Complexes covalent bonds

On the basis of these results De Schryver and co-workers reformulated Hirayama s rule as follows the possibility of intramolecular complex formation is limited by the probability to reach, within the lifetime of the excited state involved, a favorable conformation and by the extent of stabilization in the complex, covalent bond formations being the extreme. 

During maturation or aging of the living animal, collagen fibers strengthen and are further stabilized primarily by complex covalent bonds. Lysine, hydroxylysine, and histidine residues are heavily involved in the formation of these covalent bonds, that is, aldimine bonds between lysine and lysine or hydroxylysine (Balian and Bowes, 1977 Belitz et ah, 2004 Engel and Bachinger, 2005 Eyre and Wu, 2005 Nelson and Cox,... 

The more complex covalent bond, which occurs by the overlap of electronic wave functions, is discussed in Secs. 2.6 and 2.7. In Sec. 2.8, how the interaction of wave functions of more than one atom results in the formation of energy bands in crystalline solids is elucidated. 

Figure 19 Lanthanide HQ complexes covalently bonded to the SBA-15 mesoporous host LnQs -SBA-15 (Ln = Er, Nd, Yb). (Reprinted with permission from Ref. 127. Copyright (2008) American Chemical Society.)...

High-frequency shoulders on Vs(CO) and Vas(CO) bands in the FTIR spectra of an organometallic probe derived from tricarbonyl(l-4- n -5-pyridinocyclohexa-l,3-diene)iron hexafluorophosphate have been observed io on incubation with a-chymotrypsin and were assigned to an iron-complex covalently bonded to enzyme-NH2 groups. Curve-fitting analysis of the spectra enabled the bound complex to be detected even in the presence of the unbound probe and its FTIR spectrum to be calculated and compared with spectral data arising from similar interactions between the tricarbonyliron moiety and twelve aminoacids and polylysine. 

The theory predicts high stabilities for hard acid - hard base complexes, mainly resulting from electrostatic interactions and for soft acid - soft base complexes, where covalent bonding is also important Hard acid - soft base and hard base - soft acid complexes usually have low stability. Unfortunately, in a quantitative sense, the predictive value of the HSAB theory is limited. Thermodynamic analysis clearly shows a difference between hard-hard interactions and soft-soft interactions. In water hard-hard interactions are usually endothermic and occur only as a result of a gain in entropy, originating from a liberation of water molecules from the hydration shells of the... 

This class of inhibitors usually acts irreversibly by permanently blocking the active site of an enzyme upon covalent bond formation with an amino acid residue. Very tight-binding, noncovalent inhibitors often also act in an irreversible fashion with half-Hves of the enzyme-inhibitor complex on the order of days or weeks. At these limits, distinction between covalent and noncovalent becomes functionally irrelevant. The mode of inactivation of this class of inhibitors can be divided into two phases the inhibitors first bind to the enzyme in a noncovalent fashion, and then undergo subsequent covalent bond formation. 

The often fast binding step of the inhibitor I to the enzyme E, forming the enzyme inhibitor complex E-I, is followed by a rate-determining inactivation step to form a covalent bond. The evaluation of affinity labels is based on the fulfillment of the following criteria (/) irreversible, active site-directed inactivation of the enzyme upon the formation of a stable covalent linkage with the activated form of the inhibitor, (2) time- and concentration-dependent inactivation showing saturation kinetics, and (3) a binding stoichiometry of 1 1 of inhibitor to the enzyme s active site (34). 

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