Ion Flight Time Research Articles

Proteome Analysis of a Food Borne Pathogen Enteroaggative Escherichia coli under Acid Stress

Enteroaggregative Escherichia coli (EAEC) is emerging as a significant diarrheal pathogen in multiple population groups. Like any other enteropathogen, EAEC may encounter life-threatening levels of inorganic acids (H+) during their natural route of infection. In this study, we showed that EAEC orchestrates acid tolerance by modulating the levels of acid-induced outer membrane proteins (OMP). EAEC (T8) was grown in vitro by mimicking in vivo pH conditions of both in stomach and intestine. The lowest pH where EAEC showed growth was 4.0 in Luria Bertani (LB) media, and surprisingly during log phase two de novo OMP of sizes 41 kDa and 48 kDa were exclusively observed at this pH. Further, acid-induced proteins at pH 4.0 were identified by 2D gel electrophoresis and Matrix Assisted Laser Desorption Ionization-Time of Flight (MALDI-TOF). The role of the most notable acid/pH response regulator rpoS in ASP expression was determined by gene knockout in the clinical strain EAEC (T8). Mutation of rpoS gene suppressed growth and downregulated the expression of ASP in EAEC (T8) at pH 4.0 confirming probable role of rpoS in EAEC.

Inflammatory Biomarker Profiling in Elderly Patients With Acute Hip Fracture Treated With Heparins

Hip fracture is common in the elderly patients with associated high risk of venous thromboembolic complications. Pathogenic activation results in the generation of various surrogate markers in plasma. This study is designed to identify unique biomarkers in elderly patients with hip fracture using protein chip array enzyme-linked immunosorbent assay (ELISA) methods. Plasma from a randomized hip fracture study (PK-532; n = 341) treated with either enoxaparin (40 mg once daily) or unfractionated heparin (UFH; 5000 IU twice daily) were collected prior to and at 1, 3, 5, and 7 days. A total of 52 samples were analyzed using proteomic surface-enhanced laser desorption/ ionization-time of flight (SELDI-TOF) mass spectrometry to identify unique biomarkers in the molecular weight range of 0 to 150 kd. Twenty-nine healthy volunteer's and pooled plasma from total hip replacement/total knee replacement patients with a unique biomarker at 11.9 kd were used as quality controls. In the 29 healthy individuals, the biomarker profile did not reveal the presence of any unique peak in comparison to the reference normal human plasma (NHP). Plasma obtained prior to surgery exhibits unique biomarkers in 4 of 52 (7.6%) of the samples. On day 1 postoperatively, 41 of 51 (80.3%) showed a distinct peak at 11.9 kd. On day 3, 43 of 49 (87.8%) patients showed the presence of this biomarker most often at its strongest intensity. In all, 22 of 44 (50%) showed this biomarker on day 5 and 4 of 23 (17.9%) on day 7. C-reactive protein (CRP), tumor necrosis factor alpha (TNF-alpha), and serum amyloid A were also increased after surgery. Tissue factor pathway inhibitor (TFPI) antigen levels were increased due to the treatment modalities.

Modification of Reflectron Time-of-Flight Mass Spectrometer for Photodissociation of Mass-Selected Cluster Ions

We introduce a modification of reflectron time-of-flight mass spectrometer for laser photodissociation of mass-selected ions. In our apparatus, the ions of interests were selected by a mass gate near the first space focus point and decelerated right after the mass gate, were then crossed by a laser beam for dissociation. The daughter ions and surviving parent ions were re-accelerated and analyzed by the reflectron time-of-flight mass spectrometer. Compared to the designs reported by other research groups, our selection-deceleration-dissociation-reacceleration approach has better daughter-parent-ions-separation, easier laser timing, and better overlapping between the ion beam and laser beam. We also conducted detailed calculations on the parent ion and daughter ion flight times, and provided a simplified formula for the calibration of daughter ion mass.

Using I-TOF spectrometry to measure photon energies at FELs

Due to the stochastic nature of the Self Amplifying Spontaneous Emission (SASE) process and the resulting pulse-to-pulse fluctuations of the Free Electron Laser (FEL) photon energies, experimenters working with FELs need to get real-time feedback about the photon properties for their experiments. Investigations of narrow atomic or molecular resonances, phase transitions, or any other kind of effect heavily dependent on photon energy would need to know the precise FEL photon energy for each individual photon bunch. Furthermore, any spectrometer developed to deliver the information of these properties should not significantly interfere or degrade the FEL beam. Therefore, the group at the Free Electron Laser in Hamburg (FLASH) has developed an online photoionization spectrometer that uses ion time of flight (I-TOF) measurement methods on noble gases to measure the photon energy of each pulse. This paper presents the first test results for the viability of this online photoionization spectrometer (OPS).

Observation of Zr22+, Cd22+, Hf22+, W22+, and Pt22+ in the gas phase

Five homonuclear diatomic dications Zr22+, Cd22+, Hf22+, W22+, and Pt22+ have been observed in the gas phase by mass spectrometry. These exotic doubly positively charged molecules were produced indirectly in the ion extraction region of a secondary ion mass spectrometer during sputtering of zirconium, cadmium, hafnium, tungsten, and platinum metal foils, respectively, by energetic high-current Ar+ ion surface bombardment. They were detected in positive ion mass spectra at half-integer m∕z values for ion flight times of the order of ∼10−5s. To our knowledge, these species had not been observed before. This experimental work confirms two theoretical investigations that had predicted that W22+ and Cd22+ are long-lived metastable species in the gas phase, but contradicts two theoretical studies that had suggested that Pt22+ should be unstable with respect to fragmentation. Therefore an advanced theoretical investigation of the ground state of Pt22+ was also performed. Our calculation shows that the ground state of Pt22+ is metastable with an internuclear equilibrium distance of 2.36Å, a dissociation energy (with respect to the top of the barrier) of 2.32eV, and an ionization potential of Pt2+ of about 15.8eV. The latter theoretical result strongly suggests that Pt22+ dication formation in our experiment may have taken place via the resonant electron transfer process Pt2++Ar+→Pt22++Ar.

Oxygen-Containing Diatomic Dications in the Gas Phase

A variety of oxygen-containing diatomic dications XO(2+) can be produced in the gas phase by prolonged high-current (16)O(-) ion surface bombardment (oxygen ion beam sputtering) of a wide range of sample materials. These gas-phase species were detected by mass spectrometry at half-integer m/z values for ion flight times of the order of ~10-5 s. Examples provided here include ion mass spectra of AsO(2+), GaO(2+), SbO(2+), AgO(2+), CrO(2+) and BeO(2+). A detailed theoretical study of the diatomic dication system BeO(2+) is also presented.

Serum Amyloid A As a Prognostic Marker in Melanoma Identified by Proteomic Profiling

Currently known prognostic serum biomarkers of melanoma are powerful in metastatic disease, but weak in early-stage patients. This study was aimed to identify new prognostic biomarkers of melanoma by serum mass spectrometry (MS) proteomic profiling, and to validate candidates compared with established markers. Two independent sets of serum samples from 596 melanoma patients were investigated. The first set (stage I = 102; stage IV = 95) was analyzed by matrix assisted laser desorption and ionization time of flight (MALDI TOF) MS for biomarkers differentiating between stage I and IV. In the second set (stage I = 98; stage II = 91; stage III = 87; stage IV = 103), the serum concentrations of the candidate marker serum amyloid A (SAA) and the known biomarkers S100B, lactate dehydrogenase, and C reactive protein (CRP) were measured using immunoassays. MALDI TOF MS revealed a peak at m/z 11.680 differentiating between stage I and IV, which could be identified as SAA. High peak intensities at m/z 11.680 correlated with poor survival. In univariate analysis, SAA was a strong prognostic marker in stage I to III (P = .043) and stage IV (P = .000083) patients. Combination of SAA and CRP increased the prognostic impact to P = .011 in early-stage (I to III) patients. Multivariate analysis revealed sex, stage, tumor load, S100B, SAA, and CRP as independent prognostic factors, with an interaction between SAA and CRP. In stage I to III patients, SAA combined with CRP was superior to S100B in predicting patients' progression-free and overall survival. SAA combined with CRP might be used as prognostic serological biomarkers in early-stage melanoma patients, helping to discriminate low-risk patients from high-risk patients needing adjuvant treatment.

Serum-based diagnosis and mouse models for preeclampsia

The overarching goal of this study is to use serum as a blueprint to establish in vivo and in vitro diagnostic and treatment models for pregnancy disorders such as preeclampsia and intrauterine growth restriction (IUGR). When combined, preeclampsia and IUGR complicate 10–15% of all pregnancies worldwide and are a major cause of maternal, fetal and neonatal mortality and morbidity. Despite intense investigation, these complications have remained enigmatic and the mechanisms that contribute to their emergence are largely unknown. We have recently initiated serum-based studies to establish in vitro and in vivo predictive and mechanistic assays for IUGR/preeclampsia. Our data strongly suggest that serum from preeclampsia/IUGR patients contains an „activity“ that disrupts cross-talk between placental invading trophoblasts and endothelial cells as well as causes preeclampsia and IUGR like symptoms in pregnant mice. The latter phenotype is particularly apparent in interleukin-10 (IL-10) deficient mice. A major task has been to expand these studies using a larger sample of human samples to verify these novel preliminary results and to identify the dysregulated protein component(s) by state of the art proteomic techniques such as mass spectrometry based Surface Enhanced Laser Desorption and Ionization-Time of Flight (SELDI-TOF). Results will be described with a focus on identification and biochemical and physiological properties of a novel protein that is invariably deficient in preeclampsia serum. Supported by the NIH-NCRR grant P20RR018728 and the Rhode Island Research Alliance Collaborative Research Award and STIFT (Foundation for Technology, Innovation and Research in Thuringia, Germany).

A study of oleic acid and 2,4-DHB acid aerosols using an IR-VUV-ITMS: insights into the strengths and weaknesses of the technique

An investigation of oleic acid and 2,4-dihydroxybenzoic (DHB) acid aerosols was carried out using an aerosol mass spectrometer with pulsed lasers for vaporization and ionization and an ion trap for mass analysis. The extent of ion fragmentation was studied as a function of both vaporization energy and ionization wavelength. Low CO2 laser energies in the vaporization stage and near-threshold single photon ionization resulted in the least fragmented mass spectra. For DHB, only the molecular ion was observed, but for oleic acid fragmentation could not be eliminated. Tandem MS of the main fragment peak from oleic acid was carried out and provided a tool for compound identification. Photoionization efficiency curves were also collected for both DHB and oleic acid and the appearance energies of both parent and fragment ions were measured. Evidence for fragmentation occurring post-ionization is given by the similar appearance energies for both the parent and fragment ions. The results from this study were compared with those from similar experiments undertaken with time-of-flight (TOF) mass analyzers. The degree of fragmentation in the ion trap was considerably higher than that seen with TOF systems, particularly for oleic acid. This was attributed to the long storage interval in the ion trap which allows time for metastable ions to decay. Differences in the degree of fragmentation between the ion trap and TOF studies also provided further evidence for fragmentation occurring post-ionization. For 2,4-dihydroxybenzoic acid, the long delay prior to mass analysis also allowed time for reactions with background gases, in this case water, to occur.

Proteomic Analysis in Cyclosporin A-Induced Overgrowth of Human Gingival Fibroblasts

Cyclosporin A (CsA) has been used as an immunosuppressive drug to prevent organ transplant rejection and to treat autoimmune diseases. CsA has a proliferative effect on human gingival fibroblasts (HGF) in vitro. However, the molecular mechanisms underlying CsA-induced proliferation in HGF remain to be elucidated. This study was aimed to investigate the CsA responsive proteins in HGF using systematic proteomic approach. Cell viability was determined by MTT assay and reactive oxygen species (ROS) was measured by fluorescent spectrometer. Proteins profiled by two-dimensional gel electrophoresis (2-DE) were identified by matrix-assisted laser desorption ionization time-of flight (MALDI-TOF) and electrospray ionization quadrupole time-of-flight mass spectrometry (EIQ-TOF MS). To confirm the expression changes of proteins by proteomics analysis, Western blot was performed using specific antibody. CsA increased the cell viability of HGF in a dose- and time-dependent manner. Significantly, seventeen proteins were overexpressed in the CsA-treated HGF, whereas three proteins were found to be expressed less than the untreated cells. The identified proteins were mainly related with cell proliferation, metabolism, and oxidation. The overexpression of peroxiredoxin 1 (Prx 1) confirmed by Western blotting and reduction of cytosolic reactive oxygen species (ROS) levels in the CsA-treated HGF demonstrated that Prx 1 may play a crucial role in the HGF proliferation induced by CsA. Upregulation of Galectin 3 in CsA-treated HGF indicated that it is related to CsA-induced proliferation. These proteomic analysis data will provide an efficient approach in understanding the mechanisms of HGF proliferation by CsA.

Highlights of the Annual Meeting of the Italian Association for Cell Cultures (AICC): new drug delivery strategies and technological platforms for diagnosis and therapy of tumors (Part I)

The application of computational analysis to biochemistry and molecular biology techniques has allowed the design of advanced technological platforms such as DNA microarrays, proteomic analysis and tissue microarrays. These are useful in the diagnosis of tumors and in driving the choice of opportune therapeutic options. The use of target-based agents requires the determination of the molecular features of the tumors. During the AICC meeting four new advanced technological platforms were analyzed: DNA microarrays, ProtoArrays, tissue microarrays and proteomic analysis with differential in-gel electrophoresis (DIGE) and matrix-associated laser desorption ionization-time of flight (MALDI–TOF). The use of DNA microarray in the study of neo-angiogenic components in tumor tissues was described and the ex vivo analysis in humans of circulating endothelial cells was proposed. The role of both genomic and pharmacogenomic profiles in the diagnosis and prediction of response to therapy and toxicity of both breast and colorectal cancer was described. A developmental model for the optimization of zoledronic acid (ZOL) antitumor activity based on either nanotechnological modification or molecular target fishing with DNA microarrays was discussed. The interdisciplinary network between pharmacologists, cellular and molecular biologists, biochemists and translational and clinical oncologists was encouraged and approved during the meeting in order to improve the efficacy of antitumor strategies.

Construction of a novel stigmatic MALDI imaging mass spectrometer

A novel design concept and the status of development of a matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometer in the stigmatic mode utilizing a perfect focusing property of a multi-turn time-of-flight (TOF) ion optics is reported. A multi-turn TOF ion optics consisting of four toroidal electric sectors has ideal features for a mass analyzer of stigmatic imaging mass spectrometry. The constructed multi-turn TOF mass analyzer, MULTUM-IMG, demonstrated a strong focusing property allowing MALDI-generated ions to revolve in a closed orbit for hundreds turns. Images of ion ensembles extracted from matrix surfaces patterned by several sizes of thin grid meshes (100–1000 lines/in.) or from arrays of small pigment dots (approximately 30 μm in diameter) fabricated on Si plates were examined by attaching an imaging detector to a MALDI ion source directly, and were found to retain the original spatially distributing patterns of the samples. Images of a patterned ion ensemble after multiple revolutions in MULTUM-IMG were examined, and were found to retain the images at the entrance of the multi-turn TOF section. The possibility of the novel stigmatic MALDI TOF imaging mass spectrometer has been proven by examining the building blocks.

Synthesis and protonic conductivity of the oligomeric amides with different average molecular weights

New oligomeric amides were synthesized by the reaction of m-xylylenediamine and dimethyl oxalate to investigate the average molecular weight dependence of the protonic condcutivity. The average molecular weight was determined by the analysis of the Matrix Assisted Laser Desorption / Ionization Time of Flight (MALDI-TOF) mass spectra. The average molecular weight was decreased with increasing the quantity of water in the reaction condition. Some end groups of the oligomers were hydrolysed from the methoxycarbonyl groups to the calboxylic acid groups. The oligomers with the different average molecular weight exhibited different relative humidity (RH) dependence of the protonic conductivity. The protonic conductivity depends on the average molecular weight at the RH of 95%. The difference between the maximum and minimum protonic condcutivity is about three orders of the magnitude. The activation energy for the protonic conduction in one of the oligomers is 0.66 eV.

HTF9C gene of 22q11.21 region associates with schizophrenia having deficit-sustained attention

A region at chromosome 22q11.21 has been reported to potentially harbor a candidate gene for schizophrenia, ZDHHC8 (zinc finger, DHHC domain containing 8; also annotated as KIAA1292) in a number of studies. This finding has been replicated in Han Chinese, but not in other ethnicity-specific studies. For further support from within the Han Chinese ethnic group, we selected two single nucleotide polymorphisms (SNP) located at the distal 5'-end (rs1633445; intron 10 of HpaII tiny fragments locus 9C, HTF9C) and the intron 4 (rs175174) of ZDHHC8 gene to test if these were associated with schizophrenia in a study sample of Taiwan. A total of 218 schizophrenia families with at least two affected siblings participated in this study. These two SNPs were genotyped using matrix-assisted laser desorption/localization ionization time of flight (MALDI-TOF) mass spectrometry. Significant associations with schizophrenia were not shown from these two SNPs. After stratifying schizophrenia according to the deficit and the nondeficit of sustained attention assessed by the Continuous Performance Test, the rs1633445 showed significant association with schizophrenia in the presence of a deficit in sustained attention (P<0.04). SNP rs1633445 of the HTF9C gene may be associated with a deficit in sustained attention within schizophrenia, in a Taiwanese cohort. The deficit of sustained attention may be an endophenotype of schizophrenia, and warrants further study.

Detection of protein alterations in male breast cancer using two dimensional gel electrophoresis and mass spectrometry: The involvement of several pathways in tumorigenesis

Background Little emphasis has been placed today on the elucidation of protein alterations in male breast carcinogenesis. Methods Protein extracts were subjected to both isoelectric focusing (IEF) and non-equilibrium pH gradient electrophoretic (NEPHGE) analyses. Differentially expressed proteins in tumor tissues were identified by matrix assisted laser desorption /ionization time of flight (MALDI–TOF) mass spectrometry and database search. Results Some of the alterations involve variations in the expression of cytokeratins 8, 18 and 19. More interestingly, tropomyosin1, a protein known to play a role in suppression of the malignant phenotype, was found to be under-expressed in cancer tissues, implicating a possible pivotal role for this protein in male breast carcinogenesis. Co-upregulation of molecular chaperones (heat shock protein HSP27 and protein disulfide isomerase), stress related proteins (peroxiredoxin 1 and peptidylprolyl isomerase A) and glycolytic enzymes (enolase 1) occurred also in male breast tumors. Some of the remaining alterations include proteins involved in invasion and metastasis, such as galectin 1 and cathepsin D. Conclusions The present study represents a first proteomic investigation of protein alterations in infiltrating ductal carcinomas (IDCA) of the male breast. A number of protein alterations in tumor tissues have been characterised thus, providing new insights into the molecular mechanisms underlying this disease.

Fabrication of two types of micro ion sources for a micro time-of-flight mass spectrometer

This paper reports the fabrication of two types of micro ion sources for a micro time-of-flight mass spectrometer (TOFMS). The TOFMS is an analyzer that serves for the establishment of molecular weight by using the difference in the flight time of ions according to their mass-to-charge values. The micro TOFMS consists of a glass substrate with several nickel electrodes and a silicon substrate with an ion source. We have fabricated two types of ion sources using a hot filament and field emitters with a carbon nanoparticle structure. The overall size of the assembled device is 10 × 10 × 1 mm3. The hot filament is fabricated through the nickel electroplating process and some lithography processes. The carbon nanoparticle layer is grown by hot-filament chemical vapor deposition (HFCVD). The carbon nanoparticle field emitter has the triode structure with which we can control the emission current at low voltage. The maximum emission current of the hot-filament ion source is 0.28 μA. The emission current of the field emission ion source can be controlled from 0 to 18.5 μA when the gate voltage varies from 0 to 175 V.

Femtosecond laser-induced fragmentation and cluster formation studies of solid phase trinitrotoluene using time-of-flight mass spectrometry

We use surface-femtosecond laser mass spectrometry to study the fragments/products formed when trinitrotoluene (TNT) is subjected to femtosecond laser pulse irradiation and to study the conditions under which TNT is removed from a solid surface. In surface-femtosecond laser mass spectrometry a compound is deposited on a solid substrate and is desorbed into vacuum by femtosecond irradiation forming a plume of ionized and neutral species. The positive or negative ions are then accelerated by an electric potential and allowed to drift in the field-free region of a time-of-flight mass spectrometer. The mass-to-charge ratio of each ion is obtained using the value of the accelerating field and the ion flight time. In this paper we report femtosecond laser mass spectra for the positive ions formed by desorbing TNT with 130 fs pulses centered at 800 nm for fluences ranging from 7 to 1.4 × 10 5 J/m 2. The conditions under which TNT removal and ionization occur are also discussed.

An improved method for IEDF determination in pulsed plasmas and its application to the pulsed dc magnetron

In this paper we demonstrate an improved technique for obtaining time-resolved ion energy distribution functions (IEDFs) in pulsed plasma discharges using a commercial quadrupole mass energy analyser. The method involves extracting ions from the plasma at selected times during the pulse cycle through the application of a synchronized electrical bias on a grid assembly built in the barrel of the instrument. Placing the triggered mesh in the barrel of the energy analyser makes the ‘transit’ gap length (between the chopping mesh and neighbour electrodes) ∼2 mm and that results in a significant increase of the time resolution. Due to the high vacuum conditions inside the barrel, no unwanted ‘parasitic’ discharge will exist between the extractor and orifice. The electrostatic chopping by the built-in mesh allows time-resolved IEDFs to be determined without reference to the inherent time delay caused by the ion flight time through the spectrometer. The technique has been applied to the low-pressure pulsed dc magnetron discharge operating in Ar gas and at a pulse frequency of 100 kHz. Time-resolved IEDFs were obtained with a 1 µs time resolution, and these revealed large variations in the form and width of the ion energy spectra at different times during the discharge cycle, governed by the temporal evolution of the plasma potential. A calculation based on ion dynamics in the grid assembly agrees well with experimentally obtained results and demonstrates that this method can provide time resolutions as small as 200 ns.

Dissociation Dynamics of Sequential Ionic Reactions: Heats of Formation of Tri-, Di-, and Monoethylphosphine

The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta(f)H degrees 298K[P(C(2)H(5))3] = -152.7 +/- 2.8 kJ/mol, Delta(f)H degrees 298K[P(C(2)H(5))3+] = 571.6 +/- 4.0 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2] = -89.6 +/- 2.1 kJ/mol, Delta(f)H degrees 298K[HP(C(2)H(5))2+] = 669.9 +/- 2.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)] = -36.5 +/- 1.5 kJ/mol, Delta(f)H degrees 298K[H(2)PC(2)H(5)+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.

Structural Investigations On Novel Porin, OmpAb From Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic pathogen and known to cause nosocomial infections especially in ICUs of hospitals. We have previously reported that the novel outer membrane protein, OmpAb from Acinetobacter baumannii is a transmembrane porin and plays an important role in transport of small molecules, like antibiotics across the membrane. In the present study we report the N-terminal sequence, Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) analysis of OmpAb and structural investigations using UV-Vis absorption, circular dichroism (CD), and fluorescence on OmpAb. SDS-PAGE results suggest that OmpAb is actually a “heat modifiable monomer” and is of 37 kDa at room temperature. Secondary structure of OmpAb is being done for the first time that showed predominantly β-sheet structure (68%), a feature characteristic of porins. Using N- Bromosuccinimide (NBS) as oxidizing agent, the total number of tryptophans in OmpAb is estimated to be four. The present results indicate that out of the four, two tryptophans seem to be located in the integral part of the membrane, perhaps periplasmic/membrane-bound while the other two tryptophans are exposed to the solvent. We followed the fluorescence emission using conventional 280 nm and selective 305 nm excitation (established by us earlier) to explore the environment of four tryptophans in OmpAb. Emission results using selective excitation of 305 nm revealed local conformational changes of those “tryptophans which are on the surface”. On urea denaturation and pH dependent denaturation there is a loss of β-sheet structure by more than 70%, this is concomitant with the increase in fluorescence intensity and red shift in χmax em. As reflected by CD spectral data, we also found that OmpAb is fairly stable like other porins up to 70 °C. As there are no reports on the structural aspects of any outer membrane proteins of Acinetobacter baumannii, results presented here on this novel major porin, OmpAb, will help in understanding the structure-function relationship.