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Biomedical subjects

M C Payne

Publications and source records attributed to M C Payne.

At least 19 recordsLinked to original sources
PubMed2005

Surface diffusion: the low activation energy path for nanotube growth.

We present the temperature dependence of the growth rate of carbon nanofibers by plasma-enhanced chemical vapor deposition with Ni, Co, and Fe catalysts. We extrapolate a common low activation energy of 0.23-0.4 eV, much lower than for thermal deposition. The carbon diffusion on the catalyst surface and the stability of the precursor molecules, C2H2 or CH4, are investigated by ab initio plane wave density functional calculations. We find a low activation energy of 0.4 eV for carbon surface diffusion on Ni and Co (111) planes, much lower than for bulk diffusion. The energy barrier for C2H2 and CH4 dissociation is at least 1.3 eV and 0.9 eV, respectively, on Ni(111) planes or step edges. Hence, the rate-limiting step for plasma-enhanced growth is carbon diffusion on the catalyst surface, while an extra barrier is present for thermal growth due to gas decomposition.

Journal Article↗
PubMed2004

"Learn on the fly": a hybrid classical and quantum-mechanical molecular dynamics simulation.

We describe and test a novel molecular dynamics method which combines quantum-mechanical embedding and classical force model optimization into a unified scheme free of the boundary region, and the transferability problems which these techniques, taken separately, involve. The scheme is based on the idea of augmenting a unique, simple parametrized force model by incorporating in it, at run time, the quantum-mechanical information necessary to ensure accurate trajectories. The scheme is tested on a number of silicon systems composed of up to approximately 200 000 atoms.

Journal Article↗
PubMed2001

Thermodynamics of catalytic formation of dimethyl ether from methanol in acidic zeolites.

We present a theoretical study of the formation of the first intermediate, dimethyl ether, in the methanol to gasoline conversion within the framework of an ab initio molecular dynamics approach. The study is performed under conditions that closely resemble the reaction conditions in the zeolite catalyst including the full topology of the framework. The use of the method of thermodynamic integration allows us to extract the free-energy profile along the reaction coordinate. We find that the entropic contribution qualitatively alters the free-energy profile relative to the total energy profile. Different transition states are found from the internal and free energy profiles. The entropy contribution varies significantly along the reaction coordinate and is responsible for stabilizing the products and for lowering the energy barrier. The hugely inhomogeneous variation of the entropy can be understood in terms of elementary processes that take place during the chemical reaction. Our simulations provide new insights into the complex nature of this chemical reaction.

Journal Article↗
PubMed2001

Can atomic force microscopy achieve atomic resolution in contact mode?

Atomic force microscopy operating in the contact mode is studied using total-energy pseudopotential calculations. It is shown that, in the case of a diamond tip and a diamond surface, it is possible for a tip terminated by a single atom to sustain forces in excess of 30 nN. It is also shown that imaging at atomic resolution may be limited by blunting of the tip during lateral scanning.

Journal Article↗
PubMed1999

First principles investigation of singly reduced cytochrome P450.

1. The application of novel ab initio quantum mechanical methods to the states in the catalytic cycle of cytochrome P450 following the first reduction step is described. 2. A good correlation was found between the calculated energy of reduction and the experimentally determined redox potential for a range of substrate- and substrate analogue-bound systems. 3. On reduction of the haem system, the ground state of Fe remains Fe3+. On binding of a CO molecule, Fe adopts a low-spin Fe2+ state, in agreement with experiment. However, on binding of an O2 molecule, calculations indicate that the system adopts a ferric superoxide ground state, in which the Fe is in a low-spin Fe3+ state.

Camphor 5-Monooxygenase↗
PubMed1998

Evidence for stabilization of the low-spin state of cytochrome P450 due to shortening of the proximal heme bond.

The cytochrome P450 superfamily of enzymes is ubiquitous, being responsible for the metabolism of a wide range of endogenous and xenobiotic compounds. However, the detailed mechanism of the catalytic cycle of these enzymes is still not fully understood. We describe results, obtained from first principles molecular simulations, which indicate that the low-spin state of the Fe3+ ion, present in the heme moiety at the active site of a cytochrome P450 enzyme, may be stabilized by shortening of the proximal bond of the heme. Calculations indicate that a bond length of less than approximately 2.05 A between the heme Fe3+ ion and the cysteine S, which forms the proximal ligand, would result in the stabilization of the low-spin state of the Fe3+, inhibiting the progress of the P450 catalytic cycle. Our investigation uses novel first principles modeling techniques which treat the entire system quantum-mechanically.

Cytochrome P-450 Enzyme System↗
PubMed1998

An ab initio approach to the understanding of cytochrome P450-ligand interactions.

1. We describe the application of novel ab initio quantum mechanical methods to the study of ligand interactions with cytochrome P450cam (CYP101). 2. We find that our techniques accurately describe the transition from a low-spin state to a high-spin state of the haem Fe3+ on binding of a substrate. Furthermore, our methods correctly predict that a large fraction of low-spin character is retained on binding of an inhibitor. 3. We demonstrate the use of 'computational experiments' to elucidate key features of the mechanism of interaction. This leads us to identify a new mechanism for the suppression of the low- to high-spin transition on binding of an inhibitor, namely the shortening of the bond between the Fe atom and the coordinated S atom of the cysteine axial ligand.

Camphor 5-Monooxygenase↗
PubMed1996

Nicotine alleviation of nicotine abstinence syndrome is naloxone-reversible.

In a recently introduced rodent model of nicotine abstinence syndrome, the observed signs closely resembled those typical of rat opiate abstinence syndrome. Signs were precipitated by naloxone and potently reversed by morphine as well as nicotine itself, suggesting that nicotine might relieve nicotine abstinence syndrome through releasing endogenous opioids. To test this hypothesis, rats were continuously infused subcutaneously (SC) for 7 days with 9 mg/kg per day nicotine tartrate. Each rat was observed for abstinence signs at 18 and 21 h after termination of infusion. Three minutes before the 21-h test, all rats received 0.35 mg/kg nicotine tartrate, SC; 5 min before the nicotine injection, subjects received 9 or 4.5 mg/kg naloxone or saline alone, SC. Abstinence reversal scores were calculated as signs at 21 h as a percentage of signs at 18 h. Naloxone prevented nicotine alleviation of nicotine abstinence in a dose-related manner. However, naloxone in the absence of a nicotine injection had no effect on abstinence severity in either highly dependent or moderately dependent rats (infused with 9 or 5 mg/kg per day nicotine tartrate, respectively). These results support the hypothesis that endogenous opioids play a role in nicotine dependence and abstinence.

Animals↗
PubMed1994

The renal transplant perfusion index: reduction in the error and variability.

For 15 years, perfusion indices derived from scintigraphic studies have proved useful in the serial evaluation of renal transplants and they have recently been confirmed as being more sensitive than Doppler ultrasound as an indicator of vascular rejection and cyclosporin toxicity. In the calculation of these indices, correction for administered activity is often accomplished using activity measurements made over a convenient artery which, therefore, has a critical influence on the value of the index obtained. In this communication, a theoretical assessment is made of the error and variability introduced into the calculation of the perfusion index, because of inadequate spatial sampling of activity in these narrow arteries and the consequential inconsistencies in the measurement of the arterial tracer activity. Using numerical simulation, it is shown that the errors in repeat studies on the same patient may be as high as 39% and between patients as high as 53%. These figures can be reduced to below 18% and 21%, respectively, by constructing a region of interest (ROI) to extend over as much of the arterial width as possible rather than relying only on the maximum pixel count. Further reduction to below 12% and 10% is possible by utilising a 128 x 128 acquisition matrix instead of 64 x 64 and drawing the ROI over the aorta instead of the iliac artery.

Humans↗
PubMed1985

Effects of range on category scaling of loudness judgments.

In two experiments subjects judged the loudness of a 1000-Hz tone within a range of three loudnesses. The first used a within-subjects design with ranges of 1-2 sones, 1-4 sones, and 1-8 sones. The second used a between-subjects design with ranges of 1-2 sones, 1-8 sones, and 1-16 sones. Except for two values in the 1- to 2-sone range the two experiments produced comparable results. As range was increased the tendency to round away from the end points increased and the judged mid-point occurred at a higher value. Comparisons are made with rounding patterns that occur in studies of linear scales. The present results are interpreted to support Poulton's (1975) position, that the mid-point of subjective estimates depends on the range of stimuli judged, rather than that of Stevens (1975), that the mid-point on a prosthetic scale is assigned to a value that is lower than the mid-point on the subject's subjective scale.

Acoustic Stimulation↗
PubMed1985

Legionnaires' disease in the Caribbean. An outbreak associated with a resort hotel.

Outbreaks of legionnaires' disease (LD) in tourists visiting Italian and Spanish resorts have been recently reported. An unusual number of reports of LD in tourists visiting the US Virgin Islands prompted an investigation of risk factors for development of LD in this area. Twenty-seven cases of LD were identified between 1979 and 1982 through press reports, personal communication, the national LD surveillance system, a review of hospital records, and a mail survey. Twenty-four of 27 persons with the disease had visited St Croix and 12 of them had stayed at a single hotel in 1981. Available evidence suggested that infection was due to Legionella pneumophila serogroup 1; L pneumophila serogroups 1 and 3 and several new Legionella species were isolated from the potable water system at the hotel. Following hyperchlorination of the potable water system, no further cases of LD in hotel visitors have been identified to date.

Disease Outbreaks↗
PubMed1981

Effects of noise frequency on performance and annoyance for women and men.

Effects of noise frequencies on both performance on a complex psychomotor task and annoyance were investigated for men (n = 30) and women (n = 30). Each subject performed a complex psychomotor task for 50 min. in the presence of low frequency noise, high frequency noise, or ambient noise. Women and men learned the task at different rates. Little effect of noise was shown. Annoyance ratings were subsequently obtained from each subject for noise of various frequencies by the method of magnitude estimation. High frequency noises were more annoying than low frequency noises regardless of sex and immediate prior exposure to noise. Sex differences in annoyance did not occur. Not direct relationship between learning to perform a complex task while exposed to noise and annoyance by that noise was demonstrated.

Adolescent↗