Optical Rotation

Optical rotation is an important characteristic for determining the purity of an amino acid. The value measures changes that occur with changes in temperature, pH, the kind of solvent, and other conditions that affect related optically active substances. For polarimetrie measurement of the specific rotation, the sodium d line (wavelength, 589 nm) is used as a light source and expressed as [a]d.

The optical rotation for various amino acids is minimum at neutral conditions. For acidic and alkaline conditions, the value of the optical rotation of l-amino acids rotates toward a positive direction. For d-amino acids, conversely, the value rotates toward a negative direction. This rule, called Lutz's law or the Clough-Lutz-Jirgenson law, is used to determine the stereo allocation of an amino acid molecule. The stereo allocation also is attributed to the difference in the ionic form of the amino acid. Therefore, when acid is added, a change in the optical rotation value is observed up to 2 Eq, but above 2 Eq, no change in the optical rotation is observed.

When the temperature is elevated, the dextrorotary value of the optical rotation of L-aspartic acid decreases. At 75°C, the value is 0; conversely, the levorotary value increases with increases in temperature.

Optical rotatory dispersion (ORD), is the phenomenon that changes in optical rotation are observed when the wavelength (X) is changed. When [a] is plotted as a function of X2, it is the so-called rotatory dispersion curve, if it obeys the Drude monominal formula, it is called monorotation dispersion, [a] = AD.L2 — Àq, where A is the rotation constant, X is the wavelength of the incident light, and X0 is the dispersion constant that corresponds to the wavelength of the nearest optically active absorption band (33).

In general, amino acids show a normal dispersion, but tyrosine shows an anomalous dispersion due to two rotatory contributions of opposite sign, ie, with decreasing wavelength, the rotation is minimal, becomes zero, changes sign, and approaches + °° (an effect known as the Cotton effect). In the case of tyrosine, good agreement is obtained from three constant Drude equations.

Further methods for investigating the structure of amino acid enantiomers include circular dichroism, which shows differing absorption for left- and right-handed circularly polarized light.

Isoleucine, threonine, and hydroxyproline contain two chiral carbon atoms each; therefore, they appear in four stereoisomer«; forms. Cystine, which also contains two chiral carbons, has only three stereoisomers: l-, d-, and meso-cystine. The meso form has a plane symmetry.

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