Aromatic amino acids, complexation

By means of calorimetric measurements reflecting the dependence of the stability constants of metal complexes on the composition of the solvent mixture, Gergely and Kiss have shown that the stability increase resulting from a decrease in the relative permittivity shows up primarily in an increase in the — dff values, whereas the dS values decrease or remain unchanged compared with those measured in aqueous medium. A comparison of the AH values for aliphatic and aromatic amino acid complexes of copper(II) has revealed that the change in the medium acts to a lesser extent on the AH values of the aromatic amino acid complexes. This is probably due to the presence of the hydrophobic phenyl group in the aromatic ligands, as a consequence of which the extent of hydration even in aqueous medium is lower than in the case of alanine. 

Similar ligand-ligand interactions have been reported for a large number of ternary -amino acid complexes, built up of two different amino acid.s. A compilation of 72 examples is presented in reference 39. The extra stabilisation due to ligand-ligand interactions in these complexes depends on the character of the amino-acid side chains and amounts to 0.34 - 0.57 kJ/mole for combinations of aromatic and aliphatic side chains and 0.11 - 6.3 kJ/mole when arene - arene interactions are possible. ... 

Emission-Excitation Matrix (EEM) fluorescence spectroscopy as a nondestructive and sensitive analytical technique was successfully applied in this study to characterize DOM in landfill leachte. The DOM is composed of complex mixture of organic compounds with different fluorescence properties. In particular, the EEM profiles of DOM show two well-defined peaks at Ex/Em=320-350 /400-420 nm, Ex/Em=320-350 /420-450 nm reasonably due to the presence of two different groups of fluorophores. An additional and less intense band at Ex/Em=280-290 /320-350 nm can be assigned to aromatic amino acids and phenol-like compounds. 

The reaction of aromatic amino acids, dipeptides, or protected amino acids with [Ru(Cp )(CH3CN)3]PF6 in THF (THF = tetrahydro-furan) leads to the formation of robust Ru(Cp ) sandwich complexes in which side-chain phenyl (e.g., Phe, Tyr) or indole groups (Trp) are bound 7]6 to a Ru(Cp ) center (cf. 31 AA = Phe) (75, 76). The similar... 

Compared to absorbance detection, direct detection of proteins rich in aromatic amino acids by the intrinsic fluorescence of tryptophan and tyrosine residues provides enhanced sensitivity without the complexity of pre- or postcolumn derivatization. The optimal excitation wavelengths for these amino acids are in the 270- to 280-nm range. 

Despite their lack of stabilizing disulfide bridges Potl inhibitors feature a common, stable fold. The N-terminus is coiled, although in some structures a small /3-strand has been identified. After a turn the structure adopts an a-helical structure, followed by a turn and an other /3-strand. The sequence then features an extended turn or loop motif that contains the reactive site of the inhibitor before it proceeds with a /3-strand running almost parallel to the /3-strand after the a-helix. After another turn and coiled motif a short /3-strand antiparallel to the other /3-strands precedes the coiled C-terminus. Usually the N-terminal residue in the reactive site is an acidic residue followed by an aromatic amino acid, that is, tyrosine or phenylalanine. Figure 11 shows the complex of chymotrypsin inhibitor (Cl) 2 with subtilisin, the hexamer of Cl 2 from H. vulgare and a structural comparison with a trypsin inhibitor from Linum usitatissimum ... 

Formation of the bis-histidyl-heme complex also produces characteristic alterations in the protein s conformation, particularly in the environment of aromatic amino acids, notably tryptophan. Exposure of Trp residues to solvent decreases (113), Trp fluorescence is quenched (111), and an unusual band of positive ellipticity at 230 nm attributable to Trp is nearly doubled in intensity (Fig. 4) (104, 111, 124). 

The structural similarity of the catalytic domains of the enzymes of the AAH family, together with the identical reaction that they catalyze (i.e., hydroxylation of aromatic substrates) and the common dependency on BH4 and 02 (Section I), suggests that the mechanisms by which these enzymes operate are similar. It is assumed that the general AAH reaction mechanism follows a two-step reaction route in which a high-valent iron-oxo (FeIV=0) complex is formed in the first step, and that this intermediate is responsible for the hydroxylation of the aromatic amino acid substrate in the second step (15,26-28,50). The first step starts with 02 binding and activation and proceeds via a Fe-0-0-BH4 bridge and a subsequent heterolytic cleavage of the... 

All amino acids are derived from intermediates in glycolysis, the citric acid cycle, or the pentose phosphate pathway (Fig. 22-9). Nitrogen enters these pathways by way of glutamate and glutamine. Some pathways are simple, others are not. Ten of the amino acids are just one or several steps removed from the common metabolite from which they are derived. The biosynthetic pathways for others, such as the aromatic amino acids, are more complex. 

Among the essential amino acids, the aromatic amino acids (phenylalanine, tyrosine, and tryptophan) form by a pathway in which chorismate occupies a key branch point. Phosphoribosyl pyrophosphate is a precursor of tryptophan and histidine. The pathway to histidine is interconnected with the purine synthetic pathway Tyrosine can also be formed by hydroxylation of phenylalanine (and thus is considered conditionally essential). The pathways for the other essential amino acids are complex. 

Tphe complexing of virtually all purines with aromatic molecules seems - to have far-reaching biological significance. For example, it is known that caffeine affects the rates of many enzymatic reactions (e.g., 0.01, 0.05, and 0.10M caffeine will inhibit salivary amylase 29, 54, and 72% respectively) (12), and purine can decrease the helix-coil transition temperature of the proteins bovine serum albumin and lysozyme (2). It is not unreasonable to expect the involvement of caffeine-aromatic and purine-aromatic complexes because caffeine derivatives and purine complex with the aromatic amino acids tyrosine, phenylalanine, and tryptophan (2). (In fact tryptophan forms a stable 1 to 1 crystalline complex in 0.5M theophylline glycol.)... 

Since aromatic amino acids and cysteine are absent, there is no protein absorption above 270 nm. Metallothioneins exhibit a broad absorption peak, with the maximum at 190 mn. Absorptions due to the metal-thiolate complexes show as shoulders at 250 nm (Cd), 220 nm (Zn) and 270 nm (Cu).1458,1459 Theoretical predictions based on the amino acid sequence of the peptide chain indicate that the or-helical conformation is forbidden, and /3-structure is almost impossible to attain. CD and NMR studies on both the metal-containing and metal-free protein confirmed the predictions.1459 1460 However, metallothioneins are stable to tryptic digestion and the slow exchange of many peptide hydrogens of metallothionein with those of the solvent suggest that the protein has a compact and well-defined tertiary structure. 

Complexes of pyridine-2-carboxylic acid (picolinic acid, picH) and its substituted derivatives commonly exhibit the N—O bidentate nature of this aromatic amino acid. From aqueous solutions chelates are obtained with the coordinated carboxylic group deprotonated, or neutral ligand forms may be isolated from non-aqueous media.26 Bis chelates are common in either case with bivalent metal ions. The tris chelates of trivalent cobalt27 and manganese28 have been structurally characterized recently. The latter is tetragonally distorted in a structure similar to Mnin(oxine)3. 

---