Hydrogen bridge binding

Fig, 5,15 Hydrogen bridge binding between ketones and protein, using acetone as an example... 

Fig. 13.2.10. Azolones lead structure and commercial products. Target interaction sites for the hydrogen-bridge binding (Glu272), and for the point mutation at Gly 143 that causes resistance, are sketched together with the famoxadone formula.

Fig. 1.6. Binding domain to DNA. ERs contain two structures called zinc fingers, typical of proteins that interact with DNA. One zinc atom forms four links of coordination with four cysteine residues of the protein structure, which occupy nearby positions, thus leaving a loop of some 15 to 22 aminoacids. The zinc fingers of the receptor are capable of interacting with specific sequences of DNA, the hormone response elements, with which they establish hydrogen bridges and form stable structures...

FeFe-enzyme - proton or hydrogen substrate binding and also the hydride-proton reaction exclusively occurs at the iron distal to the [4Fe-4S] cluster, suggesting that mononuclear iron complexes might also be viable catalysts. Consequently, Ott and coworkers have synthesized and characterized some stable pentacoordinated Fe(II) complexes with five ligands that nicely mimic the native ones and exhibit an open coordination site [163, 164]. This approach avoids the formation of the less reactive bridging hydrides that are found in the dinuclear complexes [153]. Catalytic H2 formation from weak acids at low overpotentials with promising TOF and catalyst stability could be demonstrated [164]. 

Hydrogen bridges between the beryllium atoms produce a polymeric structure for BeH2, as shown in Fig. 18.6. The localized electron model describes this bonding by assuming that only one electron pair is available to bind each Be—H—Be cluster. This is called a three-center bond, since one electron pair is shared among three atoms. Three-center bonds have also been postulated to explain the bonding in other electron-deficient compounds (compounds where there are fewer electron pairs than bonds), such as the boron hydrides (see Section 18.5). 

Finally, let us summarize the type of binding of volatile flavouring substances to various carbohydrates - as far as it is known (Table 5.1). In essence, this is a matter of reversible physical and physico-chemical binding (adsorption, inclusion complexes, hydrogen bridges), so that, in principle, flavour release takes place in the oral cavity. 

Anhydrous proteins can also bind volatile substances ]6J1], Fig. 5.14 shows that anhydrous zein - unlike soy protein and gelatine - binds propanol. Fig. 5.14 also shows that for each of the three proteins illustrated there is a maximum water content for propanol binding. As already mentioned, here both hydrophobic interactions (see Fig. 5.12) and hydrogen bridges (see Fig. 5.13) are involved in the binding of the alcohols ethanol and propanol, and of the ketone acetone (Fig. 5.15) to the anhydrous proteins [6],... 

Free amino acids can bind with a series of volatile flavouring substances in aqueous media. Ketones and alcohols are reversibly bound by hydrogen bridges to the amino or carboxyl groups of the amino acids (Fig. 5.17), while - as with proteins - some aldehydes react chemically with the amino groups to form Schiff bases (Fig. 5.18). This has been demonstrated for interactions of vanillin with lysine, phenylalanine and cysteine [50],... 

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