Aspartic acid rotation

Aspartic acid at 20°C rotates the plane of polarization to the right, [a] D20 = + 4.36° but with the increase in temperature, the plane of polarization is rotated to the left, [a] D90 = -1.86° although the configuration of the antipode remains unchanged. Therefore, determination of the configuration of the isomers is a specific area of stereochemistry. The reader at this state must understand the logic to tackle this problems, the experimental techniques employed have been given later. 

A contemporaneous study on the same subject utilized a chemical correlation method where (—)-A-benzylargemonine chloride, obtained by sequential optical resolution and quatemization of ( )-7V-methylpavine (5), underwent a multistep degradative process to furnish (-)-A,A-dimethyl-di-H-propyl aspartate. Comparison of this final product with L-aspartic acid of known chirality led to the absolute configuration of (—)-5 (115,158). (—)-Eschscholtzine (9) was assigned the same absolute configuration by correlation of its ORD curve and optical rotation with those of (—)-argemonine (775). 

R) - and (S)-l-phenylethylamine to the diethyl esters of fumaric and maleic acid which are carried out by heating the pure compounds, without solvent, to 115-120 °C for three days (see Table 1). The reaction mixtures are then hydrolyzed and hydrogenated to give aspartic acids in high yields (85-87%) but very low optical purities (6.3-12.2%). A number of intermediates and by-products arc isolated, especially amides and imides of the dicarboxylic acids participating in the reaction. This may explain the low overall diastereoselectivity which can be calculated from the low optical rotation of the isolated aspartic acids. However, any discussion of the reaction mechanism remains difficult because the structures of the substrates and products of the actual addition step itself are not known with certainty. It is known that... 

Figure 18.32 Proton motion across tiie membrane drives rotation of the c ring. A proton enters from the intermembrane space into the cytoplasmic half-channel to neutralize the charge on an aspartate residue in a c subunit. With this charge neutralized, the c ring can rotate clockwise by one c subunit, moving an aspartic acid residue out of the membrane into the matrix half-channel. This proton can move into the matrix, resetting the system to its initial state.

Kinetics of Racemization of Amino Acids. Phosphate was used to buffer solutions of the L-amino acids at pH 7.6. Solutions of L-aspartic acid were buffered at the various pH values by either hydrochloric acid, oxalate, succinate, phosphate, borate, or NaOH. The pH values of the buffered solutions at the elevated temperatures were estimated as described previously (38). Sodium chloride was added to the solutions to adjust the final ionic strength to 0.5. The solutions were degassed and sealed under vacuum in borosilicate glass ampules. The ampules were sterilized immediately after being sealed by heating at 100 °C for 15 to 20 minutes. The rates of racemization were determined from measurements of the rate of change of optical rotation (a) of the solutions. The measurements were made on a Perkin-Elmer 141 polarimeter at 365 nm. With the exception of the aspartic acid solutions at pH values less than 2 and the phenylalanine solutions, all samples were diluted in 1M HC1 before measuring the rotation. 

Shown in cross-eye stereo side view in Figure 8.28 are three views of the Fo-rotor, that is, of the rotating wheel of the Fo-motor. At this level of resolution each residue is given as a sphere. Furthermore, each of the 10 subunits of the Fo-rotor is represented as a double-stranded a-helical hairpin with the ends at the top (cytosolic side) and the turn at the bottom (matrix side) and with one side of the hairpin in direct interaction with the lipid bilayer and the other side forming the inner wall of the rotating wheel. In Figure 8.28A the aspartic acid residue D61 is noted from outside. This residue exists with its side chain as a carboxyl (-COOH) when adjacent to the lipid bilayer, but it releases its proton to form the carboxylate (-COO ) when it reaches the effective channel when adjacent to the a-subunit. 

At least two other secondary structures are observed with peptides a P pleated sheet and a random coil. Poly(aspartic) acid, mentioned previously, forms a random coil structure. A random coil, as its name implies, does not assume a regular structure such as the a-helix because hydrogen bonds are not easily formed. Rotation about the / and ( ) angles (see 126) leads to a random orientation of the various amino acid residues. The -pleated sheet, on the other hand, does involve intramolecular hydrogen bonding. In other words, there are hydrogen bonds between two different peptide chains rather than within a single peptide chain. 

W — Specific rotations of different samples can be compared accurately only imdw identical conditions. Some studies have been made by Luts and Jirgensons (535, 536) of the rotations of the amino acids in solutions of varying pH and the temperature coefficients for aspartic acid, cystine, glutamic add, bietidine, leudne and tyrosine have been listed by Dunn et al. (209). The latter authors have investigated the specific rotation of L-histidine in water as a function of concentraticm. More extensive information is needed relating specific rotations of the amino adds to these and other variables. 

The shift is also appreciable and positive with histidine (PK2), cystine, aspartic acid (pKj), glutamic acid (pKj), and iV-dimethylcystine (Table VI) however, with valine, proline, and thiazolidine-4-carboxylic acid the shift is negative. In the case of arginine both the ionization of the amino and of the guanidino group have a very slight effect on rotation (Herbst and Grotta, 1946). With lysine the removal of the protons from... 

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