Interaction of metal cations with alkylnitriles in the gas phase: solvation of metal ions by the hydrocarbon chain

Determination and gas-phase stability evaluation of metal complexes by nanoelectrospray ionization and collision-induced dissociation tandem mass spectrometry

RATIONALE

The structures of metal complexes determine their stable functioning in product performance. Electrospray ionization mass spectrometry (ESI-MS) is used in studying metal complexes despite exhibiting limitations in analyzing labile complexes. Therefore, identifying a method for detecting unstable complexes and evaluating their stabilities is necessary, providing a theoretical basis for material selection and performance evaluation.

METHODS

The standard complexes Zn(BTZ)2 , Fe(acac)3 , and Sn(Oct)2 were analyzed using nanoESI quadrupole orbitrap MS (nanoESI-MS) and compared with ESI-MS for two temperature modes. The three complexes and alkylamine-Ag+ complexes were analyzed using nanoESI and collision-induced dissociation MS/MS (CID-MS/MS). Breakdown plots of the survival yield against collision energies expressed in terms of the center-of-mass were constructed according to the obtained product ion spectra. Quantum chemical calculations based on density functional theory were performed to calculate the binding energies between the alkylamines and Ag+ .

RESULTS

The three standard complexes were detected in the native structures using nanoESI-MS, confirming the advantage of nanoESI over ESI for detecting unstable complexes. The gas-phase stabilities of the amine-Ag+ complexes, estimated using the breakdown plots constructed by plotting the data obtained via nanoESI and CID-MS/MS, were consistent with the established theories, previous studies, and binding energies calculated using computational methods.

CONCLUSIONS

NanoESI-MS is suitable for detecting labile complexes and enables the structural analyses of unknown complex additives. A novel approach based on nanoESI and CID-MS/MS was developed to determine the gas-phase stabilities of complexes, enabling their quantification and comparison and providing a technical basis for product improvement, which is essential in developing industrial materials.

An electrospray ionization study on complexes of amylin with Cu(II) and Cu(I)

Human amylin (hIAPP) is one of a number of different peptides known to be responsible for the formation of amyloid fibrils in the pancreas of subjects with Type 2 diabetes mellitus. It was recognized that metal ions such as Cu(II) are implicated in the aggregation process of amyloidogenic peptides. However, the role of Cu(II) ions in the aggregation and dyshomeostasis of amylin has been controversial. Considering that most of the research reported in the literature pertain to the interactions between Cu(II) and amylin, we thought of interest to compare the interactions of Cu(II) and Cu(I) ions with amylin by electrospray ionization (ESI) mass spectrometry and collisional experiments, to elucidate possible differences in structural aspects of the complexes so formed. The ESI mass spectra of solutions containing hIAPP and Cu(I) or Cu(II) ions show the formation of hIAPP-Cu complexes. In both cases, M + Cu ions with three and four positive charges are detected. However, a series of fragment ions, absent in the ESI spectrum of untreated hIAPP, become detectable. Some of them are common for both Cu(I) and Cu(II) complexes, whereas others are specific for the complexes containing Cu in different oxidation states. Some fragments imply the involvement of residues His18, Ser19, Ser20, Asn21, and Asn22 in the complex formation, but the detection of the fragment b₂₂ ³⁺ indicates the presence of copper ions in a different position. This suggests different interaction sites between Cu(II) and Cu(I) and hIAPP. In contrast to Cu(II) complex, in the Cu(I) complex, some peculiar structures are present, corresponding to the cleavage of Asn-Asn peptidic bond and to [b₃₀  + Cu(I)]⁴⁺ and [b₂₈  + Cu(I)]⁴⁺ species. These results are in agreement with the coordination vacancy in [Cu(I)-(peptide)] species, which promotes Cu(I) interaction with additional neighboring donors (mainly N-histidine, and also S-methionine or other groups depending on the peptide conformation) through formation of trigonal T-shaped intermediates.

Proton affinities and ion enthalpies

Proton affinities of a number of alkyl acetates (CH₃-C(=O)-OR) and of methyl alkanoates (R-C(=O)-OCH₃, R=H, alkyl) have been assembled from the literature or measured using the kinetic method. It was observed that the proton affinities for the isomeric species CH₃-C(=O)-OR and R-C(=O)-OCH₃ are almost identical, an unexpected result as the charge in these protonated ester molecules is largely at the keto carbon atom and so this site should be more sensitive to alkyl substitution. Analysis of the data, including those from lone pair ionisation and core-electron ionisation experiments available from the literature, indicate that after protonation, extensive charge relaxation (or polarisation) takes place (as is also the case, according to the literature, after core-electron ionisation). By contrast, after lone pair ionisation, which results in radical cations, such relaxation processes are relatively less extensive. As a consequence, changes in ion enthalpies of these protonated molecules follow more closely the changes in neutral enthalpies, compared with changes in enthalpies of the corresponding radical cations, formed by electron detachment. Preliminary analyses of published energetic data indicate that the above finding for organic esters may well be another example of a more general phenomenon.

Metal ion hydrocarbon bidentate bonding in alkyl acetates, methyl alkanoates, alcohols and 1-alkenes: a comparative study

The relative affinity of the monovalent metal ions Li⁺, Na⁺, Cu⁺ and Ag⁺ towards a series of aliphatic alkyl acetates and some selected 1-alkenes (P) was examined using the kinetic method. A detailed analysis of the dissociation characteristics of a series of mixed metal-bound dimer ions of the type P₁-M⁺-P₂ and the evaluated proton affinities (PAs) of the monomers shows that the affinity of the cation towards long-chain alkyl acetates and alkenes (having a chain length ≤ C₄) is markedly enhanced. In line with recent studies of nitriles, alcohols and methyl alkanoates, this is attributed to a bidentate interaction of the metal ion with the functional group or double bond and the aliphatic chain. In particular, the longer chain alkyl acetates, methyl alkanoates and alcohols show a remarkably similar behaviour with respect to silver ion hydrocarbon bonding. The Ag⁺ adducts of the alkyl acetates dissociate by loss of CH₃COOH. This reaction becomes more pronounced at longer chain lengths, which points to metal ion bidentate formation in [Ag⁺···1-alkene] product ions having a long hydrocarbon chain. In the same vein, the heterodimers [1- hexene···Ag⁺···1-heptene] and [1- heptene···Ag⁺···1-octene] dissociate primarily into [Ag⁺···1-heptene] and [Ag⁺···1-octene] ions, respectively. Hydrocarbon bidentate formation in [Ag⁺···1-octene] also reveals itself by the reluctance of this ion to react with water in an ion trap, as opposed to [Ag⁺···1-hexene] which readily undergoes hydration.

Interaction of metal cations with functionalised hydrocarbons in the gas phase: further experimental evidence for solvation of metal ions by the hydrocarbon chain

Relative affinity measurements of monovalent metal ions (= Li(+), Cu(+) and Ag(+)) towards aliphatic amines, alcohols and methyl alkanoates (P) have been performed using the kinetic method on the dissociation of metal bound dimer ions of the type P(1)-M(+)-P(2). It was found that the cations' affinity towards long chain (≥C(4) chain length) n- and s-alkylamines, n-alkanols and methyl n- alkanoates was unexpectedly enhanced. This is attributed to a bidentate interaction of the metal ion with the amine, alcohol or ester functional group and the aliphatic chain, paralleling earlier observations on metal bound nitriles. Methyl substitution at the functional group (s-alkylamines compared with n-alkylamines) serves to strengthen only the N•••M(+) bond, and this can be rationalized by the larger proton affinities of s-alkylamines compared to n-alkylamines. This substitution, however, has no effect on the metal ion-hydrocarbon bond. In contrast, methyl substitution remote from the functional group, as in iso-pentylamine, does lead to strengthening of the metal ion-hydrocarbon bond. The cuprous ion affinity of hexadecylamine, C(16)H(33)NH(2) was found to be as large as that for ethylenediamine (352 kJ mol(-1)), known to be a strong copper binding agent. It is argued that such a metal ion-hydrocarbon interaction does not occur in the metal bound dimers.