Initial Recombination Research Articles

Recombination in Enteroviruses, a Multi-Step Modular Evolutionary Process.

RNA recombination is a major driving force in the evolution and genetic architecture shaping of enteroviruses. In particular, intertypic recombination is implicated in the emergence of most pathogenic circulating vaccine-derived polioviruses, which have caused numerous outbreaks of paralytic poliomyelitis worldwide. Recent experimental studies that relied on recombination cellular systems mimicking natural genetic exchanges between enteroviruses provided new insights into the molecular mechanisms of enterovirus recombination and enabled to define a new model of genetic plasticity for enteroviruses. Homologous intertypic recombinant enteroviruses that were observed in nature would be the final products of a multi-step process, during which precursor nonhomologous recombinant genomes are generated through an initial inter-genomic RNA recombination event and can then evolve into a diversity of fitter homologous recombinant genomes over subsequent intra-genomic rearrangements. Moreover, these experimental studies demonstrated that the enterovirus genome could be defined as a combination of genomic modules that can be preferentially exchanged through recombination, and enabled defining the boundaries of these recombination modules. These results provided the first experimental evidence supporting the theoretical model of enterovirus modular evolution previously elaborated from phylogenetic studies of circulating enterovirus strains. This review summarizes our current knowledge regarding the mechanisms of recombination in enteroviruses and presents a new evolutionary process that may apply to other RNA viruses.

Mechanism of Degradation Rate on the Irradiated Double-Polysilicon Self-Aligned Bipolar Transistor

The latent enhanced low dose rate sensitivity (ELDRS) effect is observed in the double-polysilicon self-aligned (DPSA) technology PNP bipolar junction transistor (BJT) irradiated with a high and low dose rate gamma ray, which is discussed from the perspective of the three-stage degradation rate of the excess base current. The great degradation rate as a result of the high dose irradiation of the first stage is dominantly ascribed to the positive oxide trap charges accumulated during a short irradiation, and then due to the competition between the recombination of electrons and capture of the hole by the traps. It declined sharply into a degradation rate saturated region of the second stage. However, for the low dose rate, the small increase in the degradation rate in the first stage is caused by the holes escaping from the initial recombination and being transported to the interface to form the interface states. Then, the competition between the steadily increasing interfacial trap charge and the continuously annealed shallow level oxide trap charge leads to the stable increase of degradation under low dose irradiation. Finally, in stage three, the increases of the degradation rates for high and low dose irradiation result from the different amounts of the hydrogen molecules generated by the hole reactive with depassiviated Si suspended bonds, which can interact with the deep level defects and release protons, causing an increase of interfacial trap charges with prolonged irradiation.

OC-0294 Separating initial and general recombination in reference dosimetry of proton pencil beam scanning

OC-0294 Separating initial and general recombination in reference dosimetry of proton pencil beam scanning

Genetic Parameters and Population of Coriander Reaction to Meloidogyne incognita Race 1

The genetic parameters provide indispensable information about the genetic behavior of the study population regarding the characters evaluated, providing subsidy to the breeder on the decision about the best selection method to adopt in order to realize a genetic gain and maintain adequate variability. In this sense, the present study was carried out to verify the reaction and to estimate genetic parameters of a population of coriander composed of two cultivars (Verdão and HTV Dom Luiz) and 49 half-siblings progenies parasitized by Meloidogyne incognita race 1. The experiment was conducted in a greenhouse in the Department of Agronomy of the Federal Rural University of Pernambuco. The 51 cultivars and progenies were arranged in a randomized block design with four replicates and each experimental plot was composed of eight plants. The sowing was carried out in a tray of 128 cell- expanded polystyrene filled with commercial substrate, 1,000 eggs/cell of the pathogen were inoculated on the same day of sowing. After 30 days of inoculation, the number of galls was quantified in the root system, the pathogen’s eggs were extracted and, after the quantification of the eggs, the reproduction factor was estimated for each genotype. Data of the three characteristics were compared by Scott-Knott clustering test, then classified as resistant or susceptible, and genetic parameters estimated. The genotypes differed from each other at 1% probability level for all the characters under study. Heritabilities were high and positive with values ​​of 75.91 for number of galls in the root system and 96.72 for the number of eggs. Based on the phenotypic and genotypic correlations, it was possible to select desired genotypes based on the number of galls in the initial selection cycles, making selection and recombination possible in the same cycle. Twelve (12) progenies were selected for their resistance to the pathogen.

129. Recombination effects in parallel-plates ionization chambers under clinical continuous active scanning proton beams

PurposeSeveral factors can influence the response of an ionization chamber (IC) when exposed to radiation. One of these is the ion recombination. It depends mainly on dose rate (volumetric recombination, Vrec) and ionization density (initial recombination, Irec). This work aimed to determine the recombination correction factor (Ks) for parallel-plates (PP) ICs irradiated under continuous pencil beam scanning proton beams. MethodsPPICs were exposed to clinical proton beams produced by an isochronous cyclotron (Proteus®PLUS-Iba). To compensate for fluctuations of the beam two PPICs were set face-to-face, one used as monitor and the second was the PPIC under study, as explained in [1]. To separate Vrec and Irec, measurements were performed at different dose-rates (minimum cyclotron current 75 nA, maximum 300 nA) and different voltage (20 V–500 V). Ks was obtained as the ratio between the saturation current (Isat) and the ionization current (I) measured at the operating voltage (Ks=Isat/I) [1]. This was carried-out for three scenarios: (a) single 10 × 10 cm2 unmodulated 100 MeV layer; (b) single 10 × 10 cm2 unmodulated 226 MeV layer; (c) modulated box-like field (range 15 cm-modulation 10 cm). A theoretical model is under investigation to be compared to Ks-values determined with the two-voltage method [2] for continuous beam. Only results obtained using an PPC05 IC (Iba-Dosimetry) are reported. ResultsAll scenarios show similar results (Figure). At high voltages, data has an exponential trend, highlighting the presence of charge multiplication effects (CME). This should be due to the small gap (0.6 mm) between electrodes. At low voltages, there is a rather linear relation between 1/I and 1/V2. Using the two-voltage method, at +300 V, a Ks-value of 1.001 is found in all scenarios. Due to CME this value could be overestimated. ConclusionsAlthough the proton beam used can be considered as continuous, the variation of Ks against 1/V2 is not linear due to CME, so the two-voltage method use might be unsuitable. Further investigations are currently made to decouple recombination effect from CME and derive a model for accurately determine Ks.▪

CALCULATION OF LET DISTRIBUTIONS IN THE ACTIVE VOLUME OF A RECOMBINATION CHAMBER.

Recombination chambers are linear energy transfer (LET)-dependent detectors and their applications are closely related to the problem of estimating radiation quality. The operational principle is based on the phenomenon of the initial recombination of ions in gases. Practical usefulness of the chambers is well confirmed by experimental results, however, the correlation of the response with LET is based on an approximated theoretical description of initial recombination of ions in gases. In this work, a precise numerical model of the REM-2 recombination chamber has been prepared, in order to calculate the dose distributions versus LET inside the chamber. The results form a set of data for further validation of recombination methods.

Modeling ion recombination in liquid ionization chambers - Improvement and analysis of the two-dose-rate method.

The use of liquid ionization chambers can provide useful information to endeavors with radiation dosimetry for highly modulated beams. Liquid ionization chambers may be particularly suitable for computed tomography applications where conventional ionization chambers do not present a high enough sensitivity for the spatial resolution required to characterize common X-ray beams. Due to the sensitivity, which leads to high charge densities, liquid ionization chambers can suffer from large recombination losses leading to degradation in signal to dose rate linearity. To solve this problem, a two-dose-rate method for general recombination correction has been proposed for liquid ionization chambers. However, the valid range of recombination losses that the method can accurately account for has been found to vary depending on radiation quality. The present work provides an in-depth analysis of the performance of the two-dose-rate method. Furthermore, the soundness of applying gas theory to liquids is investigated by using the two-dose-rate method. In the present work, the two-dose-rate method for general recombination correction of liquid ionization chambers used in continuous beams is studied by employing theory for gas-filled ionization chambers. An approximate relation for the general collection efficiency containing a material-specific parameter that is traceable to liquids has been derived for theoretical and experimental investigation alongside existing theory. Furthermore, the disassociation between initial and general recombination in the method is analyzed both theoretically and experimentally. The results indicate that liquids and gases share general recombination characteristics, where the liquids investigated (isooctane and tetramethylsilane) to a large extent mimic the behavior theoretically expected in gases. Furthermore, it is shown that the disassociation between initial and general recombination in the two-dose-rate method is an approximation that depends on the relation between initial recombination and the collecting electric field strength at the dose rates used. Due to the approximation used to separate initial and general recombination the valid range of collection efficiencies for the two-dose-rate method will not only depend on the model used to describe general recombination but also on the type of liquid and radiation beam quality. As there is no robust theory for initial recombination in liquids to apply, the valid range of general collection efficiencies for the two-dose-rate method should be experimentally evaluated for each radiation dosimetry application.

Empirical Study of the Disparity in Radiation Tolerance of the Minority-Carrier Lifetime Between II–VI and III–V MWIR Detector Technologies for Space Applications

The degradation of the minority-carrier recombination lifetime of various III–V nBn and II–VI HgCdTe midwave-infrared space detector materials under stepwise 63-MeV proton irradiation up to fluence of 7.5 × 1011 cm−2 and above has been measured using time-resolved photoluminescence while samples were held at 120 K to limit thermal annealing. As expected, the recombination rate of each sample was found to increase with proton fluence at a nearly constant rate, implying a near-linear increase in defect concentration. The rate of change of the carrier recombination rate, herein called the minority-carrier lifetime damage factor, was then plotted as a function of the initial recombination rate for each sample. Juxtaposing the III–V and II–VI results revealed a distinct disparity, with the incumbent detector material HgCdTe being roughly an order of magnitude more radiation tolerant to displacement damage from proton irradiation than any of the nBn materials. The results for the latter also suggest some degree of interrelation between the damage factor and initial lifetime. The behavior of the lifetime of each material under annealing revealed that HgCdTe exhibited nearly 100% recovery at 295 K whereas III–V materials recovered to only about 50% under the same conditions.

Ion recombination correction factor in scanned light-ion beams for absolute dose measurement using plane-parallel ionisation chambers

Based on international reference dosimetry protocols for light-ion beams, a correction factor (ks) has to be applied to the response of a plane-parallel ionisation chamber, to account for recombination of negative and positive charges in its air cavity before these charges can be collected on the electrodes. In this work, ks for IBA PPC40 Roos-type chambers is investigated in four scanned light-ion beams (proton, helium, carbon and oxygen). To take into account the high dose-rates used with scanned beams and LET-values, experimental results are compared to a model combining two theories. One theory, developed by Jaffé, describes the variation of ks with the ionization density within the ion track (initial recombination) and the other theory, developed by Boag, describes the variation of ks with the dose rate (volume recombination). Excellent agreement is found between experimental and theoretical ks-values. All results confirm that ks cannot be neglected. The solution to minimise ks is to use the ionisation chamber at high voltage. However, one must be aware that charge multiplication may complicate the interpretation of the measurement. For the chamber tested, it was found that a voltage of 300 V can be used without further complication. As the initial recombination has a logarithmic variation as a function of 1/V, the two-voltage method is not applicable to these scanned beams.

Insights into immune system development and function from mouse T-cell repertoires

The ability of the adaptive immune system to respond to arbitrary pathogens stems from the broad diversity of immune cell surface receptors. This diversity originates in a stochastic DNA editing process (VDJ recombination) that acts on the surface receptor gene each time a new immune cell is created from a stem cell. By analyzing T-cell receptor (TCR) sequence repertoires taken from the blood and thymus of mice of different ages, we quantify the changes in the VDJ recombination process that occur from embryo to young adult. We find a rapid increase with age in the number of random insertions and a dramatic increase in diversity. Because the blood accumulates thymic output over time, blood repertoires are mixtures of different statistical recombination processes, and we unravel the mixture statistics to obtain a picture of the time evolution of the early immune system. Sequence repertoire analysis also allows us to detect the statistical impact of selection on the output of the VDJ recombination process. The effects we find are nearly identical between thymus and blood, suggesting that our analysis mainly detects selection for proper folding of the TCR receptor protein. We further find that selection is weaker in laboratory mice than in humans and it does not affect the diversity of the repertoire.

Ion recombination and polarity corrections for small-volume ionization chambers in high-dose-rate, flattening-filter-free pulsed photon beams.

To investigate ion recombination and polarity effects in scanning and microionization chambers when used with digital electrometers and high-dose-rate linac beams such as flattening-filter-free (FFF) fields, and to compare results against conventional pulsed and continuous photon beams. Saturation curves were obtained for one Farmer-type ionization chamber and eight small-volume chamber models with volumes ranging from 0.01 to 0.13 cm3 using a Varian TrueBeam™ STx with FFF capability. Three beam modes (6 MV, 6 MV FFF, and 10 MV FFF) were investigated, with nominal dose-per-pulse values of 0.0278, 0.0648, and 0.111 cGy/pulse, respectively, at dmax . Saturation curves obtained using the Theratronics T1000 60 Co unit at the UWADCL and a conventional linear accelerator (Varian Clinac iX) were used to establish baseline behavior. Jaffé plots were fitted to obtain Pion , accounting for exponential effects such as charge multiplication. These values were compared with the two-voltage technique recommended in TG-51, and were plotted as a function of dose-per-pulse to assess the ability of small-volume chambers to meet reference-class criteria in FFF beams. Jaffé- and two-voltage-determined Pion values measured for high-dose-rate beams agreed within 0.1% for the Farmer-type chamber and 1% for scanning and microionization chambers, with the exception of the CC01 which agreed within 2%. With respect to ion recombination and polarity effects, the Farmer-type chamber, scanning chambers and the Exradin A26 microchamber exhibited reference-class behavior in all beams investigated, with the exception of the IBA CC04 scanning chamber, which had an initial recombination correction that varied by 0.2% with polarity. All microchambers investigated, with the exception of the A26, exhibited anomalous polarity and ion recombination behaviors that make them unsuitable for reference dosimetry in conventional and high-dose-rate photon beams. The results of this work demonstrate that recombination and polarity behaviors seen in conventional pulsed and continuous photon beams trend accordingly in high-dose-rate FFF linac beams. Several models of small-volume ionization chambers used with a digital electrometer have been shown to meet reference-class requirements with respect to ion recombination and polarity, even in the high-dose-rate environment. For such chambers, a two-voltage technique agreed well with more rigorous methods of determining Pion . However, the results emphasize the need for careful reference detector selection, and indicate that ionization chambers ought to be extensively tested in each beam of interest prior to their use for reference dosimetry.

A general algorithm for calculation of recombination losses in ionization chambers exposed to ion beams.

Dosimetry with ionization chambers in clinical ion beams for radiation therapy requires correction for recombination effects. However, common radiation protocols discriminate between initial and general recombination and provide no universal correction method for the presence of both recombination types in ion beams of charged particles heavier than protons. The advent of multiple field optimization in ion beams, allowing for complex patterns of dose delivery in both temporal and spatial domains, results in new challenges for recombination correction where the resulting recombination depends on the plan delivered. Here, the authors present the open source code IonTracks version 1.0, where the combined initial and general recombination effects in principle can be predicted for any ion beam with arbitrary particle-energy spectrum and temporal structure. IonTracks uses track structure theory to distribute the charge carriers in ion tracks. The charge carrier movements are governed by a pair of coupled differential equations, based on fundamental physical properties as charge carrier drift, diffusion, and recombination, which are solved numerically while the initial and general charge carrier recombination is computed. A space charge screening of the electric field is taken into account and the algorithm furthermore allows an inclusion of a free-electron component. The algorithm is numerically stable and in accordance with experimentally validated theories for initial recombination in heavy ion tracks and general recombination in a proton beam. Given IonTracks' ability to handle arbitrary inputs, IonTracks can in principle be applied to any complex particle field in the spatial and temporal domain. IonTracks is validated against the Jaffé's and Boag's theory of recombination in pulsed beams of multiple ion species. IonTracks is able to calculate the correction factor for initial and general recombination losses in parallel-plate ionization chambers. Even if only few experimental data on recombination effects in ionization chambers are available today, the universal concept of IonTracks is not limited to the ions investigated here. Future experimental investigations of recombination in pulsed and possibly also continuous ion beams may be conducted with IonTracks, which ultimately may lead to a more precise prediction of recombination factors in complex radiation fields.

Influence of Alternate Biasing on TID Effects of Irradiated Mixed-Signal Programmable Arrays

This work investigates the influence of alternate biasing on TID effects in mixed-signal programmable devices. By reducing the time the device is biased when exposed to ionizing radiation, the severity of TID effects is also reduced, due to the increasing in the initial recombination of electron-hole pairs and the radiation induced charge neutralization effect. These effects are investigated considering a switched capacitor filter prototyped in a Field Programmable Analog Array. Eight samples of such device were exposed to gamma radiation while the devices were alternately biased, by repeatedly turning the power on and off, corresponding to different equivalent total biasing times and duty-cycles. Results show increase in radiation tolerance to the alternate biased devices, which is dependent also on the biasing duty-cycle. Considering this behavior, a novel system level radiation hardening technique, based on modular redundancy, associated to an alternate biasing scheme, is also proposed in this paper.

Ion recombination correction in carbon ion beams.

In this work, ion recombination is studied as a function of energy and depth in carbon ion beams. Measurements were performed in three different passively scattered carbon ion beams with energies of 62 MeV/n, 135 MeV/n, and 290 MeV/n using various types of plane-parallel ionization chambers. Experimental results were compared with two analytical models for initial recombination. One model is generally used for photon beams and the other model, developed by Jaffé, takes into account the ionization density along the ion track. An investigation was carried out to ascertain the effect on the ion recombination correction with varying ionization chamber orientation with respect to the direction of the ion tracks. The variation of the ion recombination correction factors as a function of depth was studied for a Markus ionization chamber in the 62 MeV/n nonmodulated carbon ion beam. This variation can be related to the depth distribution of linear energy transfer. Results show that the theory for photon beams is not applicable to carbon ion beams. On the other hand, by optimizing the value of the ionization density and the initial mean-square radius, good agreement is found between Jaffé's theory and the experimental results. As predicted by Jaffé's theory, the results confirm that ion recombination corrections strongly decrease with an increasing angle between the ion tracks and the electric field lines. For the Markus ionization chamber, the variation of the ion recombination correction factor with depth was modeled adequately by a sigmoid function, which is approximately constant in the plateau and strongly increasing in the Bragg peak region to values of up to 1.06. Except in the distal edge region, all experimental results are accurately described by Jaffé's theory. Experimental results confirm that ion recombination in the investigated carbon ion beams is dominated by initial recombination. Ion recombination corrections are found to be significant and cannot be neglected for reference dosimetry and for the determination of depth dose curves in carbon ion beams.

Footprintless disruption of prosurvival genes in aneuploid cancer cells using CRISPR/Cas9 technology.

CRISPR/Cas9 has emerged as a powerful methodology for the targeted editing of genomic DNA sequences. Nevertheless, the intrinsic inefficiency of transfection methods required to use this technique with cultured cells requires the selection and isolation of successfully modified cells, which invariably subjects the cells to stress. Here we report a workflow that allows the isolation of genomically modified cells, even where loss of functional alleles constitutes a selective disadvantage owing to impaired ability to survive stress. Using targeted disruption of the Id1 and Id3 genes in murine B16-F10 and Ret melanoma cell lines as an example, we show that the method allows for the footprintless isolation of CRISPR/Cas9-modified aneuploid cancer cells. We also provide evidence that serial CRISPR/Cas9 modifications can occur, for example when initial homologous recombination events introduce cryptic PAM sequences, and demonstrate that multiple alleles can be successfully targeted in aneuploid cancer cells. By sequencing individual alleles we also found evidence for CRISPR/Cas9-induced transposable element insertion, albeit at a low frequency. This workflow should have broad application in the functional analysis of prosurvival gene function in cultured cells.

Charge Carrier Generation In A Polymer By Using N-type Doping For The Improvement Of Electrical Properties

In this paper, the charge carrier generation in polymer blends by chemical doping has studied. In these studies, we employed n-type dopant molecule decamethylcobaltocene (DMC) which exhibit very strong electron donating nature. We have demonstrated that such type of doping favours the formation of charge transfer complex (CTC) and reduce the recombination probability. We have confirmed the CTC formation form the absorption spectroscopy. Further we have used transient photoluminescence spectroscopy to reveal the reduced initial recombination of charge transfer exciton. We interpret our results based on a reduced formation of emissive charge transfer excitons in doped blends, induced by state filling of immobile tail states in the polymer HOMO. Copyright © 2016 VBRI Press.

Recombination characteristics of therapeutic ion beams on ion chamber dosimetry

In heavy ion radiotherapy, ionization chambers are regarded as a standard for determining the absorbed dose given to patients. In ion dosimetry, it is necessary to correct the radiation quality, which depends on the initial recombination effect. This study reveals for the radiation quality dependence of the initial recombination in air in ion dosimetry. Ionization charge was measured for the beams of protons at 40–160 MeV, carbon at 21–400 MeV/n, and iron at 23.5–500 MeV/n using two identical parallel-plate ionization chambers placed in series along the beam axis. The downstream chamber was used as a monitor operated with a constant applied voltage, while the other chamber was used for recombination measurement by changing the voltage. The ratio of the ionization charge measured by the two ionization chambers showed a linear relationship with the inverse of the voltage in the high-voltage region. The initial recombination factor was estimated by extrapolating the obtained linear relationship to infinite voltage. The extent of the initial recombination was found to increase with decreasing incident energy or increasing atomic number of the beam. This behavior can be explained with an amorphous track structure model: the increase of ionization density in the core region of the track due to decreasing kinetic energy or increasing atomic number leads to denser initial ion production and results in a higher recombination probability. For therapeutic carbon ion beams, the extent of the initial recombination was not constant but changed by 0.6% even in the target region. This tendency was quantitatively well reproduced with the track-structure based on the initial recombination model; however, the transitional change in the track structure is considered to play an important role in further understanding of the characteristics of the initial recombination.

Efficiencies of Some Spherical Ion Chambers in Continuous and Pulsed Radiation: A Numerical Evaluation.

SummaryBackgroundEvaluation of the collection efficiencies of ion chambers is a necessity for the proper evaluation of radiation quantities in different applications. Overall collection efficiency is the product of three different values: collection efficiencies considering contributions of, volume recombination, back-diffusion loss and initial recombination, the later may be neglected at low charge rates.Material/MethodsFive common spherical ion chambers of different volumes and specifications were included in this study for the evaluation of volume recombination collection efficiency and back diffusion collection efficiency for continuous and pulsed radiation and at different values of the applied polarizing potential. Through current work there is an attempt of focusing on how the selection of ion chamber dimensions may affect the overall collection efficiency in addition to the proper selection of other influencing parameters.ResultsCollection efficiencies considering volume recombination (fv) for five spherical ion chambers of common types were evaluated for continuous and pulsed radiation over a wide range of polarizing potential. The relation between the ion chamber volume and its evaluated collection efficiencies were studied for both continuous and pulsed radiation; transit time values for the ion chambers included in this study were evaluated at different values of applied potential. Also, collection efficiencies considering diffusion current values (fd) were evaluated for the five chambers, and plotted versus the applied polarizing potential.ConclusionsThrough this study it was feasible to evaluate numerically the collection efficiencies of some spherical ion chambers considering volume recombination and back diffusion effects; the affecting parameters were studied and highlighted.

A numerical model of initial recombination for high-LET irradiation: Application to liquid-filled ionization chambers

Abstract In this paper we present a numerical model of initial recombination in media irradiated with high linear energy transfer (LET) ions, which relies on an amorphous track model of ionization of high LET particles, and diffusion, drift and recombination of ionized charge carriers. The model has fundamental applications for the study of recombination in non-polar liquids, as well as practical ones, like in modelling hadrontherapy dosimetry with ionization chambers. We have used it to study the response of liquid-filled ionization chambers to hadrontherapy beams: dependence of initial recombination on ion species, energy and applied external electric field.

Promotion of CO oxidation on PdO(101) by adsorbed H2O

We investigated the influence of adsorbed H2O on the oxidation of CO on PdO(101) using temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations. We find that water inhibits CO adsorption on PdO(101) by site blocking, but also provides a more facile pathway for CO oxidation compared with the bare oxide surface. In the presence of adsorbed H2O, the oxidation of CO on PdO(101) produces a CO2 TPRS peak that is centered at a temperature ~50K lower than the main CO2 TPRS peak arising from CO oxidation on clean PdO(101) (~330 vs. 380K). RAIRS shows that CO continues to adsorb on atop-Pd sites of PdO(101) when H2O is co-adsorbed, and provides no evidence of other reactive intermediates. DFT calculations predict that the CO oxidation mechanism follows the same steps for CO adsorbed on PdO(101) with and without co-adsorbed H2O, wherein an atop-CO species recombines with an oxygen atom from the oxide surface lattice. According to DFT, hydrogen bonding interactions with adsorbed H2O species stabilize the carboxyl-like transition structure and intermediate that result from the initial recombination of CO and O on the PdO(101) surface. This stabilization lowers the energy barrier for CO oxidation on PdO(101) by ~10kJ/mol, in good agreement with our experimental estimate.