ISSN:1076-5174    

DOI:10.1002/jms.1190300102

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

ISSN:1076-5174    

DOI:10.1002/jms.1190300103

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

ISSN:1076-5174    

DOI:10.1002/jms.1190300104

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe gas‐phase acidities of halogen‐substituted aromatic compounds have been determined in a flowing afterglowtriple quadrupole apparatus with use of the silane cleavage method developed by DePuy and co‐workers [C. H. DePuy, S. Gronert, S. E. Barlow, V. M. Bierbaum and R. Damrauer,J. Am. Chem. Soc., 111, 1968 (1989)]. In this method the relative yields of siloxide ion products produced in reactions of OH−with trimethylsilyl‐ or phenyldimethylsilyl‐substituted aromatic compounds are correlated with the difference in gas‐phase acidity of the accompanying neutral products. Acidities are reported for different ring‐positions in fluoro‐, chloro‐ and bromobenzene, chloro‐ and bromonaphthalene and benzyl chloride. Excellent precision is achieved in most cases, with assigned uncertainties less than 2–3 kcal/mol. Good agreement is obtained between the acidities determined with use of two different types of silane precursor. Halogen‐substitution increases the gas‐phase acidities of benzene and naphthalene by similar amounts (13–14 kcal/mol). The effects on different ring‐positions in benzene and naphthalene are shown to be primarily inductive in nature, falling‐off by a consistent 2.5–3.5 kcal/mol per bond separating the acidic site from the halogen‐bearing carbon in the chlorine and bromine‐substituted systems. Larger effects are evident in the positional acidities of fluorobenzene. Themetaandparaposition acidities of halobenzenes are shown to be linearly correlated with the acidities of the correspondingmetaandparahal

ISSN:1076-5174    

DOI:10.1002/jms.1190300105

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractTwo methionine‐specific sequence ions are described that are formed upon the dissociation of protonated methionine‐containing peptides at high (keV) collision energies. These ions, hiherto unassigned, are denoted an+ 1 − CH3S⋅ and zn+ 1 − CH3S⋅. Precursor ion scans reveal that these species originate from the an+ 1 and zn+ 1 ions, respectively, where methionine resides at positionn‐1 relative to the N‐ or C‐terminus. A comparison of the collision‐induced dissociation (CID) spectra for several related peptides demonstrates that methionine residues are involved in the genesis of these species and mechanisms for their formation are proposed. Several structural features of a peptide that are important to the formation of these species are described. The importance of identifying these ions in order to correctly sequence a methionine‐containing peptide based on its CID sp

ISSN:1076-5174    

DOI:10.1002/jms.1190300106

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe neutral products eliminated upon the collisionally activated dissociation (CAD) of C 60+zions (z= 1–4) are post‐ionized to cations or anions and collected in neutral fragment‐reionization (+NfR+,−) mass spectra. These spectra provide conclusive evidence that the decompositions C 60+ z→ C x+z+ C2n(wherex>30 and2n<30) proceed by cleavage of whole C2nclusters, not consecutive losses ofnC2. As teh charge of the fullerene precursor ion increases, the largest detectable neutral loss decreases, from C28forz= 1 to C12forz= 4. Post‐ionization of C2nto cations preferentially produces odd‐carbon cluster ions (e.g., C5+, C7+, C11+, C+15) whereas post‐ionization to anions yields abundant even‐carbon

ISSN:1076-5174    

DOI:10.1002/jms.1190300107

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractBy dissociating noncovalent adducts with infrared radiation and minimizing ion charge density during measurement, Fourier‐transform mass spectrometry with electrospray ionization yields 160 000 resolving power for porcine albumin. Its molecular weight calculated from the published sequence is 66 740, but values found for two different samples are MW = 66 736 and 66 886. The spectrum of their mixture confirms this mass difference. Similar measurements of two Protein A (45 kDa) samples also give molecular weights in substantial disagreement with literature sequences. Tandem mass spectrometry gives partial sequence information that demonstrates errors in some of the reported sequence

ISSN:1076-5174    

DOI:10.1002/jms.1190300108

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe mechanistic aspects of alkene loss from ionized ethyl‐andn‐propyl phenyl thioethers have been studied by use of deuterium labelling and tandem mass spectrometry. Loss of ethene from the molecular ion of ethyl phenyl thioether proceeds predominantly by a specific H‐shift from the β‐position of the ethyl group to the remaining part of the ion. In contrast, deuterium labelling reveals that elimination of propene from the molecular ions of then‐propyl phenyl andn‐propyl 2,6‐dimethylphenyl thioethers involves three competing reactions: (i) a 1,2‐hydride shift assisted cleavage of the bond between the propyl group and the sulfur atom yielding an ion–neutral complex of a thiophenyoxy radical and a secondary propyl carbenium ion, which reacts further by proton transfer prior to dissociation; (ii) a partially reversible 1,5‐H shift from the β‐position to the 2‐ or 6‐position of the ring; and (iii) a 1,3‐H shift from the β‐position to the sulfur atom. The preference for transfer of a hydrogen atom from the β‐position is particularly pronounce for the reactions of the metastable molecular ions of then‐propyl phenyl andn‐propyl 2,6‐dimethylphenyl thioethers. This indicates that the critical energies of the processes involving H‐shifts are lower than the critical energy for propene loss with intermediate formation of an ion–neutral complex. In addition to ethene elimination, ionized ethyl phenyl thioether expels a HS+·radical. This reaction is preceded by an interchange between the hydrogen atoms at the α‐methylene group and the hydrogen atoms at the 2‐ and 6‐positions of the ring

ISSN:1076-5174    

DOI:10.1002/jms.1190300109

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractThe optical emission spectra between 190 and 680 nm of 8 keV O 2+·and CO 2+·collisions with He and N2target gases were obtained and analyzed on a modified commercial VG ZAB‐2F mass spectrometer. Emissions observed were from O, O+, He, N 2+·(B2∑+u→X2∑+g) and N2(C3∏u→ B3∏g) in the case of O 2+·projectiles and C, CO 2+·(B2∑+u,A2∏u→X2∏g), CO+·(B2∑+→X2∑+), N 2+·(B2∑+g→X2∑g) and N2(C3∏u→B3∏g) in the case of CO 2+·projectiles. The results are discussed in relation to previous reports on N 2+·–target collisions and the collision–i

ISSN:1076-5174    

DOI:10.1002/jms.1190300110

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY

摘要:

AbstractIn a study designed to examine the nature of short‐lived, electrophilic intermediates liberated during decomposition ofN,N′‐bis(2‐chloroethyl)‐N‐nitrosourea (BCNU)in vitroand also on administration of BCNU (140 μmol i.p.) to ratsin vivo, both on‐line and off‐line LC/MS/MS techniques were employed to detect and characterize the corresponding glutathione (GSH) adducts present in incubation media and excreted into bile, respectively.In vitro, four GSH conjugates were formed and these were identified, on the basis of their product ion spectra, as products ofS‐andN‐carbamoylation and alkylation reactions. Although the relative proportions of thesein vitroadducts were found to depend on the molar ratios of GSH and BCNU, the major adduct under all conditions studied proved to beS‐(2‐chloroethylcarbamoyl)glutathione (SCG). Analysis of untreated bile samples by means of on‐line LC/MS/MS with constant neutral loss (129 u) and precursor ion (m/z179) scanning techniques again led to the detection of four GSH conjugates, although only one of these (SCG) was common to the group of adducts identifiedin vitro.All of the GSH conjugates detected in bile represented products ofS‐carbamoylation, indicating that the alkylating moiety released from BCNU undergoes reactionsin vivowith

ISSN:1076-5174    

DOI:10.1002/jms.1190300111

出版商:John Wiley&Sons, Ltd.    

年代:1995

数据来源: WILEY