Circular dichroism of N-thiobenzoyl-L-α-amino-acids. Part III. Their circular dichroism through the near-ultraviolet wavelength range
Solutions in ether and in methanol of a series of N-thiobenzoyl-L-α-amino-acids exhibit circular dichroism associated with their π→π* ultraviolet absorption maximum (log Îµ∼ 3·85) near 290 mµ. In contrast to the circular dichroism behaviour of this series near 395 mµ(n→π* transition), that near 290 mµ is little influenced in sign, and in the magnitude and wavelength of the circular dichroism maximum, by changing the solvent from ether to methanol. Thus, ether solutions of the N-thiobenzoyl derivatives of all L-α-amino-acids in the present series show positive circular dichroism near 290 mµ; the same relationship holds for methanol solutions of most of these compounds and corresponding cyclohexylammonium salts. Values of (Îµl–Îµr) at the circular dichroism maximum near 290 mµ are particularly large for the N-thiobenzoyl derivatives of α-amino-acids which carry an aromatic group in their side-chain. The derivatives of the α-imino-acids, L-proline and hydroxy-L-proline, however, show negative circular dichroism near 290 mµ, probably reflecting a different spatial relationship of chromophore to asymmetric centre from that adopted by the derivatives of the α-amino-acids.
The 243 mµ absorption (log Îµ∼ 4·05) maximum of the thiobenzamide chromophore in these compounds is also “optically active”; a circular dichroism maximum can be detected near this wavelength in solutions of the N-thiobenzoyl-L-α-imino-acids and of the derivatives of L-α-amino-acids which exhibit the larger values of (Îµl–Îµr) near 290 mµ, though the circular dichroism of most members of the series is too weak near 243 mµ for detection with the present Dichrographe.