The circular dichroism of N-thiobenzoyl-L-α-amino-acids. Part IV. Contribution of imino-thiol tautomeric forms to the circular dichroism of N-thiobenzoyl-L-α-amino-acids

Abstract

Thiobenzimidates derived from N-thiobenzoyl-L-α-amino-acids show negative c.d. associated with the high intensity absorption maximum near 270 nm. characteristic of these compounds; the 4-methyl-2-phenyl-2-thiazoline obtained from L-alanine shows the same c.d.–structure relationship. Analogous derivatives from D-α-aminophenylacetic acid did not all show the positive c.d. curves near 270 nm. expected on this basis; the variable c.d. behaviour is ascribed to superimposition of the c.d. contribution of the phenyl chromophore in this wavelength region. No member of these series shows detectable c.d. at wavelengths longer than ca. 300 nm.; therefore, the c.d. shown near 390 nm. by an N-thiobenzoyl-L-α-amino-acid includes no contribution from its imino-thiol tautomer. The fact that the c.d. shown near 290 nm. by N-thiobenzoyl-L-α-amino-acids is generally positive proves that the near-ultraviolet c.d. of this series similarly does not originate from their thiobenzimidate tautomers.