Covalent changes

NMR measurements (Crampton and Ghariani, 1968). The resonance due to ring protons which occurs at — 8-8 p.p.m. in the picrate ion is shifted strongly upheld to — 6-1 p.p.m. in concentrated sodium hydroxide solution, indicating a covalency change at both C3 and C5. 

It is interesting that in his original report (1902) Meisenheimer described the preparation from 9-nitroanthracene and methoxide of an adduct which he formulated as 45. Confirmation of this structure has come from NMR measurements which show that the resonance of the proton at CIO shifts from — 8-93 to — 4-93 p.p.m. on complex formation consistent with a covalency change at this ring position (Foster et al., 1967). Again the NMR spectra (Foster et al., 1967 Fendler et al., 1968)... 

The mechanism of action of carboxypeptidase has been subject to analysis in terms of ALPH by Deslongchamps (1983, p. 351). Unfortunately, the balance of evidence is now that the covalency changes so analysed probably do not take place, and that zinc proteinases work by a substantially different mechanism to that assumed by Deslongchamps in his stereoelectronic analysis. 

There is some confusion about the various theoretical and experimental distinctions between the Sjj 1 and SN 2 mechanisms. Since molecularity is the number of molecules necessarily undergoing covalency change during the rate determining step (Ingold, 1969), the unimolecular reaction (SN 1) involves rate determining heterolysis of the R—X bond (kx, Fig. 2) without assistance from nucleophilic attack. This definition is independent of the nature of the first intermediate, which in many cases is probably a contact ion pair rather than a free (i.e. symmetrically solvated) carbocation (see... 

If enzyme inactivation is not due to covalent changes in structure, the native active structure can be reformed in immobilized enzymes by refolding from a random coil state (22,27). In fact, if they are attached to the matrix by multiple points, even thermally inactivated multichain enzymes can be reactivated (27). Although such regeneration steps apparently have not been used commercially, their use should be considered in certain cases, especially since in many cases the inactivation may result from adsorption onto the enzyme matrix of components in the process stream. For example, we have found that immobilized sulfhydryl oxidase activity can be regenerated numerous times following treatment of UHT milk by washing with 4 M urea (28). 

Synthetic Molecular Machines and Polymer/ Monomer Size Switches that Operate Through Dynamic and Non-Dynamic Covalent Changes... 

Abstract The present chapter is focused on how synthetic molecular machines (e.g. shuttles, switches and molecular motors) and size switches (conversions between polymers and their units, i.e., conversions between relatively large and small molecules) can function through covalent changes. Amongst the interesting examples of devices herein presented are molecular motors and size switches based on dynamic covalent chemistry which is an area of constitutional dynamic chemistry. 

Keywords Constitutional changes Covalent changes Dynamic covalent chemistry Molecular machines Molecular motors Molecular switches Polymer/monomer switches Reversible polymers Size switches... 

Molecular Machines that Function Through Covalent Changes. 264... 

Covalent Changes as Intermediate Steps in Motional Dynamics Based on Rotation... 

Dynamic Covalent Changes that Make Linear Molecular Motors Work. 

In the second part of this chapter are presented systems involving reversible and controlled conversions - through covalent changes - between relatively large (polymers) and small (monomers) molecules, which can be considered as size switches. 

The covalent changes discussed in this section arise through successive chemical reactions. These sequences of reactions produce (or are designed to produce) controlled rotation-based molecular motions from a given initial station A11 to a final station Alf. This may be written as A11 —> Ax —> Ay —> Az... —> Alf. In a cyclic process, A11 and Alf are identical Ax, Ay and Az represent intermediates. 

The control of the rotational sense of axial-rotation-based machines can operate through covalent changes in the constitution of the initial station. In the cases discussed hereafter, this type of molecular machine consists of a part of molecule that performs an axial rotation around an axis that is a single covalent bond. 

Constitutional covalent changes are also performed with the aim of controlling the rotational rate. 

One may note that covalent changes that are dealt with in the functioning of this motor are, under the conditions of the reaction, irreversible (kinetically controlled reactions). Thus, the steps of this chemically driven motor do not belong to the field of dynamic covalent chemistry (that is based on covalent changes under equilibrium conditions). 

Control of Motional Features of Molecular Shuttles based on Reversible Covalent Changes. Rotaxanes... 

Other photochromic devices (namely, switches that do not involve rotaxanes) where covalent changes occur are known [90],... 

There are examples of rotaxanes acting as molecular shuttles that work through covalent changes occurring reversibly and in response to external stimuli. 

Dynamic Covalent Changes that Make Linear Molecular Motors Work. Controlling the Sense of Displacement (Walking)... 

Polymer/Monomer Size Switches Switching Between Small and Large Molecular Sizes Through Reversible and Controlled Covalent Changes... 

Once such reversible systems have been identified, it is worth the effort to find conditions for their modulation through external stimuli, ideally in a repetitive way. Examples of reversible systems come from the field of dynamic covalent chemistry [19-24] which involves covalent changes with relatively fast equilibration. 

A system where the conversion between a macrocycle and a polymer is modulated by chemical effectors/stimuli was described by Ulrich and Lehn [58,59]. They set up a reversible effector-controlled constitutional switch between a polymer and a macrocycle (Fig. 15) in a dynamic covalent system. This is a sequential one-pot size-switch or polymerization-degree-switch and it involves covalent changes in the constitution through breaking and formation of covalent bonds of the imine type. 

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