Generation of [La(peptide)]3+ complexes in the gas phase: determination of the number of binding sites provided by dipeptide, tripeptide, and tetrapeptide ligands

Improved Recovery and Selectivity of Lanthanide-Ion-Binding Cyclic Peptide Hosts by Changing the Position of Acidic Amino Acids

The development of an effective host molecule to separate lanthanide (Ln) ions and a method for predicting its guest recognition/self-assembly behavior based on primary chemical structures are highly sought after in both academia and industry. Herein, we report the improvement of one-pot Ln ion recovery and a performance prediction method for four new cyclic peptide hosts that differ in the position of acidic amino acids. These cyclic peptide hosts could recognize Ln3+ directly through a 1:1 complexation–precipitation process and exhibited high Lu3+ selectivity in spite of similar ion size and electronegativity when the positions of the acidic amino acids were changed. This unpredictable selectivity was explained by considering the dipole moment, lowest unoccupied molecular orbital, and cohesion energy. In addition, a semi-empirical function using these parameters was proposed for screening the sequence and estimating the isolated yields without long-time molecular dynamics calculations. The insights obtained from this study can be employed for the development of high-performance peptides for the selective recovery of Ln and other metal ions, as well as for the construction of diverse supramolecular recognition systems.

Experimental and computational studies of monovalent metal cation-peptide interactions in [M·GlyGlyHis]+ (M = Li, Na, K, Rb, Cs, and Ag) complexes

[M·GlyGlyHis]⁺ (M = Li, Na, K, Rb, Cs, and Ag) complexes were generated using the electrospray ionization method and were subjected to collision-induced dissociation. Metal ion loss is the primary dissociation channel for [Cs·GlyGlyHis]⁺ whereas other complexes yield metal-bound peptide sequence ions and dehydrated ions as the main products. [Li·GlyGlyHis]⁺ and [Ag·GlyGlyHis]⁺ also generate product ions that are not observed for other complexes. Density functional theory calculations suggest that metal ion-peptide ligand interaction occurs through covalent interactions in [Li·GlyGlyHis]⁺ and [Ag·GlyGlyHis]⁺, and through electrostatic attraction in [Na·GlyGlyHis]⁺, [K·GlyGlyHis]⁺, [Rb·GlyGlyHis]⁺, and [Cs·GlyGlyHis]⁺. The calculations also suggest that fragmentation behavior of these complexes is affected by charge transfer to the ligand and ion-ligand interaction energy, and to a lesser extent by the ion size.

Structures and Dissociation Products of Ce/Peptide Complexes: Competition between Coordination and Charge Delocalization

Structures of [Ce(GGG)]³⁺ and [Ce(GGG ? H)]²⁺ have been investigated by DFT calculations. The two lowest-energy structures of the triply charged metal complex have the peptide in either the iminol or conventional zwitterionic form, and these ions have almost identical energies. In the doubly charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface, but the latter is favored. In both iminol structures, the metal ion coordinates to the iminol oxygen rather than to the nitrogen, unlike in previously reported iminol-containing complexes. Triply charged [Ce(peptide)]³⁺ complexes are fragile and not easily isolated in a mass spectrometer, whereas the doubly charged [Ce(peptide ? H)]²⁺ complexes are more robust. Here, we studied the fragmentations of 37 [Ce(peptide ? H)]²⁺ and 30 [Ce(peptide)(peptide ? H)]²⁺ complexes and the results are systematically summarized. Losses of CO and/or H₂O are the most commonly observed fragmentation channels for [Ce(peptide ? H)]²⁺ complexes and these dissociation pathways are modeled by DFT calculations. For [Ce(peptide)(peptide ? H)]²⁺ complexes the neutral peptide plays the role of a solvent molecule but, unlike in the dissociations of [Ce(CH₃CN)(peptide ? H)]²⁺ complexes, the loss of the solvent molecule is not observed. Instead, fragmentation occurs by cleavage of the second amide bond of the solvating peptide molecule.

Formation and Characterization of Zr4+ Stabilized by Neutral Tridentate Ligands in the Gas Phase

Ligated tetrapositive metal ions are rare gas-phase species which tend to form complexes with lower charges due to the high 4th ionization energies of metals. We report the observation of tetrapositive Zr(TMPDA)₃⁴⁺ and Zr(TMOGA)₃⁴⁺ complexes in the gas phase by electrospray ionization of Zr(ClO₄)₄/TMPDA and Zr(ClO₄)₄/TMOGA mixtures. The Zr⁴⁺ center in both complexes is coordinated by nine atoms from three neutral diamide ligands forming nine-coordinate twisted tricapped trigonal prismatic geometry on the basis of DFT calculations. Collision-induced dissociation of both complexes resulted in the loss of protonated ligands to form tripositive Zr(TMPDA)(TMPDA-H)³⁺ and Zr(TMOGA)(TMOGA-H)³⁺ products which retain the IV oxidation state of zirconium at the cost of charge reduction from 4+ to 3+ of the whole complexes. The very high 4th ionization energy of zirconium (34.34 eV) makes tetrapositive zirconium complex the most challenging tetracation to be stabilized against charge reduction in the gas phase to date. Graphical abstract ᅟ.

The Effects of Trivalent Lanthanide Cationization on the Electron Transfer Dissociation of Acidic Fibrinopeptide B and its Analogs

Electrospray ionization (ESI) on mixtures of acidic fibrinopeptide B and two peptide analogs with trivalent lanthanide salts generates [M + Met + H](4+), [M + Met](3+), and [M + Met -H](2+), where M = peptide and Met = metal (except radioactive promethium). These ions undergo extensive and highly efficient electron transfer dissociation (ETD) to form metallated and non-metallated c- and z-ions. All metal adducted product ions contain at least two acidic sites, which suggest attachment of the lanthanide cation at the side chains of one or more acidic residues. The three peptides undergo similar fragmentation. ETD on [M + Met + H](4+) leads to cleavage at every residue; the presence of both a metal ion and an extra proton is very effective in promoting sequence-informative fragmentation. Backbone dissociation of [M + Met](3+) is also extensive, although cleavage does not always occur between adjacent glutamic acid residues. For [M + Met - H ](2+), a more limited range of product ions form. All lanthanide metal peptide complexes display similar fragmentation except for europium (Eu). ETD on [M + Eu - H](2+) and [M + Eu](3+) yields a limited amount of peptide backbone cleavage; however, [M + Eu + H](4+) dissociates extensively with cleavage at every residue. With the exception of the results for Eu(III), metallated peptide ion formation by ESI, ETD fragmentation efficiencies, and product ion formation are unaffected by the identity of the lanthanide cation. Adduction with trivalent lanthanide metal ions is a promising tool for sequence analysis of acidic peptides by ETD. Graphical Abstract ᅟ.

Locating Pb2+ and Zn2+ in zinc finger-like peptides using mass spectrometry

The binding preferences of Pb(2+)and Zn(2+) in doubly charged complexes with zinc finger-like 12-residue peptides (Pep), [Mn(Pep-2(n-1)H)](2+) have been explored using tandem mass spectrometry. The peptides were synthesized strategically by blocking the N-terminus with an acetyl group and with four cysteine and/or histidine residues in positions 2, 5, 8, and 11, arranged in different motifs: CCHH, CHCH, and CCCC. The MS(2) spectra of the Pb(2+) and Zn(2+) complexes show multiple losses of water and a single methane loss and these provide a sensitive method for locating the metal dication and so elucidating its coordination. The elimination of a methane molecule indicated the position of the metal at the Cys2 residue. Whereas lead was observed to preferentially bind to cysteine residues, zinc was found to primarily bind to histidine residues and secondarily to cysteine residues. Preferential binding of lead to cysteine is preserved in the complexes with more than one Pb(2+). Key to the mechanism of the loss of water and methane is the metal dication withdrawing electrons from the proximal amidic nitrogen. This acidic nitrogen loses its hydrogen to an amidic oxygen situated four atoms away leading to formation of a five-member ring and the elimination of water.

Electron capture dissociation of trivalent metal ion-peptide complexes

With electrospray ionization from aqueous solutions, trivalent metal ions readily adduct to small peptides resulting in formation of predominantly (peptide + M(T) - H)(2+), where M(T) = La, Tm, Lu, Sm, Ho, Yb, Pm, Tb, or Eu, for peptides with molecular weights below ~1000 Da, and predominantly (peptide + M(T))(3+) for larger peptides. ECD of (peptide + M(T) - H)(2+) results in extensive fragmentation from which nearly complete sequence information can be obtained, even for peptides for which only singly protonated ions are formed in the absence of the metal ions. ECD of these doubly charged complexes containing M(T) results in significantly higher electron capture efficiency and sequence coverage than peptide-divalent metal ion complexes that have the same net charge. Formation of salt-bridge structures in which the metal ion coordinates to a carboxylate group are favored even for (peptide + M(T))(3+). ECD of these latter complexes for large peptides results in electron capture by the protonation site located remotely from the metal ion and predominantly c/z fragments for all metals, except Eu(3+), which undergoes a one electron reduction and only loss of small neutral molecules and b/y fragments are formed. These results indicate that solvation of the metal ion in these complexes is extensive, which results in the electrochemical properties of these metal ions being similar in both the peptide environment and in bulk water.