-induced cross-ring cleavage while preserving the
noncovalent interactions (noncovalent product ions (NCPIs)). MS/MS
experiments combined with in-solution proton/deuteron exchanges (HDXs)
demonstrated an unexpected labeling of PIs, i.e., a correlated D-enrichment/D-depletion.
An increase in activation time up to 3000 ms favors such processes
when limited to two H/D exchanges. These results are rationalized
by interpartner hydride/deuteride exchanges (⟨HDX⟩)
through stepwise isomerization/dissociation of sodiated NCC-d11 anions.
In addition, the D-enrichment/D-depletion discrepancy is further explained
by back HDX with residual water in LTQ (selective for the isotopologue
NCPIs as shown by PI relaxation experiments). Each isotopologue leads
to only one back HDX unlike multiple HDXs generally observed in GP.
This behavior shows that NCPIs are zwitterions with charges solvated
by a single water molecule, thus generating a back HDX through a relay
mechanism, which quenches the charges and prevents further back HDX.
By estimating back HDX impact on D-depletion, the interpartner ⟨HDX⟩
complementarity was thus illustrated. This is the first description
of interpartner ⟨HDX⟩ and selective back HDX validating
salt-solvated structures.