Aspartic acid, specific 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. 

Recently, studies of the conformation of oligomers were extended to peptides derived from /3-methyl-L-aspartates. Their synthesis (n = 2 up to 14) was described by Goodman and Boardman (82), and later the specific rotations of their solutions in dimethyl formamide, dichloro-acetic acid and in chloroform were determined (83). The oligomers exist in a random-coil form in the first two solvents, but helices become stable in chloroform for n — 11 and 14. These peptides are unusual since their L-amino-acid residues produce a left-hand helix (84, 85) whereas most of the investigated polyamino acids crystallise as a right-hand helix (86). 

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. 

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