Alan C. Hopkinson

Formation of molecular radical cations of enkephalin derivatives via collision-induced dissociation of electrospray-generated copper (II) complex ions of amines and peptides

Fragmentation of some electrospray-generated complex ions, [63CuII(amine)M].2+, where M is an enkephalin derivative, produces the radical cation of the peptide, M.+. This ion has only been observed when M contains a tyrosyl or tryptophanyl residue plus a basic residue, typically arginyl or lysyl. A typical viable amine is diethylenetriamine. Collision-induced dissociation (CID) of the M.+ ion yields a prominent [M − 106].+ product ion for tyrosine-containing peptides, and a prominent [M − 129].+ ion for a tryptophan-containing peptide. These fragment ions are formed as a result of elimination of the tyrosyl and tryptophanyl side chains. Dissociation of these ions, in turn, produces second generation product ions, many of which are typically absent in the fragmentation of protonated peptide ions. Structures for some of these unusual ions are proposed.

Amination with Pd–NHC Complexes: Rate and Computational Studies on the Effects of the Oxidative Addition Partner

Pd-PEPPSI-IPent, a recently-developed N-heterocyclic carbene (NHC) complex, has been evaluated in amination reactions with secondary amines and it has shown superb reactivity under the most mildly basic reaction conditions. Rate and computational studies were conducted to provide insight into the mechanism of the transformation. The IPent catalyst coordinates to the amine much more strongly than the IPr variant, thus favouring deprotonation with comparatively weak bases. Indeed the reaction is first order in base and only slightly more than zeroth order in amine.

Are the Radical Centers in Peptide Radical Cations Mobile? The Generation, Tautomerism, and Dissociation of Isomeric α-Carbon-Centered Triglycine Radical Cations in the Gas Phase

The mobility of the radical center in three isomeric triglycine radical cations[G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y(*)GG](+), [GY(*)G](+), and [GGY(*)](+); CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+), respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (> or = 44.7 kcal/mol), in support of the thesis that isomerically pure [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The CID spectra of [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.

Amination with PdNHC Complexes: Rate and Computational Studies Involving Substituted Aniline Substrates

The amination of aryl chlorides with various aniline derivatives using the N-heterocyclic carbene-based Pd complexes Pd-PEPPSI-IPr and Pd-PEPPSI-IPent (PEPPSI=pyridine, enhanced precatalyst, preparation, stabilization, and initiation; IPr=diisopropylphenylimidazolium derivative; IPent= diisopentylphenylimidazolium derivative) has been studied. Rate studies have shown a reliance on the aryl chloride to be electron poor, although oxidative addition is not rate limiting. Anilines couple best when they are electron rich, which would seem to discount deprotonation of the intermediate metal ammonium complex as being rate limiting in favour of reductive elimination. In previous studies with secondary amines using PEPPSI complexes, deprotonation was proposed to be the slow step in the cycle. These experimental findings relating to mechanism were corroborated by computation. Pd-PEPPSI-IPr and the more hindered Pd-PEPPSI-IPent catalysts were used to couple deactivated aryl chlorides with electron poor anilines; while the IPr catalysis was sluggish, the IPent catalyst performed extremely well, again showing the high reactivity of this broadly useful catalyst.

Conformation Switching in Gas-Phase Complexes of Histidine with Alkaline Earth Ions

Infrared multiple photon dissociation spectroscopy of gas-phase doubly charged alkaline earth complexes of histidine reveals a transition from dominance of the zwitterion (salt bridge, SB) conformation with Ba2+ to substantial presence of the canonical (charge-solvated, CS) conformation with Ca2+. This result is a clear illustration of the importance of metal-ion size in governing the delicate balance between these two modes of complexation of gas-phase amino acids. The two conformational motifs are clearly distinguished by characteristic spectral features, confirmed by density functional theory simulated IR spectra of the low-energy conformers. As a further illustration of histidine complexation possibilities, the spectrum of the Na+His complex shows purely CS character and emphasizes the greater tendency toward SB character induced by the higher charge in the alkaline earth complexes. Calculation of the complete series of alkaline earth/histidine complexes confirms the increasing stability of the SB conformations relative to CS with increasing metal ion size, as well as showing that among SB conformations the most highly chelated conformation (SB3) is favored for small metals, whereas the most extended conformation (SB1) is favored for large metals. A decomposition of the binding thermochemistry shows that these thermochemical trends versus metal-ion size are due to differences in electrostatic binding energies, with relatively little contribution from the deformation and rearrangement energy costs of distorting the ligand framework.

Characterization of the product ions from the collision-induced dissociation of argentinated peptides

Tandem mass spectrometry performed on a pool of 18 oligopeptides shows that the product ion spectra of argentinated peptides, the [bn + OH + Ag]+ ions and the [yn − H + Ag]+ ions bearing identical sequences are virtually identical. These observations suggest strongly that these ions have identical structures in the gas phase. The structures of argentinated glycine, glycylglycine, and glycylglycylglycine were calculated using density functional theory (DFT) at the B3LYP/DZVP level of theory; they were independently confirmed using HF/ LANL2DZ. For argentinated glycylglycylglycine, the most stable structure is one in which Ag+ is tetracoordinate and attached to the amino nitrogen and the three carbonyl oxygen atoms. Mechanisms are proposed for the fragmentation of this structure to the [b2 + OH + Ag]+ and the [y2 − H + Ag]+ ions that are consistent with all experimental observations and known calculated structures and energetics. The structures of the [b2 − H + Ag]+ and the [a2 − H + Ag]+ ions of glycylglycylglycine were also calculated using DFT. These results confirm earlier suggestions that the [b2 − H + Ag]+ ion is an argentinated oxazolone and the [a2 − H + Ag]+ an argentinated immonium ion.

Structure of the Observable Histidine Radical Cation in the Gas Phase: A Captodative α-Radical Ion†

Protein-based radicals play crucial roles in some of the greatest biosynthetic challenges in nature, including photosynthesis and substrate oxidation. Radical centers have been located on aromatic and sulfur-containing amino acid residues, as well as glycine residues. Invariably these charged or neutral radical species are generated through involvement of an adjacent metal cofactor. The positions of charge and spin in the radical cations are paramount for reactivity modulation and proton-coupled electron transfer, but obtaining structural details is difficult even for the simplest models. 2] Experiments in vacuo permit the investigation of intrinsic properties of radical cations in the absence of a reactivity-modulating environment. Radical cations of amino acids and peptides have been produced in vacuo by one-electron transfer in collision-induced dissociations (CIDs) of a ternary complex system comprising copper(II), an auxiliary ligand, and the amino acid or peptide. Such ternary complexes are efficiently generated by electrospray ionization, and probed downstream by using mass spectrometry (MS). Under appropriate conditions, CID of the complex yields the radical cation of the amino acid or peptide that can be isolated and trapped for spectroscopic interrogation. Herein, we report the first infrared multiple photon dissociation (IRMPD) spectroscopic experiments on a prototypical amino acid radical cation, HisC, and its ternary complex ion. In a recent article, Ke et al. showed that, by judicious choice of the auxiliary ligand, HisC of different stabilities are formed through CID of the ternary complex ion. In particular, the use of 2,2’:6’,2’’-terpyridine (tpy) as the ligand leads primarily to a HisC that is stable on the MS timescale and can be isolated and fragmented at a subsequent MS stage; by contrast, employing acetone as the ligand results in a metastable HisC and only its fragment ions are observed. Furthermore, the former, relatively stable HisC fragments by losing a water molecule to give [b1-H]C + and then CO to give [a1-H]C , whereas the latter, metastable HisC dissociates spontaneously by losing first CO2 to give the 4-ethaniminoimidazole radical cation, which then loses methanimine to give the 4-methyleneimidazole radical cation. Density functional theory (DFT) calculations at the (unrestricted) UB3LYP/6-311 + + G(d,p) level of theory predicted five low-energy HisC structures. Scheme 1 shows these structures with additional, new information on the barriers against their interconversions (see Figures S2 and S3 in the Supporting Information for details). Ke et al. postulated that the stable and metastable HisC are His5 (the structure at the global minimum) and His2, respectively. His5 is a captodative aradical ion that differs from the canonical His1 structure in having the a-CH hydrogen migrated to the imino nitrogen of the imidazole ring; His2 is best described as a 4-ethaniminoimidazole radical cation solvated by CO2. His2–His5 are all unconventional structures, and experimental verification of the HisC structure is highly desirable for confirmation of the key roles played by spin and charge delocalization in HisC stabilization. Figure 1 compares the experimental IRMPD spectrum collected for HisC with the DFT-predicted IR spectra of His1–His5. It is apparent that only one predicted IR spectrum, that of His5, resembles the measured IRMPD spectrum. In particular, His5 is the only isomer predicted to exhibit two bands, 1596 and 1653 cm , which are assigned as NH2 scissoring and C=O stretching, respectively, that match the 1606 and 1666 cm 1 bands in the IRMPD spectrum. The lack of a strong band at around 1780–1790 cm 1 in the IRMPD spectrum rules out the presence of a significant fraction of His3 and His4. Similarly, His1 can be ruled out by the presence of the doublet, 1606 and 1666 cm , and the absence of spectroscopic details in the region of 1077– 1320 cm . His2 can be eliminated by the absence of peaks at around 810–820 cm 1 and by the low endothermicity against loss of the solvating CO2 (5 kcalmol ). We interpret the excellent match between the experimental IRMPD spectrum and the predicted IR spectrum of His5 to indicate that His5 is the only abundant species present. This degree of selectivity is feasible as His5 is positioned at the bottom of a deep well on the potential-energy surface of HisC. The barriers against His5 converting into the other His isomers and dissociating into [b1-H]C + are high (Scheme 1), [*] Dr. J. Zhao, Dr. C.-K. Siu, Y. Ke, Dr. U. H. Verkerk, Prof. A. C. Hopkinson, Prof. K. W. M. Siu Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street Toronto, ON M3J 1P3 (Canada) E-mail: kwmsiu@yorku.ca

The Extent and Effects of Peptide Sequence Scrambling Via Formation of Macrocyclic b Ions in Model Proteins

The extent and effects of sequence scrambling in peptide ions during tandem mass spectrometry (MS/MS) have been examined using tryptic peptides from model proteins. Sequencescrambled b ions appeared in about 35% of 43 tryptic peptides examined under MS/MS conditions. In general, these ions had relatively low abundances with averages of 8% and 16%, depending on the instrumentation used. A few tryptic peptides gave abundant scrambled b ions in MS/MS. However, peptide and protein identifications under proteomic conditions with Mascot were not affected, even for these peptides wherein scrambling was prominent. From the 43 tryptic peptides that have been investigated, the conclusion is that sequence scrambling is unlikely to impact negatively on the accuracy of automated peptide and protein identifications in proteomics.

A nonempirical molecular orbital study on the acidity of the carbon–hydrogen bond

The geometries of acetylene, ethylene, methane, ethane, ketene, allene, propyne, acetonitrile, ketenimine, aminoacetylene, hydroxyacetylene, and all the possible anions formed by removal of a proton from these molecules have been carefully optimized using nonempirical molecular orbital calculations employing the split‐valence shell 4‐31G basis set. These geometries have then been used in 6‐31G basis set calculations. The computed order of gas phase acidity found for these molecules is hydroxyacetylene≳ketenimine≳ketene ≳acetonitrile≳aminoacetylene≳acetylene≳propyne≳allene ≳ethylene≳ethane≳methane.

Dissociation of the N–Cα Bond and Competitive Formation of the [zn – H]•+ and [cn + 2H]+ Product Ions in Radical Peptide Ions Containing Tyrosine and Tryptophan: The Influence of Proton Affinities on Product Formation

Dissociations at the N-Cα bond of tryptophan and tyrosine residues are the prevalent pathways in the fragmentations of radical cations of tripeptides that contain such as residues. This process involves a proton transfer from the β-carbon of the tryptophan or tyrosine residue to the carbonyl oxygen of the amide group, followed by cleavage of the N-Cα bond, generating low-lying proton-bound dimers that dissociate to give each an ionic and a neutral product. Formation of the [zn−H]∢+ or [cn+2H]+ ion is a competition between the two incipient fragments for the proton in a dissociating proton-bound dimer.