Tryptophan, optical absorption

The following amino acids have been definitely excluded as part of a common structure of the Type 1 center Tryptophan has been eliminated as a ligand for the reasons given in Section IIAl. Arginine is absent in the plastocyanins. Tyrosine has been eliminated by optical absorption studies of azurin and by a recent analysis of the resonance enhanced Raman spectrum of stellacyanin 206). 

Intermediates occurring in these mechanisms have been identified by ESR measurements and by flash photolysis studies using optical absorption detection. For example, ESR measurements on wool keratins revealed the formation of sulfur-centered radicals of the structure RCH2S, which, in this case, are assumed to result from a reaction of electronically excited tyrosine moieties with cystine residues [11]. In many proteins, cross-links are formed. In the case of keratin and collagen, the cross-links are of the tryptophan-histidine and dityrosine types [11]. Cross-links formed by the combination of R-S or R-S-S radicals, both intermolecularly and intramolecularly, with incorrect sites are considered to be an important source of photoaggregation effects [8]. ESR measurements have also yielded evidence of C-H and C-N bond ruptures [8]. 

Figure 23. Optical absorption spectra of IDO at -30 C. a. native ferrous IDO b, native ferric IDO c, IDO-O2 1 IDO-02-L-tryptophan [203].

Possibility of the same type of the regulatory site of TPO is suggested by the result that some indole derivatives having substituents at the 3-position serve as effectors to enhance the L-tryptophan affinity [186, 187] or to bind to the L-Tryptophan-ferrous enzyme-02 ternary complex of IDO [203]. Effects of 3-indoleethanol (lET) and indole (IND) on the catalytic (ATni, V ax) and spectroscopic properties (optical absorption and CD) have been investigated [211]. lET and IND decrease the value for D-tryptophan but not that for L-tryptophan. On the other hand, lET enhances the V ax values while IND lowers them. This opposite effect is explained by that the binding of these effectors to the enzyme in the steady state of the catalytic reaction causes the right shift of the equilibrium between the ferric (inactive) and ferrous (active) forms (Fe Fe ) by... 

Among the properties of amino adds that are most pertinent to the biomedical scientist are their optical rotations, already discussed, which are listed for each amino acid in Table 4.1. Note the dramatic differences between optical rotations in the zwitterionic (water) and fully protonated (HC1) forms. Further, all amino acids absorb ultraviolet light in the range 190-220 nm. The C=0 bond in carboxyl residues is largely responsible. Moreover, aromatic amino acids, especially tryptophan, absorb in the 260-285 nm range. Protein concentrations in solutions are often determined via absorption at 210 or 280 nm. 

S Tryptophan, which offen occurs af only one or a few places in a protein, is a useful fluorescent probe for study of protein dynamics. The optical properties of 7-azatrypfophan, 2-azotrypfophan, and 5-hydrox-ytryptophan are even better because their absorption maxima occur at longer wavelengths. These amino acids can be biosynthetically introduced in place of tryptophan in proteins. maximum fluores-... 

This particular reaction has been chosen for the reason of its high value of standard chemical affinity for this reaction (j / = — 7.1 kcal/mole). As we noted above, due to this circumstance the system behavior can reveal the deviation from that prescribed by the classical Arrhenius mechanism. The conformational changes in malatdehydrogenase were tested by measuring the average life-time of intrinsic tryptophane fluorescence (ff). This parameter is known to be sensitive to the immediate surrounding of tryptophane residues. The chemical transformation of the substrate was detected from changes in the coenzyme redox state measured in terms of the sample optical density at 340 nm (NADH absorption maximum). 

The optical density decrease at 280 nm can be related to the optical density of tryptophan in the sample using the empirical factor 1.31, which corrects for the absorption at 280 nm of the oxidation product of tryptophan. The amount of tryptophan in the solution is calculated using an extinction coefficient for tryptophan at 280 nm of 5,500 (373). 

Coloured compounds are formed when phthalyl-tryptophan is incorporated into the same molecules as phenylalanine, the colour being ascribed to an intramolecular charge transfer complex. Optically active molecules have CD bands associated with the charge transfer bands. Using the area under the bands as a measure of complexing, association constants have been evaluated which agree with those obtained from absorption changes. 

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