Side-chain radical losses from radical cations allows distinction of leucine and isoleucine residues in the isomeric peptides Gly-XXX-Arg

Sequencing of peptides via low-energy collision-induced dissociation of protonated peptides typically yields b(n) and y(n) sequence ions. The isomeric residues leucine and isoleucine rarely can be distinguished in these experiments since they give b(n) and y(n) sequence ions of the same m/z. Siu's pioneering work on electrospray ionization of copper complexes of peptides (Chu IK, Rodriquez CF, Lau TC, Hopkinson AC, Siu KWM. J. Phys. Chem. B 2000; 104: 3393) provides a way of forming radical cations of peptides in the gas phase. This method was used to generate M(+ small middle dot) ions of the two isomeric peptides Gly-Leu-Arg and Gly-Ile-Arg in order to compare their fragmentation reactions. Both radical cations fragment to give even electron y(2) and y(1) sequence ions as well as side-chain radical losses of CH(3) and CH(3)CH(2) for isoleucine and (CH(3))(2)CH for leucine. In contrast the [M + H](+) and [M + 2H](2+) ions do not allow distinction between the isomeric leucine and isoleucine peptides.     

Can radical cations of the constituents of nucleic acids be formed in the gas phase using ternary transition metal complexes?

Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) of ternary transition metal complexes of [M(L(3))(N)](2+) (where M = copper(II) or platinum(II); L(3) = diethylenetriamine (dien) or 2,2':6',2'-terpyridine (tpy); N = the nucleobases: adenine, guanine, thymine and cytosine; the nucleosides: 2'deoxyadenosine, 2'deoxyguanosine, 2'deoxythymine, 2'deoxycytidine; the nucleotides: 2'deoxyadenosine 5'-monophosphate, 2'deoxyguanosine 5'-monophosphate, 2'deoxythymine 5'-monophosphate, 2'deoxycytidine 5'-monophosphate) was examined as a means of forming radical cations of the constituents of nucleic acids in the gas phase. In general, sufficient quantities of the ternary complexes [M(L(3))(N)](2+) could be formed for MS/MS studies by subjecting methanolic solutions of mixtures of a metal salt [M(L(3))X(2)] (where M = Cu(II) or Pt(II); L(3) = dien or tpy; X = Cl or NO(3)) and N to ESI. The only exceptions were thymine and its derivatives, which failed to form sufficient abundances of [M(L(3))(N)](2+) ions when: (a) M = Pt(II) and L(3) = dien or tpy; (b) M = Cu(II) and L(3) = dien. In some instances higher oligomeric complexes were formed; e.g., [Pt(tpy)(dG)(n)](2+) (n = 1-13). Each of the ternary complexes [M(L(3))(N)](2+) was mass-selected and then subjected to collision-induced dissociation (CID) in a quadrupole ion trap. The types of fragmentation reactions observed for these complexes depend on the nature of all three components (metal, auxiliary ligand and nucleic acid constituent) and can be classified into: (i) a redox reaction which results in the formation of the radical cation of the nucleic acid constituent, N(+.); (ii) loss of the nucleic acid constituent in its protonated form; and (iii) fragmentation of the nucleic acid constituent. Only the copper complexes yielded radical cations of the nucleic acid constituent, with [Cu(tpy)(N)](2+) being the preferred complex due to suppression, in this case, of the loss of the nucleobase in its protonated form. The yields of the radical cations of the nucleobases from the copper complexes follow the order of their ionization potentials (IPs): G (lowest IP) > A > C > T (highest IP). Sufficient yields of the radical cations of each of the nucleobases allowed their CID reactions (in MS(3) experiments) to be compared to their even-electron counterparts.     

Electrospray ionization tandem mass spectrometric studies of competitive pyridine loss from platinum(II) ethylenediamine complexes by the kinetic method

The competition between pyridine ligand loss in square planar Pt(II) complexes has been examined using the doubly and singly charged ions of complexes consisting of platinum(ethylenediamine) coordinated to two different substituted pyridines. Collision induced dissociation (CID) of [Pt(en)Py(1)Py(2)](2+) (where Py(1) = one of ten different substituted pyridines and Py(2) = pyridine) results in loss of the protonated pyridines to yield the singly charged platinum ions [Pt(en)Py(1)-H](+) and [Pt(en)Py(2)-H](+). In contrast, fragmentation of [Pt(en)Py(1)Py(2)-H](+) results in neutral pyridine loss to yield the ions [Pt(en)Py(1)-H](+) and [Pt(en)Py(2)-H](+). In the latter case, the correlation between relative losses of each pyridine compared to their gas-phase proton affinities is poor. A novel chloride ion abstraction reaction occurs for the fragmentation of [Pt(en)Py(1)Py(2)](2+) when Py(1) = o-C(5)H(4)CIN and Py(2) = C(5)H(5)N, to yield the [Pt(en)(Cl)Py(2)](+) and [o-C(5)H(4)N](+) pair of ions. In order to model this process the competition between nitrogen and chlorine binding in [Pt(NH(3))(3)(o-NC(5)H(4)Cl)](2+) has been examined using density functional theory (DFT) calculations at the B3LYP/LANL2DZ level of theory. Both adducts are minima with the N adduct being more stable than the Cl adduct by 22.7 kcal mol(-1). Furthermore, the Cl adduct exhibits a significant stretching of the C-Cl bond (to 1.935 A), consistent with the observed chloride ion abstraction reaction, which is endothermic by 9.0 kcal mol(-1) (relative to the N adduct).     

Peptide derivatization as a strategy to form fixed-charge peptide radicals

As a means of generating fixed-charge peptide radicals in the gas phase we have examined the collision-induced dissociation (CID) chemistry of ternary [Cu(II)(terpy)(TMPP-M)]2+ complexes, where terpy = 2,2':6'2'-terpyridine and TMPP-M represents a peptide (M) modified by conversion of the N-terminal amine to a [tris(2,4,6-trimethoxyphenyl)phosphonium]acetamide (TMPP-) fixed-charge derivative. The following modified peptides were examined: oligoglycines, (Gly)n (n = 1-5), alanylglycine, glycylalanine, dialanine, trialanine and leucine-enkephaline (YGGFL). The [Cu(II)(terpy)(TMPP-M)]2+ complexes are readily formed upon electrospray ionization (ESI) of a mixture of derivatized peptide and [Cu(II)(terpy)(NO3)2] and generally fragment to form transient peptide radical cations, TMPP-M+*, which undergo rapid decarboxylation for the simple aliphatic peptides. This is contrasted with the complexes containing the unmodified peptides, which predominantly undergo fragmentation of the coordinated peptide. These differences demonstrate the importance of proton mobility in directing fragmentation of ternary copper(II) peptide complexes. In the case of leucine-enkephaline, a sufficient yield of the radical cation was obtained to allow further CID. The TMPP-YGGFL+* ion showed a rich fragmentation chemistry, including CO2 loss, side-chain losses of an isopropyl radical, 2-methylpropene and p-quinomethide, and *a1 and *a4 sequence ion formation. In contrast, the even-electron TMPP-YGGFL+ ion fragments to form *a(n) and *b(n) sequence ions as well as the [*b4 + H2O]+ rearrangement ion.     

Tuning the gas phase redox properties of copper(II) ternary complexes of terpyridines to control the formation of nucleobase radical cations

Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) of ternary copper(II) complexes of [Cu(terpyX)(M)]2+ (where terpyX = is a substituted 2,2':6',2'-terpyridine ligand; M = the nucleobases: adenine, guanine, thymine and cytosine) was examined as a means of forming radical cations of nucleobases in the gas phase. The following substituents were examined: 4'-NMe2-2,2':6',6'-terpyridine; 4'-OH-2,2':6',6'-terpyridine; 4'-F-2,2':6',6'-terpyridine; 2,2':6',6'-terpyridine; 4'-Cl-2,2':6',6'-terpyridine; 4'-Br-2,2':6',6'-terpyridine; 4'-CO2H-2,2':6',6'-terpyridine; 4'-NO2-2,2':6',6'-terpyridine and 6,6'-dibromo-2',2:6',2'-terpyridine. Each of the ternary complexes [Cu(terpyX)(M)]2+ was mass selected and subjected to collision induced dissociation (CID) in a quadrupole ion trap. The types of fragmentation reactions observed for these complexes depend on the nature of the substituent on the terpyridine ligand, while the yields of the radical cations of the nucleobases follow the order of their ionization energies (IEs): G (lowest IE) > A > C > T (highest IE). In general, radical cation formation is favoured for electron withdrawing substituents (e.g. NO2) while loss of the neutral nucleobase is favoured for electron donating substituents (e.g. NMe2). Loss of the protonated nucleobase is a major fragmentation pathway for the OH substituted terpyridine system, consistent with its ability to bind to a metal centre as a deprotonated ligand. Crystal structure determinations of (6,6'-dibromo-2',2:6',2'-terpyridine)bis(nitrato)copper(II) and diaqua(4'-oxo-2,2':6',6'-terpyridine)copper(II) nitrate monohydrate were performed and correlated with the ESI results.     

Quantitative organische Mikroanalyse. Elementaranalyse

Ohne Zusammenfassung     

Designing copper(II) ternary complexes to generate radical cations of peptides in the gas phase: role of the auxiliary ligand

A series of ternary copper(II) complexes of the type [Cu(II)(L)(M)](2+), where M represents the hexapeptides GGGFLR, YGGFLR and WGGFLR and L a set of 12 nitrogen donor ligands have been evaluated for their ability to form cationic peptide radicals, M(+)*, in the gas phase. Although the fragmentation chemistry of these ions is complex, two main conclusions emerge: (i) Complexes containing a tri- or tetra-dentate ligand were found to be more effective at producing the peptide radical because in these instances competitive loss of the ligand from the complex is inhibited; (ii) The ligands ought not possess any acidic protons in order to prevent competitive loss of the protonated peptide, [M + H](+). There is significant interaction of the N-terminal aromatic residues in YGGFLR and WGGLFR with the copper(ii) ion in several of the complexes as revealed by the formation of [Cu(I)(L)(p-quinomethide)](+) and [Cu(I)(L)(3-methyleneindoline)](+) fragment ions. Following its dissociation from the ternary complex, CID of the YGGFLR(+)* radical cation shows a dependence on the ligand in the complex from which it was formed. This 'memory effect' most likely reflects differences in the coordinated peptide structure induced by the ligand in the precursor complex which are maintained following dissociation.     

Gas-phase ligand loss and ligand substitution reactions of platinum(II) complexes of tridentate nitrogen donor ligands

The source of protons associated with the ligand loss channel of HX((n - 1)+) from [Pt(II)(dien)X](n+) (X = Cl, Br and I for n = 1 and X = NC(5)H(5) for n = 2) in the gas phase was investigated by deuterium-labelling studies. The results of these studies indicate that these protons originate from both the amino groups and the carbon backbone of the dien ligand. In some instances (e.g. X = Br and I), the protons lost from the carbon backbone can be even more abundant than the protons lost from the amino groups. The gas-phase substitution reactions of coordinatively saturated [Pt(II)(L(3))L(a)](2+) complexes (L(3) = tpy or dien) were also examined using ion-molecule reactions. The outcome of the ion-molecule reactions depends on both the ancillary ligand (L(3)) as well as the leaving group (L(a)). [Pt(II)(tpy)L(a)](2+) complexes undergo substitution reactions, with a faster rate when L(a) is a good leaving group, while the [Pt(II)(dien)L(a)](2+) complex undergoes a proton transfer reaction.