Chain transition

Transcription (Section 28 11) Construction of a strand of mRNA complementary to a DNA template Transfer RNA (tRNA) (Section 28 11) A polynucleotide of n hose that is bound at one end to a unique amino acid This ammo acid is incorporated into a growing peptide chain Transition state (Section 3 1) The point of maximum energy in an elementary step of a reaction mechanism Translation (Section 28 12) The reading of mRNA by van ous tRNAs each one of which is unique for a particular ammo acid... 

An open-chain transition state model, based on the improved Cram model (Section A.2.), was proposed for the prediction of the stereochemical outcome1 2. Under kinetic control, if the substituent R1 in the benzylic position is of medium size, the syn-isomer is formed as the major product2. 

It is assumed, that the carbonyl addition takes place in an open-chain transition state, in which the ( >enolate is attacked from the unshielded face. 

In Lewis acid catalyzed carbonyl additions of allylsilanes and -stannanes, syn diastereoselectiv-ity predominates, irrespective of the double-bond configuration, indicating that open-chain transition states are involved. 

Besides the classical additions of carbon-centered nucleophiles to the electrophilic sites of the cumulenic chain, transition-metal allenyhdenes are able to promote... 

An alternative mechanism for the enzymatic hydrolysis was suggested by Post and Karplus,92 and also by Fleet,93 that invoked protonation of the endocyclic 0-5 instead of the anomeric 0-1, thus leading to an open-chain transition state, as was also discussed in Franck s interesting account.94... 

It will now be instructive to examine the n-butane reaction (76). In this case the reaction follows almost exclusively a single path leading to the formation of sec-butyl radicals. The percentage of the quenching done by the two methylene groups is very nearly the same as that for the tertiary C-H bond in isobutane (i.e. >90%). However, the primary yield of w-butyl radicals ( 2%) from w-butane is decidedly less than that for isobutyl radicals ( " 14%) from isobutane. This behavior can be readily interpreted on the basis of a cyclic transition-state structure, but not with an open-chain transition state. For the two reaction sequences, we may write ... 

This reaction has also been applied to enantio- and diastereoselective synthesis using trans-crotyltrichlorosilane as substrate [180]. Once again, the highest enantioselectivity was observed for electron-poor benzaldehydes. Interestingly, however, diastereoselectivity was lower than for benzaldehyde. One explanation is that for catalyst 179 the preferred cyclic transition state leading to the major anti products is less favored as arene-arene interactions become stronger (Scheme 6.80) [180]. Stronger arene-arene interactions result in increased participation of the open-chain transition state, and reduced diastereoselectivity. 

The preparation of low-molecular polydimethylsiloxane elastomers often requires a considerable quantity of alkali, which calls for additional neutralisation. That is why in some cases it is advisable to use special substances (so-called chain growth regulators), which can take part in the reaction of chain transition. The part of regulators can be played by oli-gomethylsiloxanes, which do not have active end groups. Below we give the consumption of some regulators (g, per 1 tonne of polydimethylsiloxane) ... 

Fig. 12 Non-chain transition metal-catalyzed radical reactions with internal or external catalyst regeneration (For b an analogous oxidative process is possible - not shown), (a) Metal-catalyzed non-chain radical reactions by single- and two-electron transfers, (b) Metal-catalyzed radical reactions using a sacrificial reducing agent...

Suppose that a compound contains two chromophoric groups that exhibit electronic bands at va and vb as shown in Fig. 1-29. Then, vibrations of chromophore A are resonance-enhanced when v0 is chosen near vA, and those of chromophore B are resonance-enhanced when vo is chosen near vB. For example, heme proteins such as hemoglobin and cytochromes (Chapter 6, Section 6.1) exhibit porphyrin core n-n transitions in the 400-600 nm region and peptide chain transitions below 250 nm. Thus, the porphyrin core and peptide chain vibrations can be selectively enhanced by choosing exciting lines, in the regions of these electronic absorptions. 

Differential scanning calorimetry (DSC) Lipid chain transition temperature, Tm ... 

The prevalence of diastereomer 303B is interpreted on the basis of a competition between cyclic and open-chain transition states. ... 

Poly(NIPAM-co-MAA) with MAA content above 5 mol% is soluble under both physiological pH (7.4) and temperature (37°C), when its carboxylic functions are ionized. However, when the pH is decreased to the pH of late endosomes and lysosomes ( pH = 5-5.5) (1), MAA protonation triggers dehydration of the polymer chains. If the polymer is anchored to a phospholipid membrane through randomly or terminally incorporated alkyl chains, transition, to a globule conformation induces reorganization of the bilayer that leads to massive content leakage (3). 

Effects of urea and guanidine HC1 on adrenodoxin have also been observed (308). The kinetics of denaturation is faster for adrenodoxin than for spinach ferredoxin, which was indicative of a more stable structure for the plant protein (309). The spectropolarimetric data was interpreted to suggest either that the strong visible region Cotton effects are intrinsic to the iron-sulfur site or that the optical activity arises by a coupling of the active site chromophore to aromatic side chain transitions in the ultraviolet rather than to a backbone a-helix (308, 309). 

Synthesis of Linear Chain Transition Metal Complexes (Abstract only) 627... 

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