Phenylalanine light absorption

Comparison of the light absorption spectra of the aromatic amino acids tryptophan and tyrosine at pH 6.0. The amino acids are present in equimolar amounts (10 3 m) under identical conditions. The measured absorbance of tryptophan is as much as four times that of tyrosine. Note that the maximum light absorption for both tryptophan and tyrosine occurs near a wavelength of 280 nm. Light absorption by the third aromatic amino acid, phenylalanine (not shown), generally contributes little to the spectroscopic properties of proteins. 

Spectra of proteins and nucleic acids. Most proteins have a strong light absorption band at 280 nm (35,700 cm ) which arises from the aromatic amino acids tryptophan, tyrosine, and phenylalanine (Fig. 3-14). The spectrum of phenylalanine resembles that of toluene (Fig. 23-7)whose 0-0 band comes at 37.32 x 10 cm. The vibrational structure of phenylalanine can be seen readily in the spectra of many proteins (e.g., see Fig. 23-llA). The spectrum of tyrosine is also similar (Fig. 3-13), but the 0-0 peak is shifted to a lower energy of 35,500 cm (in water). Progressions with spacings of 1200 and 800 cm are prominent. The low-energy band of tryptophan consists of two overlapping transitions and The Lb transition has well-resolved vibrational subbands, whereas those of the La transition are more diffuse. Tryptophan derivatives in hydrocarbon solvents show 0-0 bands for both of these transitions at approximately... 

Side chains of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan absorb ultraviolet light in the 240- to 300-nm region, while histidine and cystine absorb to a lesser extent. Figure 3-13 shows the absorption spectrum of a "reference compound" for tyrosine. There are three major absorption bands, the first one at 275 nm being a contributor to the well-... 

Absorption spectrum is the plot of light intensity as a function of wavelength. Figure 1.2 shows the absorption spectra of tryptophan, tyrosine, and phenylalanine in water. A strong band at 210-220 nm and a weaker band at 260-280 nm can be seen. 

The part of a molecule that absorbs the light and is, therefore, responsible for its colour (whether in the visible or UV region) is called the chromophore, and the wavelength dependence of the absorption defines its absorption spectrum. Figure 7-3 illustrates the absorption spectrum of the three aromatic amino acids tryptophan, tyrosine and phenylalanine. 

Phenylalanine s phenyl side chain classifies it as an aromatic amino acid. The aromatic amino acids, like most compounds carrying conjugated rings, exhibit strong absorption of light in the near-ultraviolet region of the spectrum (Figure 5.6). This absorption is frequently used for the analytical detection of proteins. 

Aromatic Amino Acids (Strong absorption of light in near UV) (Figure 5.6) Phenylalanine, Tyrosine, Tryptophan Basic Amino Acids (Strongly polar, usually on exterior of proteins) (Figure... 

In absorption such as in fluorescence, light intensity is plotted as a function of the wavelengths. Figure 1.7 displays the absorption spectra of tryptophan, tyrosine and phenylalanine in water. One can notice the presence of a strong band at 210-220 nmand a weaker one at 260-280 rm. 

UV Absorbance. UV detection of peptides and proteins is mostly performed at 200-220 nm, where the absorption is proportional to the number of peptide bonds, but sometimes around 254 or 280 nm, where the detection is based on the absorbance of the aromatic residues such as tryptophan, tyrosine, and phenylalanine. This detection method is most commonly used, but it has low sensitivity, which is a major disadvantage for the detection of analytes present at low concentrations. In CZE, UV detection is limited to micromolar or submicromolar concentrations for peptides and proteins. Several capillaries have been designed to improve the sensitivity in CE (I) a rectangular capillary extended in the direction of the light path [51], (2) a Z-shaped capillary [52], and (3) a bubble cell capillary with an locally enlarged diameter in the detection region [53]. These capillaries help somewhat to improve sensitivity, but practical difficulties of availability and implementation into existing instruments remain. 

Of the twenty amino acids found in proteins, three may be classified as aromatic in character and account for virtually all of the protein absorption of ultraviolet light above 250 nm and for all of the observed luminescence. The molecular structures of the aromatic amino acids are shown in Fig. 1. The parent aromatic molecules of tryptophan, tyrosine and phenylalanine are, respectively, indole, phenol and benzene. The parents with methyl substituents in place of the amino acid side chains have the common names skatole, p-cresol and toluene, respectively. 

The polymers presented in Chart 8.1 absorb UV light to quite different extents. Nucleic acids absorb more strongly than proteins. This can be seen in Fig. 8.1, which shows absorption spectra of aqueous solutions of DNA and bovine serum albumin, recorded at equal concentrations. In contrast to the rather strongly absorbing nucleotide residues in DNA, only a few of the amino acid residues in proteins absorb light measurably in the UV region. This pertains mainly to the aromatic amino acids phenylalanine, tyrosine, and tryptophan (see Chart 8.2). 

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

One research area that has benefited from extensive fluorescence investigations is the field of protein structure-function studies. There are three aromatic amino acids that absorb light in the ultraviolet spectral range, phenylalanine (Phe), tyrosine (lyr) and tryptophan (Trp). Because of their molar absorptivity the fluorescence of Tyr (2276 = 1405 m cm ) and Trp (e28o = 5579 cm" ) have... 

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