Tomoko Kawashima

ToF-SIMS observation for evaluating the interaction between amyloid β and lipid membranes

The adsorption behaviour of amyloid beta (Aβ), thought to be a key peptide for understanding Alzheimer’s disease, was investigated by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS). Aβ aggregates depending on the lipid membrane condition though it has not been fully understood yet. In this study, Aβ samples on different lipid membranes, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), were observed with ToF-SIMS and the complex ToF-SIMS data of the Aβ samples was interpreted using data analysis techniques such as principal component analysis (PCA), gentle-SIMS (G-SIMS) and g-ogram. DOPC and DMPC are liquid crystal at room temperature, while DPPC is gel at room temperature. As primary ion beams, Bi3+ and Ar cluster ion beams were used and the effect of an Ar cluster ion for evaluating biomolecules was also studied. The secondary ion images of the peptide fragment ions indicated by G-SIMS and g-ogram were consistent with the PCA results. It is suggested that Aβ is adsorbed homogeneously on the liquid-crystalline-phase lipid membranes, while it aggregates along the lipid on the gel-phase lipid membrane. Moreover, in the results using the Ar cluster, the influence of contamination was reduced.

Peptide fragmentation caused by Ar cluster ions depending on primary ion energy

RATIONALE Time-of-flight secondary ion mass spectrometry (TOF-SIMS) with an Ar cluster ion beam as a primary ion source provides useful information in terms of peptide analysis. It is, however, difficult to interpret the spectra. The ToF-SIMS peptide spectra obtained with Ar clusters having different energies have been investigated in order to classify the secondary ions into the peptide fragment ions and those related to contaminants or the substrate. METHODS Three peptides having different molecular weights from 600 to 1300 u were measured with Ar cluster beams having different energies per atom from 4 to 40 eV/atom. RESULTS In the spectra normalized to a geometric average of all the spectra, the amino acid fragment ions are distinguished from other secondary ions. In the mass range above 600 u, the peptide fragment ions increase with mass while those not related to the peptide decrease with mass. CONCLUSIONS Energy-dependence fragmentation helps in understanding the peptide spectra. Specific peptide fragment ions of the larger peptides are likely to be detected under lower energy than energy higher than 10 eV/atom. Although it is difficult to interpret the TOF-SIMS spectra of a peptide obtained with an Ar cluster ion beam, the secondary ions can be classified by comparing those obtained with different energy Ar cluster ion beams.

Evaluation of matrix effects on TOF-SIMS data of leu-enkephalin and 1,2-dioleoyl-sn-glycero-3-phosphocholine mixed samples

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the most powerful methods to analyze biomolecules in biological tissues and cells because it provides detailed chemical structure information and chemical images with a high spatial resolution. However, in terms of quantitative analysis, there are issues such as matrix effects that often cause secondary ion intensity changes regardless of the actual concentration in a sample. For instance, the intensity of secondary ions related to peptides is generally suppressed when lipids coexist. Since the evaluation of biomolecules is crucial to understand biological phenomena, it is required to analyze peptides or lipids without matrix effects. Therefore, the mechanism of matrix effects regarding peptides and lipids in TOF-SIMS was investigated in this study. Leu-enkephalin (YGGFL, molecular weight of 555.3 Da) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, C44H84NO8P, molecular weight 785.6 Da) were employed to prepare model samples. Model samples contain different weight ratios of these two molecules. The intensity of secondary ions related to the peptide or the lipid was compared with control samples containing pure leu-enkephalin or DOPC. As a result, it is indicated that the intensity of DOPC related secondary ions is strongly enhanced by coexisting leu-enkephalin, while the intensity of leu-enkephalin related secondary ions is suppressed by coexisting DOPC especially in a low concentration range of the peptide.

Charging Phenomena During Medium Current Ion Implantation of Carbonized Photo-Resist Surface Layers

Charging phenomenon caused by ion implantation into the photo-resist has been evaluated with use of a surface potential measurement tool to clarify the mechanism of burst-like discharge of the accumulated charge in medium current implantation machines. The molecular bond of the photo-resist is cleaved by the kinetic energy of the implanted impurity ions selectively at the portion of the smallest bond energy, and cross-linking develops from this point. Bond breaking and cross-linking is accompanied by out-gassing of hydrogen released by collisions with the dopant atom and recoil cascade atoms. Finally, a carbon-rich layer with low resistance is formed on the photo-resist surface. When arsenic ions are implanted, the degraded layer of low resistance including the carbonization is being formed even under low dose (1 × 1014/cm2 or less) conditions. This is called the medium current charging phenomenon with a current of a few hundred μA. The charges are accumulated on the photo-resist, until the low resistance layer is formed on the surface of the photo-resist. When the induced voltage reaches a critical value, they rush into the open area through the layer. As a result, an explosive local melting of silicon occurs, with the critical value depending on the mask-pattern arrangement.