Acyl Carbonyl Group Research Articles

Structural Insights into Catalytic Relevances of Substrate Poses in ACC-1.

ACC-1 is a plasmid-encoded class C β-lactamase identified in clinical isolates of Klebsiella pneumoniae, Proteus mirabilis, Salmonella enterica, and Escherichia coli ACC-1-producing bacteria are susceptible to cefoxitin, whereas they are resistant to oxyimino cephalosporins. Here, we depict crystal structures of apo ACC-1, adenylylated ACC-1, and acylated ACC-1 complexed with cefotaxime and cefoxitin. ACC-1 has noteworthy structural alterations in the R2 loop, the Ω loop, and the Phe119 loop located along the active-site rim. The adenylate covalently bonded to the nucleophilic serine reveals a tetrahedral phosphorus mimicking the deacylation transition state. Cefotaxime in ACC-1 has a proper conformation for the substrate-assisted catalysis in that its C-4 carboxylate and N-5 nitrogen are adequately located to facilitate the deacylation reaction. In contrast, cefoxitin in ACC-1 has a distinct conformation, in which those functional groups cannot contribute to catalysis. Furthermore, the orientation of the deacylating water relative to the acyl carbonyl group in ACC-1 is unfavorable for nucleophilic attack.

A simple and general approach for the synthesis of highly functionalized 6-oxo-1,6-dihydropyridines

A variety of 5-cyano-4-methylthio-6-oxo-1,6-dihydropyridine-3-carboxylates have been efficiently synthesized in a one-pot reaction from N-alkyl and N-aryl derivatives of 2-cyano-3,3-bis(methylthio)acrylamides and selected β-keto esters. The reaction proceeds via potassium hydrogen carbonate mediated conjugate addition of a β-keto ester to 2-cyano-3,3-bis(methylthio)acrylamide followed by loss of methyl mercaptan and subsequent intramolecular condensation of amide group with the acyl carbonyl group. The mechanism of the reaction has been established by isolation of the 2-acetyl-4-cyano-5-amino-3-(methylthio)-5-oxopent-3-enoate intermediate and its independent cyclization to the desired 6-oxo-1,6-dihydropyridine.

Tryptophan-Lipid Interactions in Membrane Protein Folding Probed by Ultraviolet Resonance Raman and Fluorescence Spectroscopy

Aromatic amino acids of membrane proteins are enriched at the lipid-water interface. The role of tryptophan on the folding and stability of an integral membrane protein is investigated with ultraviolet resonance Raman and fluorescence spectroscopy. We investigate a model system, the β-barrel outer membrane protein A (OmpA), and focus on interfacial tryptophan residues oriented toward the lipid bilayer (trp-7, trp-170, or trp-15) or the interior of the β-barrel pore (trp-102). OmpA mutants with a single tryptophan residue at a nonnative position 170 (Trp-170) or a native position 7 (Trp-7) exhibit the greatest stability, with Gibbs free energies of unfolding in the absence of denaturant of 9.4 and 6.7 kcal/mol, respectively. These mutants are more stable than the tryptophan-free OmpA mutant, which exhibits a free energy of unfolding of 2.6 kcal/mol. Ultraviolet resonance Raman spectra of Trp-170 and Trp-7 reveal evolution of a hydrogen bond in a nonpolar environment during the folding reaction, evidenced by systematic shifts in hydrophobicity and hydrogen bond markers. These observations suggest that the hydrogen bond acceptor is the lipid acyl carbonyl group, and this interaction contributes significantly to membrane protein stabilization. Other spectral changes are observed for a tryptophan residue at position 15, and these modifications are attributed to development of a tryptophan-lipid cation- π interaction that is more stabilizing than an intraprotein hydrogen bond by ∼2 kcal/mol. As expected, there is no evidence for lipid-protein interactions for the tryptophan residue oriented toward the interior of the β-barrel pore. These results highlight the significance of lipid-protein interactions, and indicate that the bilayer provides more than a hydrophobic environment for membrane protein folding. Instead, a paradigm of lipid-assisted membrane protein folding and stabilization must be adopted.

A calorimetric and spectroscopic comparison of the effects of ergosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We performed comparative DSC and FTIR spectroscopic measurements of the effects of cholesterol (Chol) and ergosterol (Erg) on the thermotropic phase behavior and organization of DPPC bilayers. Ergosterol is the major sterol in the biological membranes of yeasts, fungi and many protozoa. It differs from Chol in having two additional double bonds, one in the steroid nucleus at C7–8 and another in the alkyl chain at C22–23. Erg also has an additional methyl group in the alkyl chain at C24. Our DSC studies indicate that the incorporation of Erg is more effective than Chol is in reducing the enthalpy of the pretransition. At lower concentrations Erg is also more effective than Chol in reducing the enthalpies of both the sharp and broad components of main phase transition. However, at sterol concentrations from 30 to 50 mol%, Erg is generally less effective at reducing the enthalpy of the broad components and does not completely abolish the cooperative hydrocarbon chain-melting phase transition at 50 mol%, as does Chol. Nevertheless, in this higher ergosterol concentration range, there is no evidence of the formation of ergosterol crystallites. Our FTIR spectroscopic studies demonstrate that Erg incorporation produces a similar ordering of liquid-crystalline DPPC bilayers as does Chol, but an increased degree of hydrogen bonding of the fatty acyl carbonyl groups in the glycerol backbone region of the DPPC bilayer. These and other results indicate that Erg is less miscible in DPPC bilayers at higher concentrations than is Chol. Finally, we provide a tentative molecular explanation for the comparative experimental and computation results obtained for Erg and Chol in phospholipid bilayers, emphasizing the dynamic conformational differences between these two sterols.

A Calorimetric and Spectroscopic Comparison of the Effects on Ergosterol and Cholesterol on the Thermotropic Phase Behavior and Organization of Dipalmitoylphosphatidylcholine Bilayer Membranes

article i nfo We performed comparative DSC and FTIR spectroscopic measurements of the effects of cholesterol (Chol) and ergosterol (Erg) on the thermotropic phase behavior and organization of DPPC bilayers. Ergosterol is the major sterol in the biological membranes of yeasts, fungi and many protozoa. It differs from Chol in having two additional double bonds, one in the steroid nucleus at C7-8 and another in the alkyl chain at C22-23. Erg also has an additional methyl group in the alkyl chain at C24. Our DSC studies indicate that the incorporation of Erg is more effective than Chol is in reducing the enthalpy of the pretransition. At lower concentrations Erg is also more effective than Chol in reducing the enthalpies of both the sharp and broad components of main phase transition. However, at sterol concentrations from 30 to 50 mol%, Erg is generally less effective at reducing the enthalpy of the broad components and does not completely abolish the cooperative hydrocarbon chain-melting phase transition at 50 mol%, as does Chol. Nevertheless, in this higher ergosterol concentration range, there is no evidence of the formation of ergosterol crystallites. Our FTIR spectroscopic studies demonstrate that Erg incorporation produces a similar ordering of liquid-crystalline DPPC bilayers as does Chol, but an increased degree of hydrogen bonding of the fatty acyl carbonyl groups in the glycerol backbone region of the DPPC bilayer. These and other results indicate that Erg is less miscible in DPPC bilayers at higher concentrations than is Chol. Finally, we provide a tentative molecular explanation for the comparative experimental and computation results obtained for Erg and Chol in phospholipid bilayers, emphasizing the dynamic conformational differences between these two sterols.

Acceleration of the Substrate Cα Deprotonation by an Analogue of the Second Substrate Palmitoyl-CoA in Serine Palmitoyltransferase

Serine palmitoyltransferase (SPT) is a key enzyme of sphingolipid biosynthesis and catalyzes the pyridoxal 5'-phosphate (PLP)-dependent decarboxylative condensation reaction of l-serine with palmitoyl-CoA to generate 3-ketodihydrosphingosine. The binding of l-serine alone to SPT leads to the formation of the external aldimine but does not produce a detectable amount of the quinonoid intermediate. However, the further addition of S-(2-oxoheptadecyl)-CoA, a nonreactive analogue of palmitoyl-CoA, caused the apparent accumulation of the quinonoid. NMR studies showed that the hydrogen-deuterium exchange at Calpha of l-serine is very slow in the SPT-l-serine external aldimine complex, but the rate is 100-fold increased by the addition of S-(2-oxoheptadecyl)-CoA, showing a remarkable substrate synergism in SPT. In addition, the observation that the nonreactive palmitoyl-CoA facilitated alpha-deprotonation indicates that the alpha-deprotonation takes place before the Claisen-type C-C bond formation, which is consistent with the accepted mechanism of the alpha-oxamine synthase subfamily enzymes. Structural modeling of both the SPT-l-serine external aldimine complex and SPT-l-serine-palmitoyl-CoA ternary complex suggests a mechanism in which the binding of palmitoyl-CoA to SPT induced a conformation change in the PLP-l-serine external aldimine so that the Calpha-H bond of l-serine becomes perpendicular to the plane of the PLP-pyridine ring and is favorable for the alpha-deprotonation. The model also proposed that the two alternative hydrogen bonding interactions of His(159) with l-serine and palmitoyl-CoA play an important role in the conformational change of the external aldimine. This is the unique mechanism of SPT that prevents the formation of the reactive intermediate before the binding of the second substrate.

Synthesis of 4‐Fluoroalkyl‐Substituted Pyridazines from Fluorinated Diazodiketones.

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Crystal structures of the class D β-lactamase OXA-13 in the native form and in complex with meropenem

The therapeutic problems posed by class D β-lactamases, a family of serine enzymes that hydrolyse β-lactam antibiotics following an acylation-deacylation mechanism, are increased by the very low level of sensitivity of these enzymes to β-lactamase inhibitors. To gain structural and mechanistic insights to aid the design of new inhibitors, we have determined the crystal structure of OXA-13 from Pseudomonas aeruginosa in the apo form and in complex with the carbapenem meropenem. The native form consisted of a dimer displaying an overall organisation similar to that found in the closely related enzyme OXA-10. In the acyl-enzyme complex, the positioning of the antibiotic appeared to be ensured mainly by (i) the covalent acyl bond and (ii) a strong salt-bridge involving the carboxylate moiety of the drug. Comparison of the structures of OXA-13 in the apo form and in complex with meropenem revealed an unsuspected flexibility in the region of the essential serine 115 residue, with possible consequences for the catalytic properties of the enzyme. In the apo form, the Ser115 side-chain is oriented outside the active site, whereas the general base Lys70 adopts a conformation that seems to be incompatible with the activation of the catalytic water molecule required for the deacylation step. In the OXA-13:meropenem complex, a 3.5 Å movement of the backbone of the 114–116 loop towards the side-chain of Lys70 was observed, which seems to be driven by a displacement of the neighbouring 91–104 loop and which results in the repositioning of the side-chain hydroxyl group of Ser115 toward the catalytic centre. Concomitantly, the side-chain of Lys70 is forced to curve in the direction of the deacylating water molecule, which is then strongly bound and activated by this residue. However, a distance of ca 5 Å separates the catalytic water molecule from the acyl carbonyl group of meropenem, a structural feature that accounts for the inhibition of OXA-13 by this drug. Finally, the low level of penicillinase activity revealed by the kinetic analysis of OXA-13 could be related to the specific presence in position 73 of a serine residue located close to the general base Lys70, which results in a decrease of the number of hydrogen-bonding interactions stabilising the catalytic water molecule.

Electric fields in active sites: substrate switching from null to strong fields in thiol- and selenol-subtilisins.

Although known to be important factors in promoting catalysis, electric field effects in enzyme active sites are difficult to characterize from an experimental standpoint. Among optical probes of electric fields, Raman spectroscopy has the advantage of being able to distinguish electronic ground-state and excited-state effects. Earlier Raman studies on acyl derivatives of cysteine proteases [Doran, J. D., and Carey, P. R. (1996) Biochemistry 35, 12495-502], where the acyl group has extensive pi-electron conjugation, showed that electric field effects in the active site manifest themselves by polarizing the pi-electrons of the acyl group. Polarization gives rise to large shifts in certain Raman bands, e.g. , the C=C stretching band of the alpha,beta-unsaturated acyl group, and a large red shift in the absorption maximum. It was postulated that a major source of polarization is the alpha-helix dipole that originates from the alpha-helix terminating at the active-site cysteine of the cysteine protease family. In contrast, using the acyl group 5-methylthiophene acryloyl (5-MTA) as an active-site Raman probe, acyl enzymes of thiol- or selenol-subtilisin exhibit no polarization even though the acylating amino acid is at the terminus of an alpha-helix. Quantum mechanical calculations on 5-MTA ethyl thiol and selenol ethyl esters allowed us to identify the conformational states of these molecules along with their corresponding vibrational signatures. The Raman spectra of 5-MTA thiol and selenol subtilisins both showed that the acyl group binds in a single conformation in the active site that is s-trans about the =C-C=O single bond. Moreover, the positions of the C=C stretching bands show that the acyl group is not experiencing polarization. However, the release of steric constraints in the active site by mutagenesis, by creating the N155G form of selenol-subtilisin and the P225A form of thiol-subtilisin, results in the appearance of a second conformer in the active sites that is s-cis about the =C-C=O bond. The Raman signature of this second conformer indicates that it is strongly polarized with a permanent dipole being set up through the acyl group's pi-electron chain. Molecular modeling for 5-MTA in the active sites of selenol-subtilisin and N155G selenol-subtilisin confirms the findings from Raman spectroscopic studies and identifies the active-site features that give rise to polarization. The determinants of polarization appear to be strong electron pull at the acyl carbonyl group by a combination of hydrogen bonds and the field at the N-terminus of the alpha-helix and electron push from a negatively charged group placed at the opposite end of the chromophore.

Reactions of carbonyl compounds in basic solutions. Part 33.1 Effect of 2-trichloro- and tribromoacetyl substituents on the alkaline hydrolysis of methyl benzoate, as well as the hydrolysis of the corresponding benzoate anions and ionisation/ring–chain tautomerism of the corresponding acids

Rate coefficients have been measured for the alkaline hydrolysis of methyl 2-trichloro- and tribromoacetylbenzoate, as well as the alkaline hydrolysis of the corresponding 2-trichloro- and tribromobenzoate anions, in 30% (v/v) dioxane–water at several temperatures. The relative rates of hydrolysis and activation parameters demonstrate relatively rapid hydrolysis via neighbouring group participation by the acyl-carbonyl group in the ester hydrolysis and slower hydrolysis via intramolecular fission of the adducts in the benzoate anion hydrolysis. The pKa values of the corresponding benzoic acids have been measured in water and their ring–chain tautomeric equilibrium constants determined.

Reactions of Carbonyl Compounds in Basic Solutions. Part 30.1 The Effect of 2-Formyl, 2,6-Diformyl and 2-Trifluoroacetyl Substituents on the Alkaline and Neutral Hydrolysis of Methyl Benzoate and Phenyl Acetate

Rate coefficients are measured for the alkaline hydrolysis of methyl 2-formyl-, 2,6-diformyl- and 2-trifluoroacetyl-benzoates and for the alkaline and neutral hydrolysis of 2-formyl-, 2,6-diformyl-4-methyl- and 2-trifluoroacetyl-phenyl acetates in water at several temperatures: the relative rates of hydrolysis and activation parameters demonstrate neighbouring group participation by the acyl-carbonyl groups in the ester hydrolysis.

In vivo 13C chemical shift imaging of the human liver

Chemical shift imaging (CSI) was applied to measure natural abundance proton-decoupled13C-NMR spectra of the human liver. Large surface coils were designed for13C spectra acquisition (16-cm-diameter circular coil) as well as for proton imaging and decoupling (21×20-cm butterfly coil). Such sizes allowed deep observations of the abdomen. A space matrix of 8×4 voxels (4×8 cm each) was defined using 32 phase-encoding steps. Magnetic field gradients were adjusted on multicompartment phantoms to limit contamination between voxels. Spectral maps containing {1H}-13C spectra of liver from healthy volunteers with an acceptable signal-to-noise ratio were recorded within 20 min. Liver spectra exhibited well-defined resonances corresponding to fatty acyl chains, carbonyl groups, and sugars. The (C-l)-glycogen resonance was also detected under such conditions. Such a technique would be of interest in the development of metabolic investigations on the human liverin vivo.

Forces, bond lengths, and reactivity: fundamental insight into the mechanism of enzyme catalysis.

Comparison of spectroscopic, kinetic, and thermodynamic data for a series of functioning acylserine proteases suggests that the observed variation in deacylation rates can be accounted for by changes in the properties of the acyl-enzyme's ground state. The acyl-enzyme's catalytically crucial acyl carbonyl group is probed by resonance Raman spectroscopy. Its spectral frequency is used to gauge both the carbonyl bond length and the strength of hydrogen bonding (originating from groups making up the oxyanion hole) to the carbonyl oxygen atom. As the deacylation rate increases 16,300-fold through the series, a shift in carbonyl frequency, vC = O, of -54 cm-1 corresponds to a carbonyl bond length increase of 0.025 A. The decrease in vC = O is also consistent with an increase in hydrogen bond donor enthalpy of -27 kJ mol-1. Interestingly, this value resembles closely the decrease in activation energy for deacylation through the series, 24 kJ mol-1, demonstrating that the hydrogen bonds to the carbonyl oxygen atom can provide sufficient energy to account for the observed rate accelerations.

Resonance Raman and Fourier transform infrared spectroscopic studies of the acyl carbonyl group in [3-(5-methyl-2-thienyl)acryloyl]chymotrypsin: evidence for artifacts in the spectra obtained by both techniques

The acyl carbonyl group of [3-(5-methyl-2-thienyl)acryloyl]chymotrypsin (5MeTA-chymotrypsin) has been investigated by using both resonance Raman (RR) and Fourier transform infrared (FTIR) spectroscopies. The spectrum of the acyl-enzyme carbonyl group has been obtained as a function of pH over the range 3.0-10.0 in the RR experiments and over the range 3.4-7.6 (p2H) in the FTIR experiments. The carbonyl spectral profiles obtained by using FTIR spectroscopy are substantially different from the carbonyl profiles obtained by using RR spectroscopy. The FTIR spectra were obtained by subtracting the spectrum of the free enzyme from that of the acyl-enzyme. Use of the active-site inhibitor phenylmethanesulfonyl fluoride demonstrates that part of the intensity observed in the FTIR spectra of 5MeTA-chymotrypsin is due to a subtraction artifact giving rise to enzyme-associated bands, probably from peptide groups perturbed by substrate binding. The enzyme bands can be removed by subtracting the FTIR spectrum of 13C=O acyl-enzyme from that of 12C=O acyl-enzyme. Additionally, this procedure reveals that one of the acyl-enzyme carbonyl bands observed at 1727 cm-1 using RR spectroscopy is absent in the FTIR acyl-enzyme spectrum. However, a feature near 1720 cm-1 can be induced in the FTIR spectrum by actinic light in the near-UV region. Thus, it is proposed that the 1727 cm-1 RR carbonyl band results from a population of acyl-enzymes which is generated by exposure to the laser beam during RR data collection. When both the RR and FTIR data are adjusted to remove artifacts, they provide essentially identical carbonyl stretching profiles.

Length of the acyl carbonyl bond in acyl-serine proteases correlates with reactivity

Resonance Raman (RR) spectroscopy has been used to obtain the vibrational spectrum of the acyl carbonyl group in a series of acylchymotrypsins and acylsubtilisins at the pH of optimum hydrolysis. The acyl-enzymes, which utilize arylacryloyl acyl groups, include three oxyanion hole mutants of subtilisin BPN', Asn155Leu, Asn155Gln, and Asn155Arg, and encompass a 500-fold range of deacylation rate constants. For each acyl-enzyme a RR carbonyl band has been identified which arises from a population of carbonyl groups undergoing nucleophilic attack in the active site. As the deacylation rate (k3) increases through the series of acyl-enzymes, the carbonyl stretching band (vC = O) is observed to shift to lower frequency, indicating an increase in single bond character of the reactive acyl carbonyl group. Experiments involving the oxyanion hole mutants of subtilisin BPN' indicate that a shift of vC = O to lower frequency results from stronger hydrogen bonding of the acyl carbonyl group in the oxyanion hole. A plot of log k3 against vC = O is linear over the range investigated, demonstrating that the changes in vC = O correlate with the free energy of activation for the deacylation reaction. By use of an empirical correlation between carbonyl frequency (vC = O) and carbonyl bond length (rC = O) it is estimated that rC = O increases by 0.015 A as the deacylation rate increases 500-fold through the series of acyl-enzymes. This change in rC = O is about 7% of that expected for going from a formal C = O double bond in the acyl-enzyme to a formal C-O single bond in the tetrahedral intermediate for deacylation. The data also allow us to estimate the energy needed to extend the acyl carbonyl group along its axis to be 950 kJ mol-1 A-1.

Stereochemistry of nitrogen heterocycles. 68. Stereochemistry of 2,5-dimethyl-4-piperidone, 2,5-dimethyl-4-piperidol, and their N-benzoyl derivatives

The isomers of 1-benzoyl-2,5-dimethyl-4-piperidone were obtained and equilibrated (89.8% cis ⇄ 10.2% trans isomer, preferred conformations 2a, 5e, and 2a, 5a). The position of the equilibrium between the isomers of 2,5-dimethyl-4-piperidone was determined by two independent methods (89.5% trans ⇄ 10.5% cis isomer). The differences in the free energies of the isomers, the conformational energy of the Β-CH3 group, and the energy of syn-axial 1,3-interaction between the CH and the unshared electron pair were calculated; the decrease in the conformational energy of the Β-methyl group in the series of piperidine (1.6), 4-piperidone (1.47), and N-acyl-4-piperidone (1.28 kcal/mole) was explained by the successive weakening of the repulsive interaction between the Β-CH3 group and the n-pair of the nitrogen as a result of the flattening of the ring and of the conjugation between the free electron pair of the nitrogen and the π electrons of the acyl carbonyl group. The last previously unknown fourth isomer of 2,5-dimethyl-4-piperidol, which exists in the 2e,4a,5a conformation, was obtained by the reduction of cis-1-benzoyl-2,5-dimethyl-4-piperidone followed by debenzoylation. In contrast to the secondary amine, its N-benzoyl- and N-benzyl-substituted derivatives exist in the alternative 2a,4e,5e conformation. The configurations and conformations of the isomers were determined by means of the IR and NMR spectra.

Phenylalanine derivatives enhancing intestinal absorption of insulin in mice.

The adjuvant effect of N-acyl-L-and D-phenylalanine derivatives on intestinal absorption of insulin was investigated in normal mice. The correlation between the chemical structural properties of the N-acyl moiety and the absorption-promoting activity was estimated from the glucose concentrations and the insulin levels in the blood of mice after oral combined administration of insulin and adjuvant. The chemical structural properties of N-acyl-phenylalanine derivatives necessary for adjuvant effect on intestinal absorption of insulin were as follows.1. An aromatic ring is present, separated by two atoms from the acyl carbonyl group.2. Either of X or Y is oxygen or X-Y is a double bond in Fig.2.3. The N-acyl moiety has small hydrophobic substituents, such as F, Cl, or Me at Rα, Rβ, Rη and has an appropriate hydrophilic-hydrophobic balance of the overall molecule.The use of these agents to enhance insulin absorption offers the possibility of a new approach to oral insulin therapy.

Characterization of the reported [Mn(CO)(Ph2PCH2CH2PPh2)2](red isomer) as the acylmanganese complex [Mn(CO)(COC6H4PhPCH2CH2PPh2)(Ph2PCH2CH2PPh2)] by X-ray diffraction

One of the products of the reaction between [Mn2(CO)10] and Ph2PCH2CH2PPh2, previously identified as a paramagnetic species [Mn(CO)(Ph2PCH2CH2PPh2)2], is in fact diamagnetic and contains an acyl carbonyl group linking the manganese to an ortho carbon of one phenyl ring and is correctly formulated as [Mn(CO)(COC6H4PhPCH2 CH2PPh2)(Ph2PCH2 CH2PPh2)].

Mössbauer Study of Some Gamma‐ and Neutron‐Irradiated Iron Carbonyl Complexes

AbstractMössbauer effect experiments have been carried out on 60Co and reactor irradiated CFe(CO)3SCH1]2, [(C4H9)2SnFe(CO)4]3, (C6H5)1PFe(CO)4, (CH2)4[Fe(CO)2C3H5]2, and (CH2)4.(CO)2[Fe(CO)2C5H5]2. It was found that the Fe—CO bond is rather radiation resistant than are expected due to the π bonding character. The σ bonds such as C—H are labile to radiation. The presence of acyl carbonyl group increase the radiation resistant property of the molecule. The increase in the isomer shift values of the complexes with cyclopentadienyl groups were interpreted as a result of dehydrogenation.

Electron‐impact studies of organometallic molecules—X. Some simple transition metal fluorocarbon complexes

AbstractThe mass spectra of several fluoroalkyl, fluoroalkenyl and fluoroacyl complexes of manganese, rhenium, iron and ruthenium carbonyls are described. After loss of carbonyl groups, fluoroalkyl compounds eliminate an olefin, with formation of metal halide species. A trifluorovinyl complex shows a novel elimination of a carbon atom to give an ion postulated to be a difluorocarbene‐metal fluoride; the occurrence of difluorocarbene‐metal ions in the spectra of some related complexes is also discussed. The spectra of the acyl complexes show little evidence of elimination of the acyl carbonyl group; the major process is fission of the CORf bond with loss of a fluoroalkyl radical and formation of the cationic metal carbonyl, e.g. π‐C5H5M(CO)3+ (M  Fe or Ru). The relevance of thermal or photochemical model reactions to processes occurring in the mass spectrometer is discussed.