Double extended

Figure 1.50 Structures of two main polysaccharides with ordered secondary structures (hollow helices, A-form). Carrageenan has the repeat unit structure shown (a) and double extended hollow helical secondary structure (b) with both separate strands rendered in the stick bond representation (yellow and red) (pdb Icar). Agarose has the alternative repeat structure shown (c) and double extended hollow helical secondary structure (d) with both separate strands rendered in the ball and stick representation (green and red) (pdb laga).

FIGURE 8.11 Modeled structures of hyperextended molecular capsule. Top Double extended with 8 spacer molecules. Bottom Triple extended with 12 spacer molecules. Peripheral groups have been removed for viewing clarity. 

This produces a double layer of charge, one localized on the surface of the plane and the other developed in a diffuse region extending into solution. 

There are several requirements for this to be a suitable deteetion method for a given moleeule. Obviously, tire moleeule must have a transition to a bound, exeited eleetronie state whose wavelength ean be reaehed with tunable laser radiation, and the band system must have been previously speetroseopioally assigned. If the moleeules are fonned with eonsiderable vibrational exeitation, the available speetroseopie data may not extend up to these vibrational levels. Transitions in the visible ean be aeeessed direetly by the output of a tunable dye laser, while transitions in the ultraviolet ean be reaehed by Ifequeney-doubled radiation. The... 

In this section, the adiabatic picture will be extended to include the non-adiabatic terais that couple the states. After this has been done, a diabatic picture will be developed that enables the basic topology of the coupled surfaces to be investigated. Of particular interest are the intersection regions, which may form what are called conical intersections. These are a multimode phenomena, that is, they do not occur in ID systems, and the name comes from their shape— in a special 2D space it has the fomi of a double cone. Finally, a model Flamiltonian will be introduced that can describe the coupled surfaces. This enables a global description of the surfaces, and gives both insight and predictive power to the fomration of conical intersections. More detailed review on conical intersections and their properties can be found in [1,14,65,176-178]. 

Method 1. Fit a 500-ml. three-necked flask with a thermometer (the bulb of which is within 2 cm. of the bottom), and inlet tube extending to the bottom of the flask, and a double surface condenser. Use old ... 

Into a 1-litre three-necked flask, equipped with a reflux (double surface) condenser, a mechanical stirrer (preferably of the Hershberg type. Fig. II, 7, 8) and a gas lead-in tube extending to near the bottom of the flask, place 200 g. (227 ml.) of dry benzene, 20 g. of paraformaldehyde (1) and 20 g. of finely-pulverised, anhydrous zinc chloride. Support the flask on a water bath so arranged that the level of the water in it is about... 

We can gam a general understanding of the mechanism of hydrogen halide addi tion to alkenes by extending some of the principles of reaction mechanisms introduced earlier In Section 5 12 we pointed out that carbocations are the conjugate acids of alkenes Therefore strong acids such as HCI HBr and HI can protonate the double bond of an alkene to form a carbocation... 

FIGURE 10 5 (a) Isolated double bonds are separated from one another by one or more sp hybridized carbons and cannot overlap to give an extended it orbital (b) In a conjugated di ene overlap of two it orbitals gives an extended it system encompassing four carbon atoms... 

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

Prostaglandins arise from unsaturated C20 carboxylic acids such as arachidonic acid (see Table 26 1) Mammals cannot biosynthesize arachidonic acid directly They obtain Imoleic acid (Table 26 1) from vegetable oils m their diet and extend the car bon chain of Imoleic acid from 18 to 20 carbons while introducing two more double bonds Lmoleic acid is said to be an essential fatty acid, forming part of the dietary requirement of mammals Animals fed on diets that are deficient m Imoleic acid grow poorly and suffer a number of other disorders some of which are reversed on feed mg them vegetable oils rich m Imoleic acid and other polyunsaturated fatty acids One function of these substances is to provide the raw materials for prostaglandin biosynthesis... 

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

We have so far described the structure of DNA as an extended double helix The crys tallographic evidence that gave rise to this picture was obtained on a sample of DNA removed from the cell that contained it Within a cell—its native state—DNA almost always adopts some shape other than an extended chain We can understand why by doing a little arithmetic Each helix of B DNA makes a complete turn every 3 4 X 10 m and there are about 10 base parrs per turn A typical human DNA contains 10 base parrs Therefore... 

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