Glutamic acid rotation

The a-carbon of glutamic acid is chiral. A convenient and effective means to determine the chemical purity of MSG is measurement of its specific rotation. The specific optical rotation of a solution of 10 g MSG in 100 mL of 2 A/HQ is +25.16. Besides L-glutamic acid [56-86-0] D-glutamic acid [6893-26-1] and the racemic mixture, DL-glutamic acid [617-65-2] are known. Unique taste modifying characteristics are possessed only by the L-form. 

Wilson and Cannan (18) reported detailed observations on the equilibrium and velocity constants in the glutamic acid—pyrrolidone carboxylic acid system in dilute aqueous solution. They found that the conversion of glutamic acid to pyrrolidone carboxylic acid follows the equation for a reversible first-order reaction. The equilibrium constant and the rate at which the equilibrium is achieved depend on the pH of the solution and the temperature. In neutral solutions, the equilibrium favors almost complete conversion of glutamic acid to pyrrolidone carboxylic acid however, the rate of the reaction is very slow and thus only 1% conversion occurs after 2-3 hr at 100°. In weakly acid (pH 4) and alkaline (pH 10) solutions, the conversion of glutamic acid to pyrrolidone carboxylic acid is much faster and about 98% conversion occurs in less than 60 hr. In strong acid (2 N HC1) and base (0.5 N NaOH) the conversion of pyrrolidone carboxylic acid to glutamic acid proceeds rapidly and virtually to completion. Other studies have shown that the conversion of glutamic acid to pyrrolidone carboxylic acid can be carried out within 2 hr at 142° with little alteration of optical rotation (80). 

When azobenzenes are attached to polypeptides, photochromic reactions of azobenzenes can induce the change in helical properties of the polypeptides, which may be detected by CD spectrum as well as optical rotation. For 4-phenylazophen-ylamine-condensed poly(y-glutamic acid) A-9 containing up to 80 mol% of 4-phenylazophenylamide side chain, UV irradiation in organic solvents, such as... 

Fig. 12. Helix-coil transitions of equimolar oopoly-L-lysine-L-glutamic acid and ovalbumin induced by variation in solvent composition. The specific rotation is plotted as a function of the per cent of trifluoroacetic acid in the 2-chioroethanol-trifluoroacetic acid solvent system. (Doty et al., 1958.)...

Fig. 15. Rotatory artifacts that simulate Cotton effects at an absorption band. The dependence of the rotatory artifact on absorbance of p-cresol solutions placed in series with the same poly-L-glutamic acid solution is shown. The concentration of p-cresol was adjusted to give the total absorbance of chromophore plus polypeptide background that appears with each curve. The rotator, poly-L-glutamic acid, was at concentration of 0.5% at pH 7.0 in a 10-cm cell. The rotations are those actually observed, a, in degrees. The rotatory dispersion at Am 2 coincides almost exactly with that for the polypeptide alone, so that it has been omitted from the figure. At Am 4, an interference filter, /, with maximum transmission between 280 and 285 m/i, was placed in the optical path. The absorption spectrum, in arbitrary units, is typical of p-cresol plus poly-L-glutamic acid background. The emission spectrum is represented in arbitrary units, uncorrected for detector response. (Urnes et al., 1961a.)...

Fig. 16. The appearance of rotatory artifacts both at the minimum in the emission spectrum of the mercury arc and as increasing polypeptide absorption is encountered below 250 m/i. The rotations are those actually observed, a, in degrees, for a 0.6% poly-L-glutamic acid solution at pH 7 in a 10-cm cell. The absorption spectrum of poly-L-glutamic acid and the emission spectrum of the arc, uncorrected for detector response, are in arbitrary units. (Urnes et al., 1961b.)...

In the case of proline, the amide group is part of a five-membered ring and rotation about the C-N bond is not possible. Therefore, proline is considered a helix breaker and is rarely found in helices. If proline is found in an a helix, a bend or kink in the helix is usually observed at that point. Alanine, glutamic acid, leucine, and methionine are good helix formers, while proline, glycine, tyrosine, and serine are not. 

Figure 42. Differential scatter of left and right circularly polarized light by poly-L-glutamic acid particles. Curve a, mean residue rotation of PGA reference state plotted on right-hand ordinate. Curve b, contribution of differential scatter term only, (Asi. —Asr) 3300/Cl. Curve c, total change in mean residue ellipticity due to differential scatter, includes differential scatter component of the (Aol— 4or) term. Curve d, comparison of empirical lO " values with total differential scatter. Curve e, approxima-...

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