Secondary complex

Now ku < 0.8 X 109 sec.-1, only slightly smaller than the upper limit 9 < 1.1 X 109 sec.-1 Apparently the unimolecular dissociation rate constants of all secondary complexes are less than ca. 5 X 107 sec.-1, those of the tertiary complexes less than 109 sec.-1, and those of the quaternary complexes probably of the order of 1010 sec.-1 These conclusions substantiate the view 16) that the mass spectrometrically observed tertiary ions arise predominantly from dissociation of the intermediate addition complexes C6Hi2+, C6Hn+, and C6Hi0+. Higher order ions, however, should arise principally from reactions of the dissociation products of the above complexes 62). 

The hydrogenation reaction is carried out with a substituted cinnamic acid. The acetamido group is of particular importance because it functions as a secondary complexation function in addition to the alkene functionality. In the first step the alkene co-ordinates to the cationic rhodium species (containing an enantiopure phosphine DIPAMP in Figures 4.4 and 4.5 with the chirality at phosphorus carrying three different substituents, Ph, o-An, CH2) for which there are several diasteromeric structures due to ... 

Primary and secondary silver(I) complexes have been readily prepared by extraction of silver nitrate into a chloroform solution of dithizone. For the secondary complex an excess of silver nitrate was necessary.445 IR spectral data for both complexes are recorded in Table 59.446 The primary complex Ag(HDz) has been reported in hydrated445 and anhydrous446 orange-red forms. The secondary complex Ag2Dz was an anhydrous brown solid. 

In addition to the organic-extractable primary complexes dithizone also forms secondary complexes such as Ag2Dz, Cu 2Dz, HgDz which are very sparingly soluble in organic solvents and of as yet undetermined structure they are of no value in analysis.8... 

Having learned this, Dupont workers [52] have added a temporary auxiliary donor atom to an unsaturated substrate in order to be able to steer adduct formation, and so the enantioselectivity of the hydrogenation. For example, asymmetric hydrogenation of imines or ketones was a reaction that yielded rather low enantiomeric excesses. However, by converting these first into acyl hydrazones the hydrazide oxygen can function as the secondary complexation function and now extremely high enantiomeric excesses can be obtained (Fig. 6.23). 

The quantity Asp/aM i" may be regarded as a conditional solubility product, which is a function of the pH and of the reagent concentration. Equation (22-15) is also useful when one wishes to take into account the effect of hydrolysis or of masking agents that may be present. It is necessary only to make the appropriate calculation of the fraction from the hydrolysis constants [Equation (7-17)] or from the formation constants of the secondary complexes [Equation (11-16)]. 

All experimental points of curve P(t) which do not verify the condition (5) belong to the penetration thresholds of order > 3 and the pore network corresponding to a secondary complexity. The algorithm can be extended to the penetration thresholds of order n. So, in interval ] between two consecutive penetration thresholds of order n-1, we define... 

B. Chance, Enzyme-substrate compounds of horseradish peroxidase and peroxides. II. Kinetics of formation and decomposition of the primary and secondary complexes, Arch. Biochem. Biophys. 22 (1949) 224. 

However, lipophilic d,l-HMPAO is easily transformed into a charged complex, which cannot pass the BBB. Once inside the brain, this secondary complex is trapped and is released very slowly (Neirinckx et al. 1987). The Tc-HMPAO complex is also used for labeling leukocytes with technetium. 

The lipophilic Tc-d,l-HMPAO complex is decomposed rapidly in vivo, both in the blood and in the brain. Due to this instability, the secondary " Tc-d,l-HMPAO complex, a charged complex, is formed (Neirinckx et al. 1988). The secondary complex cannot pass the BBB and is trapped inside the brain and in blood cells, i.e., the ionized molecule is trapped. 

The value of fc2, the velocity constant for the decomposition of the primary complex into peroxidase and peroxide cannot be obtained by direct measurement. Measurements of the equilibrium constant for the dissociation of the primary and secondary complexes with methyl hydroperoxide gave values of 3.2 X 10-6 M. and 3 X 10-7 M. from which fc2 may be estimated to be 2.2 sec.-1 or 3.4 sec.-1 from the known formation velocity constants. 

Like peroxidase, catalase forms both primary and secondary complexes with methyl and ethyl hydroperoxide (Chance, 73). The primary complexes are green having a diffuse absorption band in the red starting at 670 mju. The secondary complexes are red and have absorption maxima in the visible region at 572 and 536 mu. The catalase-ethyl hydroperoxide complex found by Stern (74) had maxima at approximately these wavelengths and was thus the secondary complex. The Soret bands of the primary complexes are similar in shape to that of the free enzyme but are shifted toward the red by several millimicrons. At... 

A property all the primary complexes have in common is the decomposition giving free catalase which does follow first order kinetics (Chance, 71). The velocity constants for ethyl and methyl hydroperoxides are 0.04 and 0.016 sec.-1 as compared with 0.02 sec.-1 for the hydrogen peroxide complex. The secondary complexes decompose far more slowly, the first order velocity constants for ethyl and methyl hydroperoxides having the values 2.3 X 10-4 and 4 X 10-6 sec.-1, respectively. 

Chance (78) has discussed this experimental data in terms of the extended Michaelis theory which accounts for the similar peroxidatic action of peroxidase, the only difference being that with peroxidase the main reaction can proceed via the secondary complexes, whereas with catalase these complexes are inactive and the main reaction proceeds via the primary complexes. Representing the primary complexes by FeOOH and FeOOR he suggests the various reactions are ... 

The conversion of the green primary complex into the pale red secondary complex appears to be a reduction process even though it occurs in the absence of any added hydrogen donors. The most definite evidence for this is the case of peroxidase where the speed of the conversion is increased in the presence of all compounds with which the peroxide system reacts (Chance, 55). For catalase, where the conversion can only be obtained with alkyl hydroperoxides, the evidence is not so clear-cut, but at least the velocity of formation of the secondary complexes increases as the hydroperoxide concentration is increased. An alternative explanation for these effects would be that the primary and secondary complexes are in some sort of equilibrium where removal of the latter would have the effect of increasing the rate of conversion. There is no indication of any such equilibrium, however, and direct reduction of the primary complex appears to be the most likely explanation. One possible formulation for this change involves the production of a ferryl ion type of compound by the removal of an OH radical by the hydrogen donor from the 02H anion bound to the iron atom ... 

Prot Fe OOH + AHs -> Prot FepO + H20 + AH-(green primary complex) (pale red secondary complex)... 

In a search for other redox systems capable of reacting with either the free enzyme or the secondary complex it was found that chloriridite ions reduce both secondary complexes whereas ferrous tris-dipyridyl ions or ferrous tria-orlho phenanthroline ions do not. This indicates that the redox potential for the reaction... 

These experiments suggest that the secondary complexes should no longer be regarded as Michaelis-Menten enzyme substrate complexes but as reaction intermediates in the same sense that free radicals and semi-quinones are reaction intermediates, for all three classes of compounds provide a path for stepwise reactions. As a consequence the accepted mechanism for peroxidase action needs revision. 

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