Noncovalent Species

Table 5.1 summarizes the most common and often commercially available ionization methods together with brief comments regarding their utility for studies in supramolecular chemistry. Many of them are incompatible with noncovalent bonds because they are intrinsically harsh, and dissociate the weakly-bound ions of interest. In the following, we focus on those which can be used for the examination of noncovalent species. 

Ionization method Ionization principle Utility for the examination of noncovalent species... 

Usually, a rapid binding step of the inhibitor I to the enzyme E leads to the formation of the initial noncovalent enzyme-inhibitor complex E-I. This is usually followed by a rate determining catalytic step, leading to the formation of a highly reactive species [E—I ]. This species can either undergo reaction with an active site amino acid residue of the enzyme to form the covalent enzyme-inhibitor adduct E—I", or be released into the medium to form product P and free active enzyme E. 

Suitable for fragile compounds (noncovalent complexes and thermally labile species)... 

Kinetically stable rotaxane-like species have been synthesized without the aid of strong noncovalent bonds previously (I. T. Harrison, Preparation of Rotaxanes by the Statistical Method , J. Chem. Soc Perkin Trans. 11974, 301-304). However, the yields reported were veiy low. 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

Noncovalent interactions play a special role in synthetic procedures used to assemble various types of supramolecular species. These syntheses rely on the stabilization provided by non-covalent interactions between recognition sites incorporated within precursors. Various types of non-covalent interactions can be used as a recognition motif utilized to guide the synthesis.Targeted synthesis of macro- and supramolecular structures of various sizes, shapes, and functionality has now become possible. Supramolecular chemistry offers incredible applications in various fields such as medicinal chemistry (drug delivery systems),host-guest chemistry,catalysis,and molecular electronics. ... 

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