Loose complex

Ion pair (Section 11.5) A loose complex between two ions in solution. Ion pairs are implicated as intermediates in S l reactions to account for the partial retention of stereochemistry that is often observed. 

In view of the easy disruption of the nonaldehyde form of the toxin it seems probable that a loose complex in some form of acetal exists between the two hemiacetals (XIV, XV). When this is disrupted under mild conditions prehelminthosporol is formed and remains until converted under more vigorous conditions to helminthosporol, while prehelminthosporal (XIV, XIX) is too unstable and is converted to Helminthosporal. 

In an effort to characterize the species formed in vapors containing mercury and arenes upon irradiation with UV light, the formation of exciplexes between excited mercury atoms and arenes has been investigated theoretically. For benzene, these investigations suggest the formation of an exciplex involving the Hg atom in the state and an 772-bound molecule of benzene. This loose complex (3[Hg(772-arene)] Figure 9) with estimated Flg-C bonds of 2.36A... 

As the reaction temperature increases, the equilibrium constant diminishes, since complex formation is accompanied by heat liberation. Sterically hindered phenols form loose complexes because of the impeding effect of voluminous alkyl substituents in the ortho-position. Hydrogen bonding reduces the activity of phenols, which was first observed in the studies of the effects of cyclohexanol and butanol on the inhibitory activity of a-naphthol in cyclohexane [9]. This phenomenon was investigated in detail with reference to the oxidation of methylethylketone [10]. The k7 values for some inhibitors of the oxidation of ethylbenzene and methylethylketone are given below (333 K) [10,46] ... 

The first step of the reaction of H2 with B1(A7), [p2n2]Zr(p.- n -NNH)(p.-H)Zr[p2n2], is coordination of a hydrogen molecule, which takes place only when H2 approaches from above to the atoms N2, Zr1 and Zr2, shown in Figure 6. This approach leads to the formation of a weakly bound molecular complex (not shown in Figure 6), (H2)B1, which is only 1.2 kcal/mol more stable than reactants, H2 + Bl. This loose complex has a Zr2 -H4 separation of -3.4A and a N2-H4 distance of -2.6A. Such a loosely bound complex would not exist when entropy is considered. Its existence/non-existence has no effect on the sequence of reaction steps described below. 

Although Schardinger did not propose a structure for his crystalline dextrins, he made several observations that can now be attributed to their cyclic structure. For example, he discovered their ability to engage in complex-formation "With various substances, the crystalline dextrins form loose complexes which, like those produced with alcohol, ether, and chloroform, are indeed partly decomposed by water, while the iodine complexes are more stable toward water. He also found, as previously mentioned, that the crystalline dextrins were nonreducing toward copper salts and nonfermentable by yeast. This last observation he considered was "... the most essential thing that I was able to mention concerning the formation of crystalline dextrins by microbes. Both of these observations can be explained by the lack of a chain termination. 

In patients clearance of intravenous heme is rapid until hemopexin levels are depleted (148), and lack of interaction with hemopexin may explain the higher clinical efficacy of heme-arginate compared with hemin itself (149, 150). In intact animals, i.v. heme causes rapid association of hemopexin but not albumin with the liver (47, 63, 68), and heme uptake from heme-albumin complexes into isolated rat hepato-cytes occurs via diffusion of heme released from its loose complex with BSA (137). Moreover, unlike uptake from heme-hemopexin, free heme uptake by cells occurred even at 4°C, as expected for nonspecific membrane association and in total disagreement with a membrane-receptor-mediated or active transport uptake process. 

The essence of this theory on the mechanism of ECF is that reaction occurs via inorganic fluorinating agents generated at the anode these include nascent fluorine, molecular fluorine, or its loose complexes with nickel fluorides, and simple or complex forms of high valence nickel fluorides [62,65,169,178,179]. However, until the precise nature and structure of the active nickel anode surface is characterised the position remains a matter for conjecture. 

Aqueous solutions of SOD from human placenta were treated with y rays at 11 K and annealed at 190 K. An EPR signal observed at 77 K was interpreted as due to a paramagnetic intermediate a loose complex Cu. .. HOi at the active site The binding of HOi (pAT, = 4.9) instead of Oj seems rather improbable in phosphate buffer pH 7.8. The activity of BESOD is, moreover, independent of pH above pH 5.3 The paramagnetic intermediate could correspond to a superoxide radical bound to the axial HjO of Cu (Scheme 4). 

The decay near 200 K of the EPR signal of in the loose complex of BESOD was not accompanied by a reduction of Cu, which occurred only upon annealing at about 270 K. Similar observations were made with BESOD reduced with H Oj. This was considered good evidence for the second mechanism (5) (6) with a constant oxidation state of Cu, Cu or Cu, and a cooperation of the two active centers in the SOD dimer... 

S °( N2H4) = 57.1 while S XCdh) = 52.5. If we add on It In 4 for spin we obtain the difference of about 7 Gibbs/mole. Actually the spin entropy is not needed to account for the transition state entropy of 4 Gibbs/mole if we assume the loose complex. 

When two molecules which are likaly to form loose complexes are mixed together, very often a new featureless band appears (Figure 3.16),... 

Fig. 4. Schematic representation of the observed dynamics. Initially uncomplexed photoacid first forms an encounter complex through diffusion. This loose complex either rearranges to a tight complex, or reacts via the hydrogen bonded network between photoacid and base. A pre-formed photoacid-base complex can directly react with extremely fast rates.

Both of the above approaches employed a metal ion as a template about which the corrin cyclization was performed, but the nickel or cobalt ions could not subsequently be removed. In order to obtain metal-free corrins, a new route was therefore devised (67AG865) which employed the novel principle of sulfide contraction (Scheme 22). Thus the sodium salt of the precorrin (284) (Scheme 23) was transformed into the thiolactam (285), and loose complexation with zinc(II) ions caused cyclization to give (286), which was treated with benzoyl peroxide and acid to give the ring-expanded compound (287). Contraction with TFA/DMF gave the corrins (288) and (289), and the major of these (289) was desulfurized with triphenylphosphine and acid to give (288). Finally, demetallation with TFA gave the required metal-free corrin (290), a source for a whole variety of metal derivatives. 

Substantial production of Hg(3P0) atoms has been seen by Callear and McGurk (180). Continuous emissions that presumably originate from loose complexes between the Hg(3P0) atoms and the hydrocarbons have been observed near 2537 A (938). No HgH has been detected from the (3Pi) atom reactions with saturated hydrocarbons (177). 

Loose complexes of Hg(3P0) with M(H20, NH 3l RH) which emit continua. 

Whether folding is a concerted or stepwise process depends on the stability of individual substructures of the protein when they are considered in isolation or loosely complexed with one another. For example, because the a helix and jS sheet of 02 are unstable without the rest of the protein, they are formed in a... 

The four electrostatic interactions which involve the dipole moments and the polarizabilities of the solute and solvent molecules are termed non-specific because they do not imply a fixed geometry or stoichiometry of the molecules. In a specific association the molecules form a loose complex, of which hydrogen bonding is the most important example. Hydrogen bonding takes place between a protic (proton donor or acid) molecule such as water and a proton acceptor (base) such as an amine (Figure 3.48). 

The selectivity of chlorination reactions carried on in solution is increased markedly in the presence of benzene or alkyl-substituted benzenes because benzene and other arenes form loose complexes with chlorine atoms. This substantially cuts down chlorine-atom reactivity, thereby making the chlorine atoms behave more like bromine atoms. 

There is a further aspect of polar additions to alkenes that we should consider, namely, that electrophilic reagents form loose complexes with the 77 electrons of the double bonds of alkenes prior to reaction by addition. Complexes of this type are called charge-transfer complexes (or ir complexes). Formation of a complex between iodine and cyclohexene is demonstrated by the fact that iodine dissolves in cyclohexene to give a brown solution, whereas its solutions in cyclohexane are violet. The brown solution of iodine in cyclohexene slowly fades as addition occurs to give colorless trims-1,2-diiodocyclohexane. 

We envisage that any alteration in the active site would be accompanied by some modification of the tertiary structure of the enzymes (Cx), which, although possibly only very minor, may be sufficient to prevent the cooperation of Cx with Cx in a loose complex on the surface of the cellulose crystallite. Equally well, of course, there may be other differences between the various insoluble growth substrates that are important determinants of the structure of the active site. These differences could be either at the molecular or at the supermolecular level. 

A "loose-complex hypothesis is highly speculative, but it is one that could fit the facts that have been established. It is compatible with the fact that the relative proportions of Cx to C i are important for maximum rate of solubilization (16). It could also explain the variation in the synergism between C i of one fungus and Cx of another, or the variation... 

NH4 itself has been less thoroughly studied, but Pople et al. showed that the tetrahedral structure is not a stable minimum but a saddle point.57 Only a very loose complex is predicted for this system, stable by only ca. 3.4 kJ mol-1. 

Figure 7 Qualitative depiction of the energy profile along the reaction co-ordinate for the Sn2 reaction Cl ICII3CI >CICn3 I Cl, which involves nucleophilic substitution of the chloride of methylchloride by a chloride ion. The potential energy curves drop as the two reactants approach until a loose complex is formed. Then the energy rises rapidly to the transition state, which has two equal C-Cl interatomic distances (zero on the abscissa). The energy profile looks quite different in the gas and solution phases. Compared to the reactants (or products), the loose complex and the TS are poorly solvated, so the energies for these are much higher in solution than in a vacuum.

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