Tryptophan, characteristics

Chapters 9, 10 and 11 describe methods for substitution directly on the ring with successive attention to Nl, C2 and C3. Chapters 12 and 13 are devoted to substituent modification as C3. Chapter 12 is a general discussion of these methods, while Chapter 13 covers the important special cases of the synthesis of 2-aminoethyl (tryptaminc) and 2-aminopropanoic acid (tryptophan) side-chains. Chapter 14 deals with methods for effecting carbo cyclic substitution. Chapter 15 describes synthetically important oxidation and reduction reactions which are characteristic of indoles. Chapter 16 illustrates methods for elaboration of indoles via cycloaddition reactions. 

Indole alkaloids from Tabernaemontana plants are all biogenetically derived from tryptophan (tryptamine) and secologanine, which constitute the indole and terpenic portions, respectively, and can be classified into nine main types depending on the structural characteristics of their skeleton (Fig. 1). 

The most direct demonstration of triplet-triplet energy transfer between the aromatic amino acids is the ODMR study by Rousslang and Kwiram on the tryptophanyl-tyrosinate dipeptide.(57) Since the first excited singlet state of tyrosinate is at lower energy than that of tryptophan, it is possible to excite tyrosinate preferentially. The phosphorescence of this dipeptide, however, is characteristic of tryptophan, which is consistent with the observation that the triplet state of tyrosinate is at higher energy than that of tryptophan, making tryptophan the expected triplet acceptor. 

Tyrosine fluorescence emission in proteins and polypeptides usually has a maximum between 303 and 305 nm, the same as that for tyrosine in solution. Compared to the Stokes shift for tryptophan fluorescence, that for tyrosine appears to be relatively insensitive to the local environment, although neighboring residues do have a strong effect on the emission intensity. While it is possible for a tyrosine residue in a protein to have a higher quantum yield than that of model compounds in water, for example, if the phenol side chain is shielded from solvent and the local environment contains no proton acceptors, many intra- and intermolecular interactions result in a reduction of the quantum yield. As discussed below, this is evident from metal- and ionbinding data, from pH titration data, and from comparisons of the spectral characteristics of tyrosine in native and denatured proteins. 

The fluorescence of purified histones has been studied by several different groups, 90 95) with the most detailed studies being on calf thymus histone HI. Histone HI, which binds to the outside of core particles, contains one tyrosine and no tryptophan. This protein exhibits a substantial increase in fluorescence intensity in going from a denatured to a folded state.<90) Collisional quenching studies indicate that the tyrosine of the folded HI is in a buried environ-ment.(91) Libertini and Small(94) have identified three emissions from this residue when in the unfolded state with peaks near 300, 340, and 400 nm. The 340-nm peak was ascribed to tyrosinate (vide infra), and several possibilities were considered for the 400-nm component, including room temperature phosphorescence, emission of a charge transfer complex, or dityrosine. Dityrosine has the appropriate spectral characteristics, but would require... 

For single-tryptophan proteins there is some correlation between blue-shifted fluorescence emission maximum and phosphorescence lifetime (Table 3.2). Another correlation is that three of the proteins which exhibit phosphorescence, azurin, protease (subtilisin Carlsberg), and ribonuclease Tlt are reported to show resolved fluorescence emission at 77 K. Both blue-shifted emission spectra and resolved spectra are characteristic of indole in a hydrocarbon-like matrix. 

Kai and Imakubo(76) found that the temperature at which emission from the exposed tryptophan is no longer observed appears to be characteristic of the protein, having values of 180 K for trypsin, 200 K for aldolase, and 230 K for alkaline phosphatase. 

The characteristics that discourage the use of RPLC for preparative isolation of bioactive proteins favor its use as an analytical tool for studying protein conformation. Chromatographic profiles can provide information on conformational stability of a protein and the kinetics of folding and unfolding processes. Information about solvent exposure of certain amino acid residues (e.g., tryptophan) as a function of the folding state can be obtained by on-line spectral analysis using diode array UV-vis detection or fluorescence detection. 

When tryptophan is dissolved in water, it shows the fluorescence characteristics illustrated in O Figure 5-2. However, one especially useful property of tryptophan fluorescence is that its emission spectrum is highly sensitive to the polarity of its environment. In less polar solvents (alcohols, alkanes, etc.), the emission... 

In addition to the HECT domain, there is another domain in many E3s called the WW domain. The WW domain is thus named because of the characteristic tryptophan (W is the single letter code for the amino acid tryptophan) believed to be critical for protein-protein interaction. The WW domain-containing E3s also tend to have a C2 domain. The presence of C2 domain is highly relevant to nervous system function because C2 domain responds to the elevation of intracellular Ca and helps in translocation to the plasma membrane. Therefore, presence of this domain in neuronal HECT E3s might be critical in ligating ubiquitin to neurotransmitter receptors or proteins associated with them. 

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). 

Characteristic amino add composi- Absence of tryptophan, histidine and arginine tion... 

Modification of Tryptophan Residues. Tryptophan residue also could be determined quantitatively by a modification with a sulfenylating agent such as 2-nitrophenyl sulfenyl chloride in 30% acetic acid ( ). Since both oxidized and sulfenylated tryptophan gave characteristic absorption at 365 nm, the extent of tryptophan modification was calculated from the mean difference between the sulfenylated protein and the kynurenine. The absorption contributed by kynurenine was comparatively weak. 

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