Michael T. Bowers

Amyloid-β protein oligomerization and the importance of tetramers and dodecamers in the aetiology of Alzheimer’s disease

In recent years, small protein oligomers have been implicated in the aetiology of a number of important amyloid diseases, such as type 2 diabetes, Parkinsons disease and Alzheimers disease. As a consequence, research efforts are being directed away from traditional targets, such as amyloid plaques, and towards characterization of early oligomer states. Here we present a new analysis method, ion mobility coupled with mass spectrometry, for this challenging problem, which allows determination of in vitro oligomer distributions and the qualitative structure of each of the aggregates. We applied these methods to a number of the amyloid-β protein isoforms of Aβ40 and Aβ42 and showed that their oligomer-size distributions are very different. Our results are consistent with previous observations that Aβ40 and Aβ42 self-assemble via different pathways and provide a candidate in the Aβ42 dodecamer for the primary toxic species in Alzheimers disease.

Ion-Polar molecule collisions: the effect of ion size on ion-polar molecule rate constants; the parameterization of the average-dipole-orientation theory

Abstract Proton transfer rate constants from CH 5 + , C 2 H 5 + , C 3 H 7 + and C 4 H 9 + to NH 3 , CH 3 NH 2 , and (CH 3 ) 2 NH have been measured at thermal energies by ion cyclotron resonance techniques. The data are compared with the predictions of the average-dipole-orientation theory. In all cases, the CH 5 + , C 2 H 5 + and C 3 H 7 + ions transfer a proton with essentially unit efficiency. The C 4 H 9 + ions transfer protons with efficiencies between 0.6 and 0.9 depending on the substrate molecule. There is no evidence that the size of the ion has an appreciable effect on the magnitude of the proton transfer rate, at least within the accuracy of the ICR experiments. The average-dipole-orientation theory has been parameterized to permit determination of capture limit rate constants by reading a graph and making a simple calculation. A graph covering the range of polarizabilities and dipole moments of most molecules is included.

Theory of ion‐polar molecule collisions. Comparison with experimental charge transfer reactions of rare gas ions to geometric isomers of difluorobenzene and dichloroethylene

A classical theory for the ion‐permanent dipole interaction is developed that takes into consideration the thermal rotational energy of the polar molecule. The theory is formulated in terms of an r‐dependent average orientation angle θ (r) between the dipole and the line of centers of collision. The technique allows quantitative determination of the capture cross section as a function of ion‐polar molecule relative velocity. In addition, capture limit rate constants are readily calculated both at thermal energies and as a function of relative energy. Charge transfer rate constants from various rare gas ions to difluorobenzene and dichloroethylene isomers have been measured at thermal energies using ion cyclotron resonance spectroscopy. Rate constants are considerably larger than predicted by the capture theories for both polar and nonpolar isomers indicating long range electron jump is a prevalent mechanism for charge transfer in these systems. The average‐dipole‐orientation theory developed in this pape...

Structures of carbon cluster ions from 3 to 60 atoms: Linears to rings to fullerenes

A new method for identifying and characterizing cluster ion isomers is presented. Results indicate that most carbon clusters have more than one stable form, with 29≤n≤45 indicating three or four different structures. Fullerenes first appear at C+30 and begin to dominate above C+45. From small to large the isomeric progression is from linear to rings to fullerenes.

Effect of the long-range potential on ion mobility measurements

The temperature dependence of ion mobilities in helium was studied by using the ion chromatography method to investigate the effect of long-range terms in the ion-buffer gas interaction. Experimental cross sections thus determined increased significantly as the temperature was lowered from 300 to 80 K for all ions investigated, which were fullerene C60+, cationized PEG polymers, cationized crown ethers, and protonated and sodiated oligoglycines. The temperature dependence of the collision cross sections was successfully modeled by employing simple atom-atom interaction potentials including a repulsive R−12 term and the attractive long-range R−6 and R−4 terms, R being the distance between the colliding particles.

Ion mobility–mass spectrometry reveals a conformational conversion from random assembly to β-sheet in amyloid fibril formation

Amyloid cascades that lead to peptide β-sheet fibrils and plaques are central to many important diseases. Recently, intermediate assemblies of these cascades were identified as the toxic agents that interact with cellular machinery. The location and cause of the transformation from a natively unstructured assembly to the β-sheet oligomers found in all fibrils is important in understanding disease onset and the development of therapeutic agents. Largely, research on this early oligomeric region was unsuccessful because all the traditional techniques measure only the average oligomer properties of the ensemble. We utilized ion-mobility methods to deduce the peptide self-assembly mechanism and examined a series of amyloid-forming peptides clipped from larger peptides or proteins associated with disease. We provide unambiguous evidence for structural transitions in each of these fibril-forming peptide systems and establish the potential of this method for the development of therapeutic agents and drug evaluation.

Collisions in a noncentral field: A variational and trajectory investigation of ion–dipole capture

Variational rate theory and trajectory methods are used to investigate the classical dynamics of capture on a noncentral long range potential. The ion–dipole surface is investigated in detail. An upper bound to the true thermal capture rate constant is derived using the variational approach. The upper bound is independent of the moment of inertia of the rotor and has only a multiplicative inverse square root dependence on the system reduced mass. For the ion–dipole surface the upper bound is in excellent agreement with trajectory calculations of the true capture rate constant and with experimental data on thermal heavy particle transfer rate constants between ions and polar neutrals. The trajectory calculations are done using an angular momentum conserved coordinate system which eliminates the need to solve two sets of Hamilton’s equations. A precise criterion for capture is derived and is used in the trajectory calculations. Reduced variables are introduced for the ion–dipole surface and it is shown that...

Gas-Phase Ion Chromatography: Transition Metal State Selection and Carbon Cluster Formation

Gas-phase ion chromatography can separate ions that have the same mass but differ in isomeric structure or electronic configuration. The main features of this technique are briefly outlined, and applications to a series of problems in transition metal chemistry and carbon cluster chemistry are described. Examples in transition metal chemistry include state-selective reactivity, excited state deactivation, and state-selective ligand binding energies. For clusters, ion chromatography was used to determine the structure of pure carbon cluster ions as a function of size from C4 to C84. The results indicate that carbon grows first in linear chains, transforms to monocyclic planar rings at about C10, and forms new families of planar bi-, tri-, and tetracyclic rings at C20, C30, and C40, respectively. Fullerenes, which mysteriously appear at C30 and dominate by C50, are generated by heating the planar ring systems above an isomerization barrier rather than by growth of graphite precursors.

Design of a new electrospray ion mobility mass spectrometer

Abstract The design of a new ion mobility mass spectrometer is presented. The design features an electrospray ion source; an ion funnel to transmit ions efficiently from the source to the mobility cell and to accumulate ions in the pulsed ion mode; a mobility cell, and a quadrupole mass analyzer. Each part of the instrument is described in detail. Preliminary results obtained with the new instrument are presented to demonstrate its capabilities. Equilibrium experiments showed that the Δ G °(300 K) values for the addition of the first water molecule to the doubly protonated peptides bradykinin, angiotensin II, and LHRH are in the range from −3.5 to −2.5 kcal/mol. The corresponding values for the singly protonated ions are >−0.5 kcal/mol for angiotensin II and LHRH, but equal to −2.6 kcal/mol for bradykinin. The stronger bonding in bradykinin may be due to the presence of a salt bridge structure. Ion arrival time distributions showed that singly protonated peptides can form aggregates of the form ( n M + n H) n + . The mobilities of these ions indicated that they are near spherical. Heating the drift cell to ∼450 K caused dissociation of the (2M + 2H) 2+ ion into two (M + H) + units on the 1 ms experimental time scale. A theoretical fit to the experimental data yielded rate constants and a barrier for dissociation of 30 ± 2 kcal/mol for bradykinin and 39 ± 3 kcal/mol for LHRH.

Structural Stability from Solution to the Gas Phase: Native Solution Structure of Ubiquitin Survives Analysis in a Solvent-Free Ion Mobility–Mass Spectrometry Environment

The conformations of desolvated ubiquitin ions, lifted into the gas phase by electrospray ionization (ESI), were characterized by ion mobility spectrometry (IMS) and compared to the solution structures they originated from. The IMS instrument combining a two-meter helium drift tube with a quadrupole time-of-flight mass spectrometer was built in-house. Solutions stabilizing the native state of ubiquitin yielded essentially one family of tightly folded desolvated ubiquitin structures with a cross section matching the size of the native state (1000 Å(2)). Solutions favoring the A state yielded several well-defined families of significantly unfolded conformations (1800-2000 Å(2)) matching in size conformations between the A state and a fully unfolded state. On the basis of these results and a wealth of data available in the literature, we conclude that the native state of ubiquitin is preserved in the transition from solution to the desolvated state during the ESI process and survives for >100 ms in a 294 K solvent-free environment. The A state, however, is charged more extensively than the native state during ESI and decays more rapidly following ESI. A state ions unfold on a time scale equal to or shorter than the experiment (≤50 ms) to more extended structures.