Stability with amino acid

In acidic solution, the degradation results in the formation of furfural, furfuryl alcohol, 2-furoic acid, 3-hydroxyfurfural, furoin, 2-methyl-3,8-dihydroxychroman, ethylglyoxal, and several condensation products (36). Many metals, especially copper, cataly2e the oxidation of L-ascorbic acid. Oxalic acid and copper form a chelate complex which prevents the ascorbic acid-copper-complex formation and therefore oxalic acid inhibits effectively the oxidation of L-ascorbic acid. L-Ascorbic acid can also be stabilized with metaphosphoric acid, amino acids, 8-hydroxyquinoline, glycols, sugars, and trichloracetic acid (38). Another catalytic reaction which accounts for loss of L-ascorbic acid occurs with enzymes, eg, L-ascorbic acid oxidase, a copper protein-containing enzyme. 

FIGURE 18.27 Pyridoxal-5-phosphate forms stable Schiff base adducts with amino acids and acts as an effective electron sink to stabilize a variety of reaction intermediates. 

The effect of pKa on the stability constant of the 1 1 monodentate complexes of Me3 SnX with amino acids in aqueous solutions was studied by potentiometric methods281. 

Hora et al. [3.19] described the complexity of protein stabilization by the example of recombinant, human Interleukin-2 (rhIL-2). Formulations with amino acids and mannitol/ sucrose are sensitive to mechanical stress e. g. by pumping. Hydroxypropyl-beta-cyclodextrin (HPcD) provides stability, but increases the sensitivity to oxygen. Polysor-bate 80 forms a mechanically stable product, but results in oxidation. In both cases contamination in the HPcD or traces of H202 in the Polysorbate may have been the starter for the oxidation. Brewster [3.20] reports, that HPcD stabilizes interleukin without forming aggregations and this results in 100 % biopotency. 

The ability of flavosemiquinone to form complexes with amino acids was studied in view of the possible relevance of such complexes in flavoproteins The results indicate that tryptophan, cysteine and possibly tyrosine could function at the active centers of flavoproteins as agents to stabilize the protein-bound flavosemiquinone... 

The reaction of silver(I) ions with amino acids has received considerable attention.397 A number of studies have attempted to analyze the effects of substituents on the stability of the silver(I) complexes. When a series of a>-amino adds NHJ(CH2) C02 (n = 1-5) were examined, it was found that lengthening of the aliphatic side chain increased the overall stability of the complex.398,399... 

Biological significance can sometimes arise in rather unexpected ways the thermal properties of chelate polymers of 2,6-diaminopimelic acid (dap 12) and 4,4 -diamino-3,3 -dicarboxybiphenyl (bbdc 13) with Zn11 have been compared241 with those of non-polymeric divalent metal chelates with amino acids. This confirms the expected enhancement of thermal stability when coordination polymerization occurs, these results possibly being relevant to the thermal stability of certain bacterial spores which contain dap. Zn11 complexes of dap are more thermally stable than those of bbdc, possibly because the latter chelate cannot pack as well, due to the intermolecular repulsions of the biphenyl groups. 

The principles of the above reactions form the basis of a series of important metabolic interconversions involving the coenzyme pyridoxal phosphate (structure 2.41). This condenses with amino acids to form a Schiff base (structure 2.42). The pyridine ring in the Schiff base acts as an electron sink which very effectively stabilizes a negative charge. 

As discussed in Chapter 2, section C2, pyridoxal phosphate condenses with amino acids to form a Schiff base (structure 8.44). Each of the three groups around the chiral carbon at the top of structure 8.44 may be cleaved to give an anion that is stabilized by delocalization of the electrons over the 7r orbitals. 

The reaction of OPA with amino acids (see Fig. 10) requires a mercaptan cofactor that is incorporated as part of the final derivative product. The choice of mercaptan can affect derivative stability and chromatographic selectivity (178). Mercaptoethanol is the most commonly used coreagent. Cysteine is not well detected, because this amino acid can react at the a-amine group or it can react via the side chain thiol. Thus, cysteine is determined only after conversion of the thiol group by oxidation or alkylation. Reaction time with OPA is very fast, 1 minute at room temperature. Detection limits are typically in the low picomole range. Representative references include... 

Complexants other than alcohols (e.g., amines, nitriles, tert-butyl compounds, surfactants, and multiple fluorophores) can also be detected by the method of Scheme 2 [279-284], Bohne and Yang have replaced alcohols with amino acids [285], although the zwitterionic charge distribution and increased steric requirements of the former lead to smaller stability constants of the ternary complex. Notwithstanding, tryptophan, leucine, and phenylalanine produce measurable changes in the I III ratio of pyrene emission, thereby allowing for their detection at mM levels. 

Kalchenko, O. I., Perret, F., Morel-Desrosiers, N., and Coleman, A. W. (2001) A comparative study of the determination of the stability constants of inclusion complexes of p-sulfonatocalix [4]arene with amino acids by RP-HPLC and H-l NMR, Perkin Trans. 2 258-263. 

More yS-amino acids are, apparently, special building blocks within natural (cyclo)peptides and depsipeptides [11]. It is not dear whether Nature aimed at an increase of proteolytic stability or at conformational modification. Chemists, however, followed this example of natural / -amino acid chemistry decades ago and started to modify known a-peptides by substitution of solitary a-amino adds with p-amino acids [12]. One major issue of this synthetic analoging was to increase the proteolytic stability of peptides [13, 14] with regard to peptidases [15]. A second goal was the controlled conformational tuning of cyclo-a-peptides with / -amino acid modifications. 

Mohn AR, Gainetdinov RR, Caron MG, Roller BH. 1999. Mice with reduced NMDA receptor expression display behaviors related to schizophrenia. Cell 98 427-436. Moitra J. SzilaK A1997.,. Krylov D, Vinson C Leucine is the most stabilizing aliphatic amino acid in the d position of a dimeric leucine zipper coiled coil. Biochemistry 36 12567-12573. 

The reaction of lactim ethers with amino acids has been studied in detail.21,23,93 These reactions proceed readily, resulting in the substituted amidino acids. Their cyclization yields fused dihydropyrimi-dinones or imidazolinones (Scheme 13). The cyclization of the aliphatic amidino acids requires drastic conditions, whereas aromatic and heterocyclic amidino acids react under mild conditions, perhaps due to the formation of the new aromatic ring which stabilizes the resulting structure. 

The role of structural zinc sites is likely two-fold. They maintain the structure of the protein in the immediate vicinity of the metal site. In this manner it may influence enzymatic activity by providing active-site residues involved in catalysis and/or effecting the chemical environment of catalytic groups through interaction with amino acids originating from within its metal-binding spacers. However, in many cases, these metal sites also effect the overall stability of the protein as judged by temperature and pH criteria. 

While studies on the crystal structures of RE(III)-amino acid complexes can give us clear pictures on the ways in which RE(III) ions and the amino acids bond to each other, their solution chemistry, which deals with the reactions in solution, the chemical species formed, their stability, as well as their distribution over certain pH ranges, can help us understand better the in vivo behaviors of RE(III) ions and their complexes with amino acids. Work on the solution chemistry of RE-amino acid complexes has been carried out since the early 1960s [9]. It has been found that the amino acids studied behave very similarly to one another, just as we have learned from their structural chemistry. Mononuclear species with 1 1 and 1 2 (RE L) stoichiometry have been reported for all of the amino acids. In some studies, the presence of mononuclear species with stoichiometry 1 3, dinuclear species with stoichiometry 2 4 and 2 6, in addition to the hydrolyzed species, such as [RE(OH)L]+, [RE(OH)] +, and RE(OH)3(s), have been confirmed [126, 135, 136]. 

Kremer, C., Torres, J., Dominguezb, S., and Mederos, A. (2005) Structure and thermodynamic stability of lanthanide complexes with amino acids and peptides. Coordination Chemistry Reviews, 249, 567-590. 

The relationship between amino acid properties and protein stability revealed that the number of carbon atoms (methyl and methylene groups) that reflects the property hydrophobicity has a strong relationship with protein stability for the mutations in the interior of the protein. Yet, hydrophobic, hydrogen bond, electrostatic, and other polar interactions are important for the stability of mutation at the surface of the protein. The atom pair potentials set up on the basis of chemical nature and connectivity successfully could predict protein stability during amino acid substitution. 

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