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At least 19 recordsLinked to original sources
PubMed1991

Effect of protonation on the isomerization properties of n-butylamine Schiff base of isomeric retinal as revealed by direct HPLC analyses: selection of isomerization pathways by retinal proteins.

Alumina adsorption chromatography and ion-pair reversed-phase chromatography were developed to analyze the isomers of unprotonated and protonated n-butylamine Schiff base of retinal (RSB and PRSB), respectively. Photoisomerization starting from the all-trans, 11-cis and 13-cis isomers was traced for RSB in n-hexane, acetonitrile, methanol and 1-butanol, and for PRSB in methanol, acetonitrile and 1-butanol. The quantum yields of photoisomerization for the all-trans, 9-cis, 11-cis and 13-cis isomers were determined for RSB and PRSB in the above solvents except 1-butanol. On the other hand, photoisomerization of isomeric retinal bound (through Schiff base linkage) to bovine serum albumin (RBSA) in aqueous solution (pH 3, 7 and 12) as well as thermal isomerization of RSB (in n-hexane), PRSB (in methanol) and RBSA (in aqueous solution, pH 7) were traced starting from the all-trans, 11-cis, and 13-cis isomers. Protonation of RSB drastically changes the pathway of photoisomerization and increases the quantum yields of isomeric RSB. The solvent polarity increases the quantum yields of RSB differently depending on the configuration. Protonation enhances thermal isomerization also. The results of the above model systems are compared with those of retinal proteins to rationalize their selection of the particular isomerization pathways.

Butylamines↗
PubMed2001

Mechanistic imperatives for aldose-ketose isomerization in water: specific, general base- and metal ion-catalyzed isomerization of glyceraldehyde with proton and hydride transfer.

The deuterium enrichment of dihydroxyacetone obtained from the aldose-ketose isomerization of D,L-glyceraldehyde in D(2)O at 25 degrees C was determined by (1)H NMR spectroscopy from the integrated areas of the signals for the alpha-CH(2) and alpha-CHD groups of the product. One mole equivalent of deuterium is incorporated into the product when the isomerization is carried out in 150 mM pyrophosphate buffer at pD 8.4, but only 0.6 mol equiv of deuterium is incorporated into the product of isomerization in the presence of 0.01 M deuterioxide ion, so that 40% of the latter isomerization reaction proceeds by the intramolecular transfer of hydride ion. Several pathways were identified for catalysis of the isomerization of glyceraldehyde to give dihydroxyacetone. The isomerization with hydride transfer is strongly catalyzed by added Zn(2+). Deprotonation of glyceraldehyde is rate-determining for isomerization with proton transfer, and this proton-transfer reaction is catalyzed by Brønsted bases. Proton transfer also occurs by a termolecular pathway with catalysis by the combined action of Brønsted bases and Zn(2+). These results show that there is no large advantage to the spontaneous isomerization of glyceraldehyde in alkaline solution with either proton or hydride transfer, and that effective catalytic pathways exist to stabilize the transition states for both of these reactions in water. The existence of separate enzymes that catalyze the isomerization of sugars with hydride transfer and the isomerization of sugar phosphates with proton transfer is proposed to be a consequence of the lack of any large advantage to reaction by either of these pathways for the corresponding nonenzymatic isomerization in water.

Aldose-Ketose Isomerases↗
PubMed2005

Hydrogen-shift isomerism: mass spectrometry of isomeric benzenesulfonate and 2-, 3- and 4-dehydrobenzenesulfonic acid anions in the gas phase.

The isomeric 3- and 4-dehydrobenzenesulfonic acid anions b and c were prepared by collision induced dissociation (CID) of the [M - H](-) ions of isomeric sulfobenzoic acids obtained by negative electrospray ionization (ESI). The CID spectra (MS(3)) of anions b and c are different from each other, and both are different from that of the isomeric benzenesulfonate anion a, obtained from benzenesulfonic acid. The stability of ions b and c shows that 1,2-proton transfer does not take place in this system under the conditions of the CID experiment. Density functional (DFT) calculations at B3LYP/6-31+G(2d,p) level of theory show that benzenesulfonate anion a is the most stable isomer, and the energies of isomers b and c are higher by more than 65 kcal mol(-1). The calculated energies of the transition states involved in the 1,2-hydrogen migration leading to the interconversion of the isomeric anions are very high (>120 kcal mol(-1)relative to ion a, barrier energies >55 kcal mol(-1)), much higher than those of transition structures leading to fragmentation. This situation does not allow isomerization of ions b and c to a, under the conditions of the CID experiments. The isomeric 2-dehydrobenzenesulfonic acid anion isomerizes to the benzenesulfonate anion a by a facile proton transfer from the SO(3)H group to the adjacent position 2. The results of this work indicate that the gas phase deprotonation of meta- and para-sulfobenzoic acids is a kinetically controlled process.

Journal Article↗
PubMed1997

Isomerization of the free enzyme versus induced fit: effects of steps involving induced fit that bypass enzyme isomerization on flux ratios and countertransport.

In a single-substrate-single-product enzyme reaction, "counter transport', which indicates that the ratio of the forward to the reverse fluxes is less than that expected from the Independence Relationship, is regarded as strong evidence for the free enzyme existing in two states, one of which combines with the substrate and the other with the product, with a slow isomerization between the two conditions. To account for positive and negative co-operativity, found with some enzymes, additional induced-fit reactions bypassing at least part of the isomerization have been proposed. The effects of such additional steps have been examined, using two models: in one, (a), the enzyme passes through an intermediate state during its isomerization, and both substrate and product may react with this state to give rise to the binary complexes; in the other, (b), the substrate may react with the enzyme as soon as the product is released and similarly with the reverse reaction, the isomerization thereby being bypassed completely. In the presence of such additional steps, the following can be concluded. (i) The data should be analysed in terms of the flux ratios, rather than observation of the amount of countertransport. (ii) The additional bypassing steps markedly change the pattern of dependence of the flux ratio on substrate and product concentrations. At high substrate and product concentrations, the ratio remains very dependent on how far the reaction is from equilibrium, and the kinetics are asymmetric. (iii) The mechanism causing the flux ratio to be less than that given by the Independence Relationship differs from that previously described, in that, at least in part, it arises from a 1:1 exchange between substrate and product. (iv) Despite this novel mechanism, there must be two states of the enzyme, combining respectively with substrate and product, and these must not be in rapid exchange. Thus countertransport remains very strong evidence for the existence of two such states. It is no longer a requirement that the enzyme states should be linked by an isomerization step. (v) Under no conditions can the flux ratio exceed that given by the Independence Relationship. (vi) Under unusual conditions the isomerization of the enzyme in model (b) may be undetectable by steady-state kinetics. (vii) Measurements of the coefficients in the flux ratio equations enable limits to be set to certain ratios of the rate constants. In addition to these conclusions, methods are described for (viii) analysing flux ratio data for the presence of induced fit steps and (ix) determining flux ratios from induced transport curves. The derivation of steady state-velocity equations show that: (x) both models may give rise to positive and negative 'co-operativity' and sigmoid substrate-velocity curves, but that, under conditions giving rise to sigmoid curves, the deviation of the flux ratio from that required by the Independence Relationship may be difficult to demonstrate because of the asymmetry of the system. Under all conditions the fluxes at equilibrium should obey hyperbolic kinetics.

Algorithms↗
PubMed1980

Isomeric incorporation of the haem into monomeric haemoglobins of Chironomus thummi thummi. 1. Isolation of chemically homogeneous haemoglobins. Evidence for the isomerism of the haem in the component III.

With high-resolution NMR spectroscopy it has been possible to detect various types of isomerisms in the monomeric component III of the haemoglobins of Chironomus thummi. In the component III prepared from commercially available larvae of Ch. thummi a chemical heterogeneity has been found to occur at the position E6 (Ile or Thr). The imidazole C-2 proton resonance of the adjacent His-E7 is split because of this alternative occupancy. From larvae of an inbreeding of the single subspecies Chironomus thummi thummi a component III material has been isolated in which the position E6 is occupied by one single amino acid. An additional isomerism of this haemoglobin III is produced by an isomeric incorporation of the haem group into the haem pocket. This isomerism has been established from the complex signal pattern of the mesoproton resonances of the diamagnetic ligated haemoglobin III. Other evidences of this isomerism have been obtained from an analysis of th high-field proton resonances.

Animals↗
PubMed1979

Enzymic cis-trans isomerization of nitrofuran derivatives: isomerizing activity of xanthine oxidase, lipoyl dehydrogenase, DT-diaphorase and liver microsomes.

Xanthine oxidase (xanthine:oxygen oxidoreductase, EC 1.2.3.2) supplemented with an electron donor could catalyze the cis-trans isomerization of 3-(5-nitro-2-furyl)-2-(2-furyl)acrylamide, 3-(5-nitro-2-furyl)-2-phenylacrylamide and 3-(5-nitro-2-furyl)-2-(2-furyl)acrylonitrile. The direction of isomerization (cis leads to trans, cis in equilibrium trans or trans leads to cis) is dependent on the chemical structure of these nitrofuran derivatives. Lipoyl dehydrogenase (NADH:lipoamide oxidereductase, EC 1.6.4.3), DT-diaphorase (NAD(P)H:(quinone-acceptor) oxidoreductase, EC 1.6.99.2) and liver microsomes could also catalyze the conversion of cis-3-(5-nitro-2-furyl)-2-(2-furyl)acrylamide to its trans isomer in the presence of an appropriate electron donor. Such isomerizing activity of these enzymes is much higher than their nitro-reducing activity. In addition, the cis-trans isomerization of some nitrofuran derivatives was demonstrated with the liver slices and the small intestines of rats. A new cis-trans isomerization mechanism which is based on transfer of a single electron by an enzyme system to a nitrofuran derivative to give the radical-anion was proposed. This postulated mechanism was supported by the preliminary experiments using pulse radiolysis technique.

Animals↗
PubMed2006

Isomeric distribution and catalyzed isomerization of cobalt(III) complexes with pentadentate macrocyclic ligands. importance of hydrogen bonding.

We have investigated the isomeric distribution and rearrangement of complexes of the type [CoXLn]2+,3+ (where X = Cl-, OH-, H2O, and Ln represents a pentadentate 13-, 14-, and 15-membered tetra-aza or diaza-dithia (N4 or N2S2) macrocycle bearing a pendant primary amine). The preparative procedures for chloro complexes produced almost exclusively kinetically preferred cis isomers (where the pendant primary amine is cis to the chloro ligand) that can be separated by careful cation-exchange chromatography. For L13 and L14 the so-called cis-V isomer is isolated as the kinetic product, and for L15 the cis-VI form (an N-based diastereomer) is the preferred, while for the L14(S) complex both cis-V and trans-I forms are obtained. All these complexes rearrange to form stable trans isomers in which the pendent primary amine is trans to the monodentate aqua or hydroxo ligand, depending on pH and the workup procedure. In total 11 different complexes have been studied. From these, two different trans isomers of [CoClL14(S)]2+ have been characterized crystallographically for the first time in addition to a new structure of cis-V-[CoClL14(S)]2+; all were isolated as their chloride perchlorate salts. Two additional isomers have been identified and characterized by NMR as reaction intermediates. The remaining seven forms correspond to the complexes already known, produced in preparative procedures. The kinetic, thermal, and baric activation parameters for all the isomerization reactions have been determined and involve large activation enthalpies and positive volumes of activation. Activation entropies indicate a very important degree of hydrogen bonding in the reactivity of the complexes, confirmed by density functional theory studies on the stability of the different isomeric forms. The isomerization processes are not simple and even some unstable intermediates have been detected and characterized as part of the above-mentioned 11 forms of the complexes. A common reaction mechanism for the isomerization reactions has been proposed for all the complexes derived from the observed kinetic and solution behavior.

Journal Article↗
PubMed2004

The dynamics of conformational isomerization in flexible biomolecules. II. Simulating isomerizations in a supersonic free jet with master equation dynamics.

Infrared-induced conformational isomerization of N-acetyl-tryptophan methyl amide is studied theoretically using a microcanonical Rice-Ramsperger-Kassel-Marcus description of the rates on potential energy surfaces calculated using the AMBER and OPLS-AA force fields. The results are compared with the experimental data from Dian et al. in the preceding paper [J. Chem. Phys. 120, 133 (2004)]. An exhaustive search of the potential energy surfaces locates all minima and transition states on these surfaces. A simple model is proposed for the vibrational cooling, and an appropriate cooling rate is chosen as the standard conditions for the master equation simulations by comparison with experiment. The two potential energy surfaces differ in the relative energies of minima and the heights of key transition states, leading to differences in the dominant pathways and rates of conformational isomerization. The predicted quantum yields depend sensitively on the cooling rate, varying from the slow cooling limit in which equilibrium populations are achieved to the fast quenching limit in which conformational isomerization is shut off. The excitation energy is varied from 5 to 20 kcal mol(-1). At the lowest energies, isomerization is completely quenched, while at the highest energies, equilibrium conditions are achieved. In between these extremes, the quantum yields are sensitive to the excitation energy, and can be used to locate the rate-limiting barriers to isomerization.

Computer Simulation↗
PubMed2004

Structural mechanism governing cis and trans isomeric states and an intramolecular switch for cis/trans isomerization of a non-proline peptide bond observed in crystal structures of scorpion toxins.

Non-proline cis peptide bonds have been observed in numerous protein crystal structures even though the energetic barrier to this conformation is significant and no non-prolyl-cis/trans-isomerase has been identified to date. While some external factors, such as metal binding or co-factor interaction, have been identified that appear to induce cis/trans isomerization of non-proline peptide bonds, the intrinsic structural basis for their existence and the mechanism governing cis/trans isomerization in proteins remains poorly understood. Here, we report the crystal structure of a newly isolated neurotoxin, the scorpion alpha-like toxin Buthus martensii Karsch (BmK) M7, at 1.4A resolution. BmK M7 crystallizes as a dimer in which the identical non-proline peptide bond between residues 9 and 10 exists either in the cis conformation or as a mixture of cis and trans conformations in either monomer. We also determined the crystal structures of several mutants of BmK M1, a representative scorpion alpha-like toxin that contains an identical non-proline cis peptide bond as that observed in BmK M7, in which residues within or neighboring the cis peptide bond were altered. Substitution of an aspartic acid residue for lysine at residue 8 in the BmK M1 (K8D) mutant converted the cis form of the non-proline peptide bond 9-10 into the trans form, revealing an intramolecular switch for cis-to-trans isomerization. Cis/trans interconversion of the switch residue at position 8 appears to be sequence-dependent as the peptide bond between residues 9 and 10 retains its wild-type cis conformation in the BmK M1 (K8Q) mutant structure. The structural interconversion of the isomeric states of the BmK M1 non-proline cis peptide bond may relate to the conversion of the scorpion alpha-toxins subgroups.

Amino Acid Sequence↗
PubMed2004

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis-->trans tunneling rate.

Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans-->cis reaction is promoted using resonant excitation of a number of modes in the 3500-7000 cm(-1) region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier (> or =4400 cm(-1)), the measured quantum yields are in average 2%-3%, and this is one order of magnitude smaller than the corresponding values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans-->cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis-->trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans-->cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans-->cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis-->trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process.

Journal Article↗
PubMed1986

Monoterpene cyclases: use of the noncyclizable substrate analog 6,7-dihydrogeranyl pyrophosphate to uncouple the isomerization step of the coupled isomerization-cyclization reaction.

Enzymes from Salvia officinalis capable of catalyzing the isomerization and subsequent cyclization of geranyl pyrophosphate to the monoterpenes (+)-alpha-pinene and (+)-bornyl pyrophosphate were examined with the noncyclizable substrate analog 6,7-dihydrogeranyl pyrophosphate in an attempt to dissect the cryptic isomerization step from the normally coupled reaction sequence. The analog inhibited the cyclization of geranyl pyrophosphate and was itself catalytically active, affording acyclic terpene olefins and alcohols as products. The enzymatic products generated from 6,7-dihydrogeranyl pyrophosphate qualitatively resembled the solvolysis products of 6,7-dihydrolinalyl pyrophosphate, yet they constituted a far higher proportion of olefins, suggesting that enzymatic product formation occurs in an environment relatively inaccessible to water. Since the normal cyclization of geranyl pyrophosphate is considered to proceed via preliminary isomerization to the bound tertiary intermediate (3R)-linalyl pyrophosphate, the results suggest that the analog undergoes the normal pyrophosphate ionization-migration step, giving rise in this case to (3R)-6,7-dihydrolinalyl pyrophosphate which is reionized, and because the subsequent cyclizations are precluded, the resulting cation is either deprotonated or captured by water. In divalent metal ion requirement, pH optimum, and other characteristics, the enzymatic transformation of the analog resembles the normal monoterpene cyclase reaction.

Catalysis↗
PubMed2001

Importance of packing coefficients of host cavities in the isomerization of open host frameworks: guest-size-dependent isomerization in cholic acid inclusion crystals with monosubstituted benzenes.

The crystal structures of inclusion compounds of cholic acid (CA) with 28 monosubstituted benzenes have been systematically investigated. All of the crystals belong to the monoclinic space group P2(1) and have bilayer structures with one-dimensional molecular channels that can include guest compounds. They are classified into four types of host frameworks that depend on the conformations and stacking modes of the host compound. The host frameworks and the host-guest ratios depend primarily on the molecular volumes of the guest compounds. The packing coefficient of the host cavity (PCcavity), which is the volume ratio of the guest compound to the host cavity, is used to clarify the relationship between the guest volume and isomerization of the host frameworks. The value of PCcavity, for stable inclusion compounds lies in the range of 55-70%. Compounds out of this range induce isomerization of the host frameworks. The packing coefficients of other host-guest compounds, in which the guest components are included in the host cavities through steric dimensions and van der Waals forces, are also in this range. These results indicate that PCcavity is a useful parameter correlation for guest recognition and isomerization of the host frameworks.

Journal Article↗
PubMed2002

Solid-state reaction study of the trans-to-cis isomerization of (eta-C(5)H(4)Me)Re(CO)[P(OPh)(3)]Br(2): a new mechanism for the isomerization reaction.

Single-crystal X-ray structures have been determined at intermediate stages in the single-crystal to single-crystal trans-to-cis thermal isomerization of (eta(5)-C(5)H(4)Me)Re(CO)[P(OPh)(3)]Br(2) at 150 degrees C. Unit cell parameters and site occupancy factors linked to the induced disorder were monitored and found to follow first-order kinetics in the initial (fast component) stage of the reaction. A rate constant of 30 x 10(-6) s(-1) (0.11 h(-1)) was obtained to first-order approximation for the reaction. The irreversibility of the isomerization from single crystals of the cis form was also confirmed. Fourier and difference Fourier maps suggest a novel 2-fold rotation mechanism for the trans-to-cis isomerization reaction. This mechanism entails a unidirectional anticlockwise movement of the CO ligand and only one Br atom.

Journal Article↗
PubMed2004

Molecular isomerization induced by ultrashort infrared pulses. II. Pump-dump isomerization using pairs of time-delayed half-cycle pulses.

We investigate population transfer across the barrier in a double-well potential, induced by a pair of time-delayed single-lobe half-cycle pulses. We apply this setup both to a one-dimensional (1D) quartic model potential and to a three-dimensional potential representing HCN-->HNC isomerization. Overall the results for the two systems are similar, although in the 3D system some additional features appear not seen in the 1D case. The generic mechanism of population transfer is the preparation by the pump pulse of a wave packet involving delocalized states above the barrier, followed by the essentially 1D motion of the delocalized part of wave packet across the barrier, and the eventual de-excitation by the dump pulse to localized states in the other well. The correct timing is given by the well-to-well passage time of the wave packet and its recurrence properties, and by the signs of the field lobes which determine the direction and acceleration or deceleration of the wave packet motion. In the 3D system an additional pump-pump-dump mechanism linked to wave packet motion in the reagent well can mediate isomerization. Since the transfer time and the pulse durations are of the same order of magnitude, there is also a marked dependence of the dynamics and the transfer yield on the pulse duration. Our analysis also sheds light on the pronounced carrier envelope phase dependence previously observed for isomerization and molecular dissociation with one-cycle and sub-one-cycle pulses.

Journal Article↗
PubMed2004

Cumulative isomerization probability studied by various transition state wave packet methods including the MCTDH algorithm. Benchmark: HCN-->CNH isomerization.

The 3D cumulative isomerization probability N(E) for the transfer of a light particle between two atoms is computed by one time-independent and two time-dependent versions of the transition state wave packet (TSWP) method. The time-independent method is based on the direct expansion of the microcanonical projection operator on Chebyshev polynomials. In the time-dependent TSWP methods, the propagations are carried out by the split operator scheme and the multiconfiguration time-dependent Hartree (MCTDH) algorithm. This is the very first implementation of the TSWP method in the Heidelberg MCTDH package [G. W. Worth, M. H. Beck, A. Jackle, and H.-D. Meyer, The MCDTH package, Version 8.2 (2000); H.-D Meyer, Version 8.3 (2002). See http://www.pci.uni-heidelberg.de/tc/usr/mctdh/]. The benchmark is the HCN-->CNH isomerization for zero total angular momentum. Particular insights are given into the tunneling region. In larger systems, the time-dependent version of TSWP making use of the MCTDH algorithm will permit to treat more and more modes quantum mechanically, for very accurate results. Therefore, it was important to calibrate the implementation. Besides, we also assess the efficiency of a reduced dimensionality approach by comparing the new exact 3D calculations of N(E) for the HCN-->CNH isomerization with results obtained via 1D or 2D active subspaces. This suggests that, it should be possible to take directly benefit of the present 3D approaches, adapted for triatomic Jacobi coordinates to compute N(E) for H-transfer in larger systems, via three active coordinates. The prerequisite is then the simplification of the reduced 3D kinetic energy operator with rigid constraint to take the form corresponding to a pseudo triatomic system in Jacobi coordinates with effective masses. This last step is checked in the methoxy radical and malonaldehyde. Finally, different ways to obtain reliable eigenvectors of the flux operator associated with a dividing surface are revisited.

Journal Article↗
PubMed2006

The infrared-driven cis-trans isomerization of HONO. II: Vibrational relaxation and slow isomerization channel.

In a recent paper [R. Schanz et al., J. Chem. Phys. 122, 044509 (2005)], we investigated the IR-driven cis-trans isomerization of HONO in a Kr matrix with the help of femtosecond IR spectroscopy. We found that isomerization occurs on a 20 ps time scale, however, with a cis-->trans quantum yield of only 10% that is significantly below the value reported in the literature (close to 100%). At the same time, we concluded that vibrational energy has not completely dissipated out of the molecule at the maximum delay time we reached in this study (500 ps). In order to verify whether additional, slower reaction channels exist, we extend the study here to delay times up to 100 ns. At a temperature of 32 K, we indeed find an additional isomerization channel on a 2 ns timescale, which increases the total cis-->trans quantum yield to approximately 30%. The trans-->cis quantum yield is approximately 7%. There is still a discrepancy between the quantum yields we observe and the literature values, however, we provide experimental evidence that this discrepancy is due to the different temperatures of our study. Vibrational cooling occurs on a 20 ns time scale, and cascades in a highly nonstatistical manner through one single normal mode (most likely the ONO bending mode nu(5)). Intermolecular energy dissipation into the rare gas matrix is more efficient than intramolecular vibrational energy redistribution and the matrix environment can certainly not be considered a weak perturbation.

Journal Article↗
PubMedDate not supplied

[Gel-permeating chromatography of interacting multi-component systems. I. Study of protein isomerization by the method of gel-permeation chromatography. II. Method of determining the kinetic constant of isomerization].

A technique is suggested of determining the isomerization constants by means of gel-permeation chromatography. Chromatograms of proteins and expressions for statistical moments of their distribution along the chromatographic column obtained in the authors previous work [1], are used. A necessary condition for optimal setup of the GPCh experiments is found. Fulfilment of this condition provides a maximal accuracy of determination of the isomerization constants.

Chromatography, Gel↗
PubMed1998

Analysis of isomeric mixtures using blackbody infrared radiative dissociation: determining isomeric purity and obtaining individual tandem mass spectra simultaneously.

A new method that makes possible, for the first time, simultaneous acquisition of individual dissociation mass spectra of isomeric ions in mixtures is presented. This method exploits the exquisite sensitivity of blackbody infrared radiative dissociation kinetics to minor differences in ion structure. Instead of separating precursor ions based on mass (isomers have identical mass), fragment ions are related to their original precursor ions on the basis of rate constants for dissociation. Mixtures of the peptide isomers des-R1 and des-R9 bradykinin are dissociated simultaneously at several temperatures. By fitting the kinetic data to double-exponential functions, the dissociation rate constant and abundance of each isomer in the mixture are obtained. To overcome the difficulty of fitting double-exponential functions, a novel global analysis method is used in which several dissociation data sets collected at different temperatures are simultaneously fit. The kinetic data measured at multiple temperatures are modeled with the preexponentials (corresponding to the abundance of each isomer) as "global" parameters which are constant for all data sets and the exponentials (rate constants) as "local" variables which differ for each data set. The use of global parameters significantly improves the accuracy with which abundances and dissociation rate constants of each individual compound can be obtained from the mixture data. Fragment ions produced from a mixture of these two isomers are related back to their respective precursor ions from the kinetic data. Thus, not only can the composition of the isomeric mixture be determined but an individual tandem mass spectrum of each component in the mixture can be obtained.

Bradykinin↗