Carbonyl Stretch Research Articles

Ultrafast Infrared Studies of Bond Activation in Organometallic Complexes

The ability to directly monitor the progress of chemical reactions as they take place is one of the major recent developments in physical chemistry. Many of the advances in our understanding of chemical processes have come about as a result of time-resolved spectroscopic techniques. Conventional methods of time resolution, however, are limited to microsecond and slower regimes, curtailing the applicability of such techniques in many cases of interest. Ultrafast spectroscopy, made possible by the invention and subsequent refinement of pulsed laser technology, has become one of the most powerful tools available to chemists by providing insight into those physical and chemical phenomena which occur too rapidly to be studied by traditional methods. Processes amenable to investigation by ultrafast techniques include electron transfer reactions, vibrational and electronic relaxation in molecules large and small, reaction dynamics in natural and artificial photosynthetic systems, and surface phenomena in both bulk and microscopic systems, to name only a few. Predominate in the field are UV-pump visible-probe techniques, a fact which derives largely from the early development of practical UV and visible sources. UV-vis experiments, widely applied to the study of physical phenomena, convey information about those (primarily electronic) transitions which involve the absorption of visible probe photons. Chemically important information, including structures, can be obtained by examining the IR region of the spectrum, which offers better spectral resolution, a greater number of spectral features, and increased sensitivity to the chemical environment. One problem which is amenable to study using ultrafast vibrational spectroscopy is bond activation, the weakening of otherwise stable chemical bonds so as to increase their reactivity. This is an area of particular interest to organic chemists since many organic reactions involve breaking the extremely stable (>90 kcal/mol) carbon-hydrogen (C-H) and silicon-hydrogen (Si-H) bonds of alkanes and silanes, respectively. A mild (i.e., room temperature and ambient pressure) path to C-H bond activation is also of obvious interest to the petrochemical industry. Si-H bond activation by certain transition-metal-containing compounds was discovered by Jetz and Graham in 1971.1 This was followed in 1982 by the discovery of a related activation reaction of alkane C-H bonds involving Cp*(L)IrH2 (Cp* ) (CH3)5C5) and Cp*Ir(CO)2. Efforts to understand the dynamics of these activation reactions have been made by several groups using gas-phase,5 lowtemperature/high-pressure liquid noble gas,6,7 or matrix isolation8-10 experiments. These techniques slow the reactions, thereby rendering the intermediates observable using conventional Fourier transform IR (FTIR), UV-vis, and fluorescence spectroscopies. Though fruitful, these methods share the disadvantage of observing the activation reactions under conditions quite different from those under which they are normally carried out. Femtosecond infrared spectroscopy provides an ideal tool for investigating the real-time dynamics of these reactions because it provides sufficient temporal resolution to observe all early intermediate steps under ambient reaction conditions. Carbonyl stretches in the 1850-2050 cm-1 range have large absorption cross sections and display significant shifts (>10 cm-1) with changing chemi-

Structure, conformation, anomeric effects and rotational barriers in the HERON amides, N,N ′-diacyl-N,N ′-dialkoxyhydrazines

N,N′-Diacyl-N,N′-dialkoxyhydrazines are HERON amides that exhibit theoretical and physical properties of bisheteroatom-substituted amides. Amide nitrogens are pyramidal and they adopt a preferential conformation which permits an anomeric interaction in which one nitrogen overlaps strongly with the adjacent N–O σ* orbital. A crystal structure of N,N′-di(p-chlorobenzoyl)-N,N′-diethoxyhydrazine 4d has been obtained which confirms these properties in the solid state. Infrared data for ten such hydrazines indicate unusually high carbonyl stretch frequencies in accord with pyramidality at nitrogen. Diastereotopic resonances and dynamic 1H NMR studies indicate both a significant N–N rotational barrier of between 65 and 72 kJ mol–1, which is consistent with a strong anomeric interaction, as well as a much smaller than usual amide rotation barrier of 54 kJ mol–1, a direct consequence of pyramidality at nitrogen.

Is Arginine Zwitterionic or Neutral in the Gas Phase? Results from IR Cavity Ringdown Spectroscopy

Is Arginine Zwitterionic or Neutral in the Gas Phase? Results from IR Cavity Ringdown Spectroscopy

Electron Transfer through the Hydrogen-Bonded Interface of a β-Turn-Forming Depsipeptide

Hydrogen-bonding networks are believed to play an important role in electron-transfer pathways in a protein medium. A porphyrin−quinone donor−acceptor compound with a depsipeptide bridge which forms a β-turn has been synthesized to study hydrogen bond-mediated electron transfer. The placement of the donor and acceptor has been chosen to favor electron transfer through the hydrogen bond interface of the β-turn. Use of ester linkages also allows control of the hydrogen-bonding pattern within the β-turn-forming depsipeptide. Infrared spectroscopy in the amide A (NH stretch) and amide I (carbonyl stretch) regions indicates that the β-turn conformation is about 85% populated in dichloromethane and essentially completely disrupted in dimethyl sulfoxide at 296 K. The electron-transfer rate constant, ket, was evaluated using the singlet excited-state lifetimes of the porphyrin in the presence and absence of an electron acceptor. The lifetimes were obtained using time-correlated single-photon-counting fluorescence spectroscopy. Very fast electron transfer (ket = (1.1 ± 0.1) × 109 s-1) was observed in the presence of the β-turn conformation. When the β-turn structure was disrupted using the solvent DMSO, electron transfer was no longer competitive with the intrinsic fluorescence emission. Analysis of the data in terms of Marcus theory and the pathway model for electronic coupling yielded a value for the hydrogen bond coupling decay factor, εhb, of 0.8 ± 0.4, which is of the same order of magnitude as the theoretically predicted value of 0.36.

A study of glycine adsorption on a Cu{110} surface using reflection absorption infrared spectroscopy

The chemisorption of glycine (NH 2CH 2COOH) and its fully deuterated analogue d 5-glycine, vacuum deposited on a clean Cu{110} surface, have been investigated using reflection absorption infrared spectroscopy (RAIRS). At room temperature (300 K) a saturated monolayer is dissociatively adsorbed producing glycine in its anionic form; a glycinate species. The orientation of the adsorbed species changes as the coverage increases with bonding occurring initially via equivalent oxygen atoms of the carboxylate group, orientated broadly perpendicular to the surface, similar to the bonding found in simple carboxylic acids. At saturation coverage, at 300 K, a different orientation of the carboxylate group is observed with the carboxylate binding in a more unidentate fashion. The IR spectra are then very similar to those of solid copper glycinate. Low energy electron diffraction (LEED) shows a poor (3×2)g pattern. On annealing this fully covered surface to 420 K, there is no significant desorption and the IR spectra show dramatic changes in relative band intensities with the spectra becoming more similar to those obtained at low coverage. The (3×2)g LEED pattern sharpens considerably. At this stage, we suggest that anionic glycine (glycinate) species are adsorbed with the O 2–C–C–N backbone essentially parallel to the surface with interadsorbate bonding dominated by CH⋯O and N–H⋯O hydrogen bonds similar to those found in bulk glycine. The (3×2) structure is a compromise between optimal adsorbate site, intermolecular hydrogen bonding and maximum adsorbate density. With the substrate held just below room temperature, multilayers of the zwitterionic glycine form which desorb in two stages with the second layer leaving the surface at a higher temperature than the other layers. At liquid nitrogen temperatures (85 K), some of the glycine is present on the surface in its acid form, as evidenced by the presence of a carbonyl (CO) stretch.

Studies in the setting of polyelectrolyte cements: part VII. The effect of divalent metal chlorides on the properties of zinc polycarboxylate and glass-ionomer dental cements.

A study is reported in which a zinc polycarboxylate and a glass polyalkenoate dental cement, were prepared from aqueous solutions of divalent metal chlorides, namely ZnCl2, CaCl2, MgCl2 and SrCl2, all at 1.0 mol dm-3 concentration, as well as from pure water. Calcium chloride was employed at additional concentrations, i.e. 2.0, 0.5 and 0.1 mol dm-3. As was previously found for monovalent salts, setting of the zinc polycarboxylate was speeded up and water uptake generally enhanced by the presence of the divalent metal salts. However, the divalent salts were found to reduce the compressive strength at 24 h (from 86 MPa to about 60 MPa). The glass polyalkenoate showed broadly similar effects to those observed in the presence of monovalent salts, with the setting time being increased, water uptake inhibited and compressive strength at 24 h reduced; however, by contrast, the working time was generally reduced. These results occur because the rate of the neutralization process is increased by the divalent salts, a consequence of the reduced pH of the poly(acrylic acid) caused by these salts. Infrared spectroscopy demonstrated interactions between the metal chlorides and poly(acrylic acid), with various chelate structures being apparent from the position of the asymmetric carbonyl stretch.

Crystallisation and phase behaviour of poly(butylene terephthalate)/polyarylate blends

Crystallisation behaviours of poly(butylene terephthalate) (PBT) homopolymer and PBT in PBT/polyarylate (PAr) blend have been studied using FT-i.r. spectrometry. In addition, the phase behaviour of the PBT/PAr blend has been investigated using a dynamic mechanical analyser. Several conformational sensitive bands in PBT were investigated using FT-i.r. spectra. The peak of carbonyl stretch mode in PBT shifted to a lower wavenumber and some of the intensities of methylene rocking modes or bending modes increased as crystallisation of PBT proceeded. PAr reduced the crystallisation rate of PBT in PBT/PAr blend. In dynamic mechanical thermal analysis, PBT and PAr were completely miscible in amorphous state, irrespective of the composition of the blend. Crystallisation-induced amorphous phase separation of PBT/PAr blend was detected. Single T g was observed in all compositions in amorphous state. Different phase separation behaviour was observed in PBT- and PAr-rich blends.

Theoretical Investigation of the Cyclic Peptide System Cyclo[(d-Ala-Glu-d-Ala-Gln)m=1-4

Using newly developed theoretical methods, we present preliminary results for some calculated properties of the cyclic peptide system cyclo[(d-Ala-Glu-d-Ala-Gln)m=1-4]. These calculations are motivated by the fact that the cyclo[(d-Ala-Glu-d-Ala-Gln)2] cyclic peptide structure was the first cyclic peptide structure synthesized by an experimental group through a self-assembly process. In this paper, we calculate the electronic structure and vibrational mode properties of the isolated ring structures for cyclo[(d-Ala-Glu-d-Ala-Gln)m=1-4] and of the cyclic peptide nanotube system cyclo[(d-Ala-Glu-d-Ala-Gln)2]. The HOMO−LUMO gap is wide (∼5.0 eV) yielding a transparent material with possibly unique bioelectronic device applications. In addition, we find that the C−O carbonyl stretch modes and the N−H amide-I stretch modes of these isolated ring structures to be highly localized at around 1773 and 3192 cm-1, respectively.

Theoretical study of electronically excited cis- and trans-glyoxal

The equation-of-motion coupled cluster method for excitation energies in the singles and doubles approximation (EOMEE-CCSD) is applied to an investigation of the structure and harmonic frequencies of planar conformers of glyoxal in their first excited singlet states. For the trans-isomer, agreement between calculated harmonic frequencies and observed fundamentals is generally satisfactory, although the theoretical values are slightly more than 10% too high for the carbonyl stretching modes. Parallel calculations of the corresponding ground state properties allow for an empirical prediction of the excited state frequencies in which calculated differences in normal-mode frequencies are simply added to the ground state fundamentals. Estimates made by this procedure are within 20 cm −1 of the actual positions for all modes except the carbonyl stretches and the in-phase CH stretch, for which the experimental assignment is uncertain. For the cis isomer, the results presented here should be useful in analysis of future experiments. Significantly, the experimentally inferred dipole moment of 4.8 Debye for this isomer appears to be in error.

Thermal decomposition reactions of acetaldehyde and acetone on Si(100)

We have studied the thermal interactions of acetone and acetaldehyde on Si(100), both sputtered and annealed, using high resolution electron energy loss spectroscopy, (HREELS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). There is no carbonyl stretch in HREELS and the C and O(1s) XPS peaks reflect two different carbonyl processes, one involving bond cleavage, the other a reduction of the C–O bond order. Hydrogen TPD gives a peak at 840–850 K which is as much as threefold more intense than from H-saturated Si(100). SiO desorbs near 1050 K and XPS shows total loss of oxygen and retention of carbon. Approximately 34% of the acetaldehyde monolayer and 62% of the acetone monolayer decomposes on annealed Si(100) to produce silicon carbide. In contrast, after sputtering with 500 eV Ar ions, these percentages are reduced to 14% and 25%, respectively. We conclude that Si dimers play an important role in the chemistry of carbonyl groups.

Ultrafast infrared study of the ultraviolet photodissociation of Mn2(CO)10

The 310 nm photodissociation of dimanganese decarbonyl, Mn2(CO)10, in cyclohexane has been studied using ultrafast infrared spectroscopy. Subpicosecond IR detection near 5 μm, in the region of carbonyl stretch vibrations, is carried out using a frequency-resolved broad (>100 cm−1) IR probe pulse. The evolution of infrared spectra provide information on Mn2(CO)10 and its photoproducts. In the terminal carbonyl region (1970–2050 cm−1), bleach signals are observed due to loss of the parent compound. A broad, featureless absorption appears promptly and narrows upon vibrational cooling (on the 50 ps time scale) to reveal IR spectral features consistent with those previously assigned to two photoproducts, Mn(CO)5 and Mn2(CO)9. The bleach signals partially recover, indicating ground state recovery of vibrationally excited Mn2(CO)10, which is consistent with previous reports of geminate recombination. Mn2(CO)9 signal appearance times are 54(4) ps for the terminal CO band at 2052 cm−1 and 31(5) ps for the bridge band at 1760 cm−1. The direct observation of the bridging band establishes 31 ps as an upper limit for the bridge bond formation. The time dependence of the signal is probably due primarily to vibrational cooling of low frequency modes. The times are somewhat longer than the shorter of two times measured in the visible region and attributed to low frequency mode thermalization by Zhang and Harris [J. Chem. Phys. 95, 4024 (1991)]. Vibrational coherence effects were observed at early times (close to the vibrational dephasing time).

Chemical Modification and Metal Binding Studies of Datura innoxia

The esterification of carboxylate functionalities present in the cell walls of Datura innoxia results in a decrease in metal uptake by as much as 40%, depending on the metal studied. These findings suggest that carboxylate groups are important in metal ion adsorption to this biomaterial. Base hydrolysis of the native plant material resulted in a slight increase in metal ion uptake for Cu2+ and Sr2+ and a decrease in uptake for Cd2+. These results are attributed to the hydrolysis of esters native to the plant material, which increases the carboxylate content but also results in conformational changes in the macromolecules that comprise the cell fragments. Both the esterified product and the hydrolyzed material were examined via infrared spectroscopy. A peak occuring at 1735 cm-1 (attributed to the carbonyl stretch) confirmed the esterification process. The infrared spectra of the hydrolyzed samples indicate further ionization of carboxylate groups or hydrolysis of esters native to D. innoxia.

The structural basis for pseudoreversion of the E165D lesion by the secondary S96P mutation in triosephosphate isomerase depends on the positions of active site water molecules.

The structural basis for the improvement in catalytic efficiency of the mutant E165D chicken triosephosphate isomerase by the secondary mutation, S96P, has been analyzed using a combination of X-ray crystallography and Fourier transform infrared spectroscopy. All X-ray structures were of the complex of phosphoglycolohydroxamate (PGH), an intermediate analog, with the isomerase, and each was solved to a resolution of 1.9 A. Comparison of the structure of the double mutant, E165D.S96P, with that of the single mutant, E165D, as well as with the wild-type isomerase shows only insignificant differences in the positions of the side chains in all of the mutants when compared with the wild-type isomerase, except that in both the E165D and E165D.S96P mutants, the aspartate side chain was approximately 0.7 A further away from the substrate analog than the glutamate side chain. Significant differences were observed in the crystal structure of the E165D.S96P double mutant in the positions of ordered water molecules bound at the active site. The loss of two water molecules located near the side chain at position 165 was observed in isomerases containing the S96P mutation. The resulting increase in hydrophobicity of the pocket probably causes an increase in the pKa of the catalytic base, D165, thereby improving its basicity. A new ordered water molecule was observed underneath the bound PGH in the E165D.S96P structure, which likely decreases the pKa's of the substrate protons, thereby increasing their acidity. An enzyme derived carbonyl stretch at 1746 cm-1 that is only observed in the IR spectrum of the E165D.S96P double mutant isomerase with bound substrates has been assigned to a stable ground state protonated D165-enediol(ate) intermediate complex. Thus, the gain in activity resulting from the S96P second site change probably results from a combination of improving the basicity of the enzyme, improving the acidity of the substrate protons, and stabilization of a reaction intermediate. All three of these effects seem to be caused by changes in bound water molecules.

Modeling Vibrational Spectra of Amino Acid Side Chains in Proteins: The Carbonyl Stretch Frequency of Buried Carboxylic Residues

Modeling Vibrational Spectra of Amino Acid Side Chains in Proteins: The Carbonyl Stretch Frequency of Buried Carboxylic Residues

A Comparative Study of ATP and GTP Complexation with Trivalent Al, Ga and Fe Cations. Determination of Cation Binding Site and Nucleotide Conformation by FTIR Difference Spectroscopy

The interactions of adenosine-5'-triphosphate (Na2H2ATP) and guanosine-5'-triphosphate (Na2H2GTP) with trivalent Al, Ga and Fe cations are investigated in aqueous solution at pH = 6-7 with metal/nucleotide ratios (r) 1/10, 1/2, 1 and 4. Fourier Transform infrared (FTIR) difference spectroscopy is used to characterize the nature of metal cation binding and nucleotide conformational variations, in aqueous solution. Spectroscopic evidences show that at low cation concentration (r = 1/10), metal binding is mainly through the -PO2- and -PO3(2-) groups of the nucleotide moiety (outer-sphere), while at r > 1/10, in addition to direct metal-phosphate binding (chelation), minor metal-base complexes are also formed via indirect metal-NH2 or metal-carbonyl interaction (through H2O). At r = 1, metal macrochelate formation occurs, involving direct cation coordination to phosphate groups and imidazol N-7 atom with an indirect metal-NH2 or metal-carbonyl interaction. At r = 4, polymerization of these metal complexes is observed through nucleotide phosphate chain. The amount of cation/phosphate binding is considerably larger than that of metal/base binding. Evidence for metal-phosphate coordination comes from major spectral changes (shifting and intensity variations) of the alpha-, beta- and gamma-PO2- vibrations in the region 1250-900 cm-1, while metal-base binding is characterized by spectral alterations of the bands at 1700 (guanine carbonyl stretch), 1695 (adenine N-1-H+ mode), 1650-1660 (NH2 bending), 1610-1613 (pyrimidine), 1575 (pyrimidine), 1535-1540 (imidazol), 1470-1480 (imidazol) and 1377 cm-1 (imidazol). The ribose moiety shows C2'-endo/anti conformation for the free ATP and its metal-ATP complexes with an infrared marked band at 821 cm-1. A mixture of both C2'-endo/anti and C3'-endo/anti conformations are occurring for the free GTP and its trivalent metal-complexes with marker bands at 820 and 800 cm-1.

Conformational Rigidity in a Self-Assembled Monolayer of 4-Mercaptobenzoic Acid on Gold

Self-assembled monolayers of 4-mercaptobenzoic acid exhibit infrared spectral features for the carboxylic acid functionality that are characteristic of isolated vapor-phase molecules (e.g., sharp O-H stretch band, high-frequency non-hydrogen-bonded carbonyl stretch band) and inconsistent with molecules that are dimerized or that reside in condensed-phase hydrogen-bonding environments. This is in sharp contrast with the behavior of ω-mercaptoalkanoic acid monolayers, which exhibit spectral features characteristic of hydrogen-bonded and/or dimerized acid groups (e.g., no obvious O-H stretch band, lower frequency carbonyl stretch band). It is postulated that these characteristic differences reflect differing degrees of conformational rigidity in the monolayers, with the aromatic monolayers being sufficiently rigid and oriented as to prevent the intermolecular hydrogen bonding/dimerization that is commonly observed in aliphatic monolayers.

Jet-cooled fluorescence excitation spectrum, carbonyl wagging, and ring-puckering potential energy functions of 3-cyclopenten-1-one in its S1(n,π*) electronic excited state

The jet-cooled fluorescence excitation spectrum of 3-cyclopenten-1-one has been recorded in the 308–330 nm region, and the electronic origin for the S1(n,π*) state of A2 symmetry was observed at 30 229 cm−1. The observed spectrum consists of more than 80 bands involving primarily ν3 (carbonyl stretch), ν29 (carbonyl out-of-plane wagging), and ν30 (ring puckering). Bands were also assigned to combinations with seven other vibrational modes. The energies for the v=0 to 11 quantum states of ν29 were measured and used to determine a one-dimensional potential energy function. This function has energy minima at wagging angles of ±24° and a barrier to inversion of 939 cm−1. Four bands associated with ν30 were observed and were used to determine an asymmetric single-minimum one-dimensional ring-puckering potential energy function for the S1(n,π*) state. The ring-puckering energy levels in the ν29 vibrational excited states are little changed from the v=0 state indicating that there is little interaction between the carbonyl wagging and the ring-puckering motions.

Anomalies in the Isotropic Raman Spectra of Liquid Mixtures of Isotopomers of Formamide. Intermolecular Interactions in the Carbonyl Stretching Region

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAnomalies in the Isotropic Raman Spectra of Liquid Mixtures of Isotopomers of Formamide. Intermolecular Interactions in the Carbonyl Stretching RegionA. Mortensen, O. Faurskov Nielsen, J. Yarwood, and V. ShelleyCite this: J. Phys. Chem. 1995, 99, 13, 4435–4440Publication Date (Print):March 1, 1995Publication History Published online1 May 2002Published inissue 1 March 1995https://doi.org/10.1021/j100013a011RIGHTS & PERMISSIONSArticle Views76Altmetric-Citations29LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (706 KB) Get e-Alerts

Ab Initio Molecular Orbital Study of Substituent Effects in Vaska Type Complexes (trans-IrL2(CO)X): Electron Affinities, Ionization Potentials, Carbonyl Stretch Frequencies and the Thermodynamics of H2 Dissociative Addition

Ab initio electronic structure calculations are used to study substituent effects in Vaska-type complexes, trans-IrL{sub 2}(CO)X (1-X) (X = F, Cl, Br, I, CN, H, CH{sub 3}, SiH{sub 3}, OH, and SH; L = PH{sub 3}). Both the electron affinity and the ionization potential of 1-X are computed to increase upon descending the halogen series of complexes, which indicates, surprisingly, that the complexes with more electronegative halogens are more difficult to reduce and easier to oxidize. The computed electron affinity trend is consistent with the half-wave reduction potential trend known for 1-X (L = PPh{sub 3}; X = F, Cl, Br, and I). Computed carbonyl stretch frequencies for 1-X are greater than experimental values (L = PPh{sub 3}), but observed trends are well reproduced. The redox and spectroscopic trends are discussed in terms of the substituent effects on the electronic structure of 1-X, particularly as revealed in the molecular orbital energy level diagrams of these complexes. The reaction energy for H{sub 2} addition to 1-X, leading to the cis,trans-(H){sub 2}IrL{sub 2}(CO)X (2-X) product, has been computed. After electron correlation effects are included (MP4(SDTQ)), the reaction enthalpy computed for 1-CI is {minus}18.4 kcal/mol (L = PH{sub 3}) as compared to amore » reported experimental value of {minus}14 kcal/mol (L = PPh{sub 3}). Compared with available experimental data, the electronic effects of L(L = PH{sub 3}, NH{sub 3}, or AsH{sub 3}) and X on the thermodynamics of the H{sub 2} addition reaction are accurately reproduced by the model calculations at all levels of theory (HF and MPn). Formation of the hypothetical products cis,trans- and trans,trans-(H){sub 2}IrL{sub 2}(CO)X(2-X and 3-X) (X = BH{sub 2}, NH{sub 2}, and PH{sub 2}) is used to demonstrate that {pi}-acceptor substituents promote the H{sub 2} addition reaction to 1-X while {pi}-donor substituents disfavor addition.« less

Analysis of hydrogen bonding in enzyme-substrate complexes of chloramphenicol acetyltransferase by infrared spectroscopy and site-directed mutagenesis.

Chloramphenicol acetyltransferase (CAT) reversibly transfers an acetyl group between CoA and the 3-hydroxyl of either chloramphenicol (Cm) or 1-acetylchloramphenicol (1AcCm). The products of the forward reactions, 3-acetylchloramphenicol (3-AcCm) and 1,3-diacetylchloramphenicol (1,3Ac2-Cm), are the substrates for the reverse reaction. The role of the 3-acetyl carbonyl group in the binding of the substrates 3AcCm and 1,3Ac2Cm to CAT has been investigated using infrared spectroscopy. Comparison of difference spectra (3-[12C = O]acetyl- minus 3-[13C = O]acetyl-) obtained for the binary complexes of 3AcCm with wild-type CAT, and with a variant wherein serine-148 is replaced by alanine (S148A), reveals a large (9 cm-1) down frequency shift for the 3-acetyl carbonyl stretch in the wild-type complex, indicative of a hydrogen bond between this carbonyl and the hydroxyl group of Ser-148. The carbonyl bandwidth in the wild-type complex is reduced by 33% compared to that for the complex with S148A, indicating restriction of carbonyl mobility and dispersion in the former, an observation consistent with the proposed hydrogen bond between the ester carbonyl and the hydroxyl of Ser-148. Repetition of the experiment using 1,3Ac2Cm as the ligand reveals a frequency shift of only 3 cm-1 between wild-type and S148A complexes, indicating only a small change in the strength of carbonyl interaction.(ABSTRACT TRUNCATED AT 250 WORDS)