The circular dichroism of N-thiobenzoyl-L-α-amino-acids. Part II. Further examples of solvent-dependent circular dichroism
Circular dichroism studies have been extended to include a further eleven N-thiobenzoyl-D- and L-α-amino-acids. Two L-compounds of this group show positive circular dichroism near 400 mµ in ethereal solution; therefore, a generalisation linking the absolute configuration of an α-amino-acid with the sign of the circular dichroism shown near this wavelength by its N-thiobenzoyl derivative fails for measurements with ethereal solutions.
The double-humped circular dichroism curves exhibited by N-thiobenzoyl-L-alanine and by N1-benzyl-Nα-thiobenzoyl-L-histidine, in methanol, are indicative of an equilibrium between solvated and non-solvated species in these solutions. Consequently, the inference is made (without recourse to low-temperature studies) that the circular dichroism behaviour of N-thiobenzoylamino-acids, in methanol, more generally reflects their involvement in solvational equilibria.