Ozone holes


Figure 11.6 A schematic view of the presumed binding mode of the tetrahedral transition state intermediate for the deacylation step. The four essential features of the serine proteinases are highlighted in yellow the catalytic triad, the oxyanion hole, the specificity pocket, and the unspecific main-chain substrate binding. Figure 11.6 A schematic view of the presumed binding mode of the tetrahedral transition state intermediate for the deacylation step. The four essential features of the serine proteinases are highlighted in yellow the catalytic triad, the oxyanion hole, the specificity pocket, and the unspecific main-chain substrate binding.
Tight binding and stabilization of the tetrahedral transition state intermediate is accomplished by providing groups that can form hydrogen bonds to the negatively charged oxygen atom attached to Ci. These groups are in a pocket of the enzyme called the oxyanion hole (see Figure 11.6). The positive charge that develops on the His residue after it has accepted a proton also stabilizes the negatively charged transition state. These features also presumably destabilize binding of substrate in the normal state.  [c.209]

These four features all occur in an almost identical fashion in all members of the chymotrypsin superfamily of homologous enzymes, which includes among others chymotrypsin, trypsin, elastase, and thrombin. Reasonably, one might imagine that such a combination of four characteristic features had arisen only once during evolution to give an ancestral molecule from which all serine proteinases diverged. However, subtilisin, a bacterial serine proteinase with an amino acid sequence and, as we will see, a three-dimensional structure quite different from the mammalian serine proteinases, exhibits these same four characteristic features. Subtilisin is not evolutionar-ily related to the chymotrypsin family of enzymes nevertheless, the atoms in subtilisin that participate in the catalytic triad, in the oxyanion hole, and in substrate binding are in almost identical positions relative to one another in the three-dimensional structure as they are in chymotrypsin and its relatives. Starting from unrelated ancestral proteins, convergent evolution has resulted in the same structural solution to achieve a particular catalytic mechanism. The serine proteinases, in other words, provide a spectacular example of convergent evolution at the molecular level, which we can best appreciate by explaining in detail the structures of chymotrypsin and subtilisin.  [c.210]

This inhibitor does not form a covalent bond to Ser 195 but one of its carboxy oxygen atoms is in the oxyanion hole forming hydrogen bonds to the main-chain NH groups of residues 193 and 195. The tyrosyl side chain is positioned in the specificity pocket, which derives its specificity mainly from three residues, 216, 226, and 189, as we shall see later. The main chain of  [c.211]

Figure 11.10 Topological diagram of the two domains of chymotrypsin, illustrating that the essential active-site residues are part of the same two loop regions (3-4 and 5-6, red) of the two domains. These residues form the catalytic triad, the oxyanion hole (green), and the substrate binding regions (yellow and blue) including essential residues in the specificity pocket. Figure 11.10 Topological diagram of the two domains of chymotrypsin, illustrating that the essential active-site residues are part of the same two loop regions (3-4 and 5-6, red) of the two domains. These residues form the catalytic triad, the oxyanion hole (green), and the substrate binding regions (yellow and blue) including essential residues in the specificity pocket.
The C-terminal part is green. The catalytic triad Asp 32, His 64, and Ser 221 as well as Asn 15S, which forms part of the oxyanion hole are shown in purple. The main chain of part of a polypeptide Inhibitor is shown in red. Main-chain residues around 101 and 127 (orange circles) form the nonspecific binding regions of peptide substrates.  [c.216]

The active site of subtilisin is outside the carboxy ends of the central p strands analogous to the position of the binding sites in other a/p proteins as discussed in Chapter 4. Details of this active site are surprisingly similar to those of chymotrypsin, in spite of the completely different folds of the two enzymes (Figures 11.14 and 11.9). A catalytic triad is present that comprises residues Asp 32, His 64 and the reactive Ser 221. The negatively charged oxygen atom of the tetrahedral transition state binds in an oxyanion hole,  [c.216]

Figure 11.14 Schematic diagram of the active site of subtilisin. A region (residues 42-45) of a bound polypeptide inhibitor, eglin, is shown in red. The four essential features of the active site— the catalytic triad, the oxyanion hole, the specificity pocket, and the region for nonspecific binding of substrate—are highlighted in yellow. Important hydrogen bonds between enzyme and inhibitor are striped. This figure should be compared to Figure 11.9, which shows the same features for chymotrypsin. (Adapted from W. Bode et al., EMBO /. Figure 11.14 Schematic diagram of the active site of subtilisin. A region (residues 42-45) of a bound polypeptide inhibitor, eglin, is shown in red. The four essential features of the active site— the catalytic triad, the oxyanion hole, the specificity pocket, and the region for nonspecific binding of substrate—are highlighted in yellow. Important hydrogen bonds between enzyme and inhibitor are striped. This figure should be compared to Figure 11.9, which shows the same features for chymotrypsin. (Adapted from W. Bode et al., EMBO /.
The single mutation Asp 32-Ala reduces the catalytic reaction rate by a factor of about lO compared with wild type. This rate reduction reflects the role of Asp 32 in stabilizing the positive charge that His 64 acquires in the transition state. A similar reduction of kcat and kcat/ m (2.5 x 10 ) is obtained for the single mutant Asn 155-Thr. Asn 155 provides one of the two hydrogen bonds to the substrate transition state in the oxyanion hole of subtilisin.  [c.218]

Bryan, P., et al. Site-directed mutagenesis and the role of the oxyanion hole in subtilisin. Proc. Natl. Acad. Sci. USA 83 3743-3745, 1986.  [c.220]

X-ray crystallographic studies of serine protease complexes with transition-state analogs have shown how chymotrypsin stabilizes the tetrahedral oxyanion transition states (structures (c) and (g) in Figure 16.24) of the protease reaction. The amide nitrogens of Ser and Gly form an oxyanion hole in which the substrate carbonyl oxygen is hydrogen-bonded to the amide N-H groups.  [c.519]

The oxyanion hole of chymotryp.sin. stabilize.s the tetrahedral oxyanion tran.sition. state.s of die mechani.sm in Figure 16.24.  [c.519]

There is a tendency for the water produced by the combustion to condense in the narrow neck of the combustion tube, instead of passing right over into the absorption tube. To avoid this, two movable copper hooks Q are mounted on a copper rod, which can slide in and out of a hole cut in the mortar P these may be placed over the beak of the combustion tube and conduct sufficient heat from the mortar to vaporise the water once again so that it is driven over by the Oxygen stream into the absorption tube R.  [c.470]

The type of carbon dioxide absorption tube S (Fig. 85) which U recommended is shown in detail in Fig. 86. In this tube the composite filling of soda-lime and a small packing of magnesium perchlorate are kept in separate chambers which are a-put in contact only while gas is flowing, by rotating a ground-glass joint tap Tj which divides the tube into two compartments the perchlorate being packed in what is virtually an enlarged hollow tap-stopper a (Fig. 86) at one end of the tube. By keeping these two elements of tube-packing out of contact with each other except when the tube is being used to absorb carbon dioxide, the "life of the tube, before repacking becomes necessary, is much increased. In filling the carbon dioxide absorption tube S of this type (Fig 86), the hollow stopper a, having a small hole at the lower end, is charged in turn with a glass-wool plug b, perchlorate c, and a second glass-wool plug d, and the glass stopper e is then fixed into a with warm glass cement. The absorption stopper a is carefully greased at the neck T, and at the lower end opposite S, and then fitted into the main absorption tube S like an ordinary tap the unit now constitutes the tap T,. The main soda-lime chamber f is then filled from the other end of S with a glass-wool plug g, soda-time h, and a second glass-wool plug j, and the greased tap T finally inserted. The presence of perchlorate is necessary as the FiG. 86. oda-lime contains a little moisture which is driven off by the continued passage of oxygen, and this would cause low results for carbon if the water were not prevented from leaving the tube by the "safety" layer of perchlorate.  [c.471]

The tap T2 is now turned off and the apparatus tested for leaks. If no bubbles appear at the end of the capillary X there is no leak between Y and the tap Ti of the bottle W. The best way of observing X is to shake off any bubbles at its end and then to observe the water meniscus inside the capillary. If it continues to move, there is a leak. The tap Ti is now opened and X observed again rapid cessation of flow indicates no leak up to the first tap T3 on the carbon dioxide absorption train. The process is repeated for the other taps, T4, Tg, and Tg, of the absorption tubes water may flow for a few seconds on opening as the Mariotte bottle has then been put in contact with with the whole length of the hot combustion tube up to the tap Tj of the purification train. If any leak is encountered at any stage all the taps should be closed, the faulty tap or tube adjusted, and the process repeated. The tap Tj is then opened and the oxygen stream flowed to pass through the apparatus. If necessary the level on the pressure gauge D is adjusted to 6 cm. by the micro screw-clip E, and then the side-arm Y of the Mariotte bottle lowered until there is a rate of flow of 5 ml./min. If it is found necessary to lower Y more than a few centimetres there is an obstruction in the system. This is nearly alw ays caused by one or more of the taps on the absorption tubes being not quite completely open when opening the taps it is essential to ensure that side-arm and hole in the tap are exactly in line, as the thumb-grip on the top of the tap is not always set exactly at right-angles to the tap-hole. A 5 or 10 ml. measuring cylinder should be used when adjusting the rate of flow so that the value is measured with reasonable accuracy.  [c.478]

At the end of the sweeping out, the tap Ti is first closed, and then the taps T3, T4, Ts and Tj in this order. The tubes R and S are then detached from the beak of the combustion tube, the guard tube V is then detached from them and replaced on the combustion tube beak. The furnace and thermostatic mortar are then switched off and the combustion tube allowed to cool with the tap to the oxygen supply open. The bung J is removed, and the boat withdrawn by means of a piece of rigid copper wire with a small hook in the end that fits into the small hole in the lip at the back of the boat the bung is then replaced and the boat transferred to its block in the desiccator.  [c.481]

There is a tendency for the water produced by the combustion to condense in the narrow neck of the combustion tube, instead of passing right over into the absorption tube. To avoid this, two movable copper hooks Q are mounted on a copper rod, which can slide in and out of a hole cut in the mortar P these may be placed over the beak of the combustion tube and conduct sufficient heat from the mortar to vaporise the water once again so that it is driven over by the oxygen stream into the absorption tube R.  [c.470]

The tap T2 is now turned off and the apparatus tested for leaks. If no bubbles appear at the end of the capillary X there is no leak between Y and the tap T[ of the bottle W. The best way of observing X is to shake off any bubbles at its end and then to observe the water meniscus inside the capillary. If it continues to move, there is a leak. The tap Ti is now opened and X observed again rapid cessation of flow indicates no leak up to the first tap Tj on the carbon dioxide absorption train. The process is repeated for the other taps, T<, Tj, and T5, of the absorption tubes water may flow for a few seconds on opening T5 as the Mariotte bottle has then been put in contact with with the whole length of the hot combustion tube up to the tap Tj of the purification train. If any leak is encountered at any stage all the taps should be closed, the faulty tap or tube adjusted, and the process repeated. The tap T2 is then opened and the oxygen stream flowed to pass through the apparatus. If necessary the level on the pressure gauge D is adjusted to 6 cm. by the micro screw-clip E, and then the side-arm Y of the Mariotte bottle lowered until there is a rate of flow of 5 ml./min. If it is found necessary to lower Y more than a few centimetres there is an obstruction in the system. This is nearly always caused by one or more of the taps on the absorption tubes being not quite completely open when opening the taps it is essential to ensure that side-arm and hole in the tap are exactly in line, as the thumb-grip on the top of the tap is not always set exactly at right-angles to the tap-hole. A 5 or 10 ml. measuring cylinder should be used when adjusting the rate of flow so that the value is measured with reasonable accuracy.  [c.478]

At the end of the sweeping out, the tap Ti is first closed, and then the taps Ts, T4, T5 and T5 in this order. The tubes R and S are then detached from the beak of the combustion tube, the guard tube V is then detached from them and replaced on the combustion tube beak. The furnace and thermostatic mortar are then switched off and the combustion tube allowed to cool with the tap to the oxygen supply open. The bung J is removed, and the boat withdrawn by means of a piece of rigid copper wire with a small hook in the end that fits into the small hole in the lip at the back of the boat the bung is then replaced and the boat transferred to its block in the desiccator.  [c.481]

Octet rule (Section 1 3) When forming compounds atoms gain lose or share electrons so that the number of their va lence electrons is the same as that of the nearest noble gas For the elements carbon nitrogen oxygen and the halo gens this number is 8  [c.1290]

Many shells are horizontally spht to faciUtate refractory repairs so that the nonproductive furnace time required to replace these refractories is minimized (1). Tapered shells are sometimes used to increase the charge capacity or hot metal capacity of existing furnaces. The conventional furnace shell contains a tapping spout to direct the molten contents when the furnace is emptied. More recendy furnaces are being designed for eccentric bottom tapping (EBT) as shown in Figure 1. This design, where the tap hole is contained in the bottom of an extension to the furnace shell, allows the furnace to be completely drained by tilting the furnace only 15° as opposed to 45° for conventional furnaces. This allows a larger portion of the furnace sidewall to be water-cooled, which lowers refractory consumption. It also allows faster tapping and hence lower temperature losses during tapping. A water-cooled door is located diametrically across from the tap hole for the addition of alloys, duxes, oxygen injection, etc, and allows the removal of slag. Additional openings are used on some furnaces to faciUtate gunning, ie, spraying granular refractory material to rebuild the eroded lining oxygen injection or the introduction of oxy-fuel burners as a supplementary heat source.  [c.121]

Fine and Hollow Fibers. Controlling and designing the geometry, fineness or denier, and porosities of fibers (and occasionaUy of yams) have led to novel and high technology textile products for diverse appHcations. HoUow fibers derived from regenerated ceUulose or from synthetic fibers have been used in the development of artificial body organs such as the kidney, pancreas, and lung (see Hollow-FIBERMEMBRANEs). HoUow fibers have also frequently been employed to increase the insulation value of garments due to the benefits of the air trapped inside the fiber cavity. A variety of ultrafine fibers, ranging in tex (denier) from as Httle as 0.0011 (0.01) up to 0.011 (0.1) have been commercialized (primarily in Japan) to impart various surface characteristics that change fabric hand and appearance. Because spinning ultrafine fibers directly is not technically feasible, such fibers are produced by spinning bicomponent or biconstituent polymer mixtures, highly stretching them to form ultrafine deniers, and extracting or otherwise removing the  [c.70]

Other uses of oxyacetylene flames in mill operations are in building up or hardfacing metal, lancing (piercing a hole in a metal mass), and a variety of metal cleaning procedures. A minor but interesting fuel use of acetylene is in flame spectrophotometry where oxygen and nitrous oxide are used as oxidants in procedures for a wide variety of the elements.  [c.394]

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation.  [c.75]

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54).  [c.404]

The band at 280 nm has been shown to be a germanium center (207). Nonbonding oxygens having negative charge can act as hole traps in irradiated siUca. These oxygen-associated hole centers (OHCs) have been associated with an absorption at 163 nm. Dry OHCs, observed in vitreous siUcas having low hydroxyl content, are peroxy-radical defects (212).  [c.511]

Early processes used three-phase a-c, but increasingly the movement is to a single d-c electrode with a conducting hearth. The high power densities and intense temperatures necessitate both water cooling and improved basic refractory linings. Scrap is charged into the furnace, which usually contains some of the last heat as a Hquid heel to improve efficiency. Oxygen is blown to speed the reactions, which do not differ in any significant way from basic-oxygen furnace (BOF) steelmaking. Oxy-fuel burners are also used to accelerate scrap heating and reduce electricity consumption. Various techniques are appHed to ensure adequate separation of melt and slag to permit effective ladle treatment. A fairly common practice is to use eccentric bottom tapping (Fig. 1), which minimizes vortexing.  [c.374]

Another method to prepare 19-norsteroids is first to oxidize the C19 angular methyl substituent, followed by reductive decarboxylation or decarbonylation of the resultant C19 lactone, carboxylic acid, or aldehyde. All methods of oxidation of angular methyl groups proceed through high energy intermediates capable of oxidizing unactivated CH bonds. These high energy intermediates are generated from an intramolecular heteroatom in close proximity to the angular methyl group. Practical routes to 19-norsteroids are shown in Figure 13. The addition of hypohalous acid to A -steroids (95) gives 5a-halo-6P-carbinols (96). Cyclization of this 6P-alcohol onto the C19 angular methyl substituent can occur under a variety of different conditions. For example, ether (97) is produced by the treatment of (96) with lead tetraacetate under thermal or photolytic conditions. In addition, treatment of (96) with lead tetraacetate and iodine proceeds though an intermediate hypoiodite (138) that, after homolytic decomposition, results in ether (97) (139). Ether (97) can be either oxidized to lactone (98) or the C3-acetate hydrolyzed and oxidized to the C3-ketone (99). Elimination of the halogen of (99) followed by C6-oxygen reduction yields the C19 alcohol (100). Oxidation of (100) to the aldehyde or carboxylic acid (101), followed by decarbonylation or decarboxylation, respectively, results in the 19-norsteroid (102). Alternatively, acetate hydrolysis, C3-oxidation, and elimination of (98) forms lactone (103). Concomitant C6-reduction and decarboxylation of (103) yield the 19-norsteroid (102) (140). In a similar process, the C19 and C18 angular methyl groups can be oxidized by photolytic activation of a nearly nitrite ester (141). A free-radical activation of the C18-angular methyl moiety has been exploited in a number of synthetic approaches to aldosterone (142,143).  [c.429]

Figure 11.9 A diagram of the active site of chymotrypsin with a bound inhibitor, Ac-Pro-Ala-Pro-Tyr-COOH. The diagram illustrates how this inhibitor binds in relation to the catalytic triad, the strbstrate specificity pocket, the oxyanion hole and the nonspecific substrate binding region. The Inhibitor is ted. Hydrogen bonds between Inhibitor and enzyme are striped. (Adapted from M.N.G. James et al., /. Mol. Biol. 144 43-88, 1980.) Figure 11.9 A diagram of the active site of chymotrypsin with a bound inhibitor, Ac-Pro-Ala-Pro-Tyr-COOH. The diagram illustrates how this inhibitor binds in relation to the catalytic triad, the strbstrate specificity pocket, the oxyanion hole and the nonspecific substrate binding region. The Inhibitor is ted. Hydrogen bonds between Inhibitor and enzyme are striped. (Adapted from M.N.G. James et al., /. Mol. Biol. 144 43-88, 1980.)
Two essential features are required to stabilize the covalent tetrahedral transition state in serine proteinases—the oxyanion hole, which provides hydrogen bonds to the negatively charged oxygen atom in the transition state, and the histidine residue of the catalytic triad, which provides a positive charge. The charge on this histidine is, in turn, stabilized by the aspartic acid side chain of the catalytic triad (Figure 11.6). The histidine residue also plays a second role in the catalytic mechanism by accepting a proton from the reactive serine residue and then donating that proton to the nitrogen atom of the leaving group. The effects on the catalytic rate of the different side chains involved in the catalytic triad and the oxyanion hole have been examined by P. Carter, J.A. Wells, and D. Estell at Genentech, USA, by analyses of mutants of subtilisin with one or several of these side chains have been changed.  [c.217]

Fig. VIII-13. LEED and ESDIAD on clean and oxygen-dosed Ni(lll) (a) LEED, elean surface (b) H ESDIAD of NH3 on Ni(Ill), the halo suggesting free rotation of the surface NH3 groups (c) ESDIAD after predosing with oxygen, then heated to 600 K and cooled before dosing with NH3—only well-ordered chemisorbed NH3 is now present. (From Ref. 93.) Fig. VIII-13. LEED and ESDIAD on clean and oxygen-dosed Ni(lll) (a) LEED, elean surface (b) H ESDIAD of NH3 on Ni(Ill), the halo suggesting free rotation of the surface NH3 groups (c) ESDIAD after predosing with oxygen, then heated to 600 K and cooled before dosing with NH3—only well-ordered chemisorbed NH3 is now present. (From Ref. 93.)
Auger peaks are labelled according to the x-ray level nomenclature. For example, KLjL2 stands for a transition in which the initial core hole in the K shell is filled from the shell, while the Auger electron is emitted from the L2 shell. Valence levels are indicated by V as in the KVV transitions of carbon or oxygen. The energy of an Auger electron fonned m a KLM transition is to a good approximation given by  [c.1858]

Outside Vapor Deposition. The outside vapor deposition (OVD) process developed by Coming Glass Works (17), is depicted in Figure 11. Soot is deposited layer by layer on a rotating mandrel at a temperature such that the soot particles are partially sintered. The precursor chemicals are the same as those used in the MCVD process but are oxidized by a gas—oxygen torch by similar chemical reactions. A doped core is deposited first foUowed by a Si02 cladding, The mandrel is removed and the porous preform consoHdated at 1500—1600°C in a furnace with a controUed atmosphere containing helium, oxygen, and chlorine. The central hole may be coUapsed either during sintering or fiber drawing, eliminating the need for a substrate tube. AdditionaUy, dopants such as titania, Ti02, may be added to the outer layers of the boule to improve fatigue resistance of the fiber drawn from it.  [c.255]

The performance characteristics of the membranes available in the 1990s are shown in Figure 33. Hollow-fine fiber membranes made by Dow and Du Pont have relatively low fluxes, but because large membrane areas can be made so economically in hoUow-ftber form, these membranes can stUl compete. The highest flux, highest performance membrane is the cross-linked polyether membrane made by Toray. However, this membrane is unstable to oxidation, and aU free oxygen and chlorine must be removed from the feed water to the membrane, a process that is expensive and subject to failure. As a result, most of the reverse osmosis membrane market is divided between various types of thin-film interfacial composite membranes and ceUulose diacetate Loeb-Sourirajan membranes. CeUulose diacetate membranes stiU retain a fraction of the market because of their greater chemical and mechanical stabUity compared to interfacial composites. This advantage is graduaUy disappearing as improved interfacial composite membranes are developed (73).  [c.80]

Photoinitiated Radiation Cure. Attachment of photopolymerizable organic groups to a polysiloxane backbone provides a convenient means to utilise uv radiation to promote cure (215). Examples of organic groups iuclude mercaptans, acrylates, and oxiranes. Thiols can be added across an olefin usiag an aromatic ketone as the initiator (216,217). Unfortunately, the odors associated with thiols have limited the commercial acceptance of this technology. Sihcones containing pendant acrylate groups have been prepared by reaction of acryhc acid with an epoxy-functional sdoxane or by reaction of a halo-organo-functionahzed sdoxane with acryhc acid (218,219). Cross-linking occurs by a free-radical mechanism and the reaction conditions must be iuert. Viuyl ethers and oxiranes can be cationicady photopolymerized iu the presence of diaryhodonium or sulfonium salts (220—226). This chemistry can be carried out iu the presence of oxygen.  [c.49]


See pages that mention the term Ozone holes : [c.498]    [c.212]    [c.519]    [c.519]    [c.14]    [c.200]    [c.645]    [c.468]    [c.471]    [c.468]    [c.471]    [c.471]    [c.403]    [c.96]   
Fundamentals of air pollution (1994) -- [ c.159 , c.160 ]