So Hee Yoon

Temperature of peptide ions generated by matrix-assisted laser desorption ionization and their dissociation kinetic parameters.

Product ion yields in postsource decay and photodissociation at 193 and 266 nm were measured for some peptide ions without a basic amino acid residue ([Y(6) + H](+), [F(5) + H](+), and [YPFVEPI + H](+)) generated by matrix-assisted laser desorption ionization (MALDI). Data indicated statistical nature for the dissociation processes. Assuming that peptide ions formed by MALDI are in thermal equilibrium at temperature T and that their dissociation rate constants are specified by the critical energy (E(0)) and entropy (DeltaS(double dagger)), a method based on kinetic analysis was devised to determine these parameters simultaneously. The matrix used was found to affect the effective temperature of peptide ions, 2,5-dihydroxybenzoic acid (400-430 K) < sinapinic acid (440 K) < alpha-cyano-4-hydroxycinnamic acid (460-510 K), in agreement with previous perceptions. E(0) of around 0.6 eV and DeltaS(double dagger) of -24 eu were smaller than previous quantum chemical results for small model peptide ions.

A comparative study of in- and post-source decays of peptide and preformed ions in matrix-assisted laser desorption ionization time-of-flight mass spectrometry: Effective temperature and matrix effect

In-source decay (ISD) and post-source decay (PSD) of a peptide ion ([Y6 + H]+) and a preformed ion (benzyltriphenylphosphonium, BTPP) generated by matrix-assisted laser desorption ionization (MALDI) were investigated with time-of-flight mass spectrometry. α-Cyano-4-hydroxycinammic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) were used as matrices. For both ions, ISD yield was unaffected by delay time, indicating rapid termination of ISD. This was taken as evidence for rapid expansion cooling of hot “early” plume formed in MALDI. CHCA was hotter than DHB for [Y6 + H]+ while the matrix effect was insignificant for BTPP. The “early” plume temperature estimated utilizing previous kinetic results was 800–900 K, versus 400–500 K for “late” plume. The results support our previous finding that the temperature of peptide ions interrogated by tandem mass spectrometry was lower than most rough estimates of MALDI temperature.

Time-Resolved Photodissociation of Singly Protonated Peptides with an Arginine at the N-Terminus: A Statistical Interpretation

Time-evolution of product ion signals in ultraviolet photodissociation (UV-PD) of singly protonated peptides with an arginine at the N-terminus was investigated by using a tandem time-of-flight mass spectrometer equipped with a cell floated at high voltage. Observation of different time-evolution patterns for different product ion types—an apparently nonstatistical behavior—could be explained within the statistical framework by invoking consecutive formation of some product ions and broad internal energy distributions for precursor ions. an+1 and bn ions were taken as the primary product ions from this type of peptide ions. Spectral characteristics in post-source decay, UV-PD, and collisionally activated dissociation at low and high kinetic energies could be explained via rough statistical calculation of rate constants. Specifically, the striking characteristics in high-energy CAD and UV-PD—dominance of bn and dn formed via an+1—were not due to the peculiarity of the excitation processes themselves, but due to quenching of the bn channels caused by the presence of arginine.

Dissociation mechanisms and implication for the presence of multiple conformations for peptide ions with arginine at the C-terminus: time-resolved photodissociation study.

Time-resolved photodissociation (PD) patterns of singly protonated peptides with arginine at the C-terminus (C-arg peptide ions) have been used to classify the dissociation channels into two categories, i.e. high-energy channels generating v, w and x and low-energy ones generating b, y and z. x + 1 formed by C(alpha)-CO cleavage seems to be the intermediate ion in high-energy channels just as a + 1 is for N-arg peptide ions. Difference in time-resolved pattern indicates that the two sets of channels, high- and low-energy ones, are not in direct competition. Noncompetitive dissociation is also indicated by the observation of anomalous effect of matrix used in matrix-assisted laser desorption ionization, a cooler matrix generating more high-energy product ions both in spontaneous dissociation and in PD. Results from detailed investigation suggest that the two sets of channels start from two (or more) different conformations.

Time-Resolved Photodissociation Study of Singly Protonated Peptides with a Histidine Residue Generated by Matrix-Assisted Laser Desorption Ionization: Dissociation Rate Constant and Internal Temperature

Product ion yields in post-source decay and time-resolved photodissociation at 193 and 266 nm were measured for some peptide ions with a histidine residue ([HF6+H]+, [F6H+H]+, and [F3HF3+H]+) formed by matrix-assisted laser desorption ionization (MALDI). Compared with similar data for peptide ions without any basic residue reported previously, significant reduction in dissociation efficiency was observed. Internal temperatures (T) of the peptide ions and their dissociation kinetic parameters—the critical energy (E0) and entropy (ΔS‡)—were determined by the method reported previously. Slight decreases in E0, ΔS‡, and T were responsible for the histidine effect-reduction in dissociation rate constant. Regardless of the presence of the residue, ΔS‡ was far more negative than previous quantum chemical results. Based on this, we propose the existence of transition structures in which the nitrogen atoms in the histidine residue or at the N-terminus coordinate to the reaction centers. Reduction in T in the presence of a histidine residue could not be explained based on popular models for ion formation in MALDI, such as the gas-phase proton transfer model.

Influence of basic residues on dissociation kinetics and dynamics of singly protonated peptides: time-resolved photodissociation study.

Product ion yields in postsource decay and time-resolved photodissociation at 193 and 266 nm were measured for some peptide ions with lysine ([KF6 + H]+, [F6K + H]+, and [F3KF3 + H]+) formed by matrix-assisted laser desorption ionization. The critical energy (E0) and entropy (DeltaS(double dagger)) were determined by RRKM fitting of the data. The results were similar to those found previously for peptide ions with histidine. To summarize, the presence of a basic residue, histidine or lysine, inside a peptide ion retarded its dissociation by lowering DeltaS(double dagger). On the basis of highly negative DeltaS(double dagger), presence of intramolecular interaction involving a basic group in the transition structure was proposed.

Changes in Dissociation Efficiency and Kinetics of Peptide Ions Induced by Basic Residues and Their Mechanistic Implication

With matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry, total abundance of product ions formed by dissociation inside (in-source decay, ISD) and outside (post-source decay, PSD) the source was measured for peptide ions [Y5X + H]+, [XY5 + H]+, [Y2XY3 + H]+, and [XY4X + H]+ (X = tyrosine (Y), histidine (H), lysine (K), and arginine (R) with H for the ionizing proton). α-Cyano-4-hydroxycinammic acid was used as matrix. Product abundance became smaller in the presence of basic residues (H, K, and R), in the order Y > H ≈ K > R. In particular, product abundances in ISD of peptide ions with R were smaller than those with H or K by an order of magnitude, which, in turn, were smaller than that for [Y6 + H]+ by an order of magnitude. Product abundance was affected by the most basic residue when more than one basic residue was present. A kinetic explanation for the data was attempted under the assumption of quasi-thermal equilibrium for peptide ions in MALDI plume which undergoes expansion cooling. Dramatic disparity in product abundance was found to arise from small difference in critical energy and entropy. Results indicate similar transition structures regardless of basic residues present, where the ionizing proton keeps interacting with a basic site. Further implication of the results on the dissociation mechanism along b-y channels is discussed.

Dissociation Kinetics of Singly Protonated Leucine Enkephalin Investigated by Time-Resolved Photodissociation Tandem Mass Spectrometry

The yields of post-source decay (PSD) and time-resolved photodissociation (PD) at 193 and 266 nm were measured for singly protonated leucine enkephalin ([YGGFL + H]+), a benchmark in the study of peptide ion dissociation, by using tandem time-of-flight mass spectrometry. The peptide ion was generated by matrix-assisted laser desorption ionization (MALDI) using 2,5-dihydroxybenzoic acid as the matrix. The critical energy (E0) and entropy (ΔS‡ at 1000 K) for the dissociation were determined by Rice-Ramsperger-Kassel-Marcus fit of the experimental data. MALDI was done for a mixture of YGGFL and Y6 and the plume temperature determined by the kinetic analysis of [Y6 + H]+ data were used to improve the precision of E0 and ΔS‡ for [YGGFL + H]+. E0 and ΔS‡ thus determined (E0 = 0.67 ± 0.08 eV, ΔS‡=−24.4 ± 3.2 eu with 1 eu = 4.184 J K−1mol−1) were significantly different from those determined by blackbody infrared radiative dissociation (BIRD) (E0 = 1.10 eV, ΔS‡ = −14.9 eu), and by surface-induced dissociation (SID) (E0 = 1.13 eV, ΔS‡ = −10.3 eu). Analysis of the present experimental data with the SID kinetics (and BIRD kinetics also) led to an unrealistic situation where not only PSD and PD but also MALDI-TOF signals could not be detected. As an explanation for the discrepancy, it was suggested that transition-state switching occurs from an energy bottleneck (SID/BIRD) to an entropy bottleneck (PSD/PD) as the internal energy increases.