Enolate peptide

Protons that could logically be involved in a membrane Bohr effect are those present on imidazole rings coordinated to Fe or Cu in redox proteins. Removal of an electron from the metal ion could be accompanied by displacement of electrons within the imidazole, within a peptide group that is hydrogen-bonded to an imidazole, or within some other acidic group. A hypothetical example is illustrated in Eq. 18-12 in which a carboxyl group loses a proton when "handed" a second. If the transiently enolized peptide linkage formed in... 

The smooth conversion of the enol acetate (151) into an A -acyl derivative (152) under extremely mild conditions points to the high acylating capacity of these esters. This cleavage of isoxazolium salts is also caused by other anions of carboxylic acids, and thus they can be readily converted to reactive enol esters. A very convenient and specific synthesis of peptides due to Woodward et is based on... 

Fig. 3 Protonation states, isomerism and mesomerism of the HBI chromophore (p-hydroxybenzi-lidene-imidazolinone). The chromophore is shown in its most stable Z ( cw ) conformation, conventionally associated to a 0° value of the dihedral angle t, while the E ( trans ) conformation corresponds to t = 180°. For model compound HBDI (4 -hydroxy-benzylidene-2,3-dimethyl-imidazolinone), Ri = R2 = CH3, for chromophore in GFP, Ri, and R2 stand for the peptidic main chains toward N-terminus and C-terminus, respectively, (a) Possible protonation states of HBI (a) neutral, (b) anionic, (c) enolic, (d) cationic, and (e) zwitterionic. (b) Two resonance structures of the anionic form of HBI...

BASE-CATALYZED ENOLIZATION OF OXAZOLONES FORMED FROM ACTIVATED PEPTIDES... 

Fig. 6.25. Simplified mechanism of two degradation reactions between peptides and reducing sugars occurring in solids, a) Maillard reaction between a side-chain amino (or amido) group showing the formation of an imine (Reaction a), followed by tautomerization to an enol (Reaction b) and ultimately to a ketone (Reaction c). Reaction c is known as the Amadori rearrangement (modified from [8]). b) Postulated mechanism of the reaction between a reducing sugar and a C-terminal serine. The postulated nucleophilic addition yields an hemiacetal (Reaction a) and is followed by cyclization (intramolecular condensation Reaction b). Two subsequent hydrolytic steps (Reactions c and d) yield a serine-sugar conjugate and the des-Ser-peptide...

It has been shown that the Claisen rearrangement of lithium enolates of amino acid enynol esters allows the synthesis of very sensitive y, 5-unsaturated amino acids with conjugated enyne side chains.The chelate-enolate Claisen rearrangement has also been applied to the synthesis of unsaturated polyhydroxylated amino acids, polyhydroxylated piperidines, and unsaturated peptides. ... 

Note that we can write a similar resonance picture for esters, and we shall actually need to invoke this when we discuss enolate anions (see Section 10.7). However, electron donation from oxygen is not as effective as from the less electronegative nitrogen. We shall also see that this resonance effect in amides has other consequences, such as increased acidity of the amide hydrogens (see Section 10.7) and stereochemical aspects of peptides and proteins (see Section 13.3). In addition, the amide derivatives have... 

The enzymic processes appear exactly equivalent, except that protons are removed and supplied through the involvement of peptide side-chains. It is unlikely that a distinct enolate anion is formed instead, we should consider the process as concerted with a smooth flow of electrons. Thus, as a basic group removes a proton from one part of the molecule, an acidic group supplies a proton at another. 

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