Low Temperature Circular Dichroism of Tyrosyl and Tryptophanyl Diketopiperazines

Abstract Diketopiperazines have been used as a model system to evaluate the factors determining the shapes and intensities of the near ultraviolet circular dichroism (CD) bands of the tyrosyl and tryptophanyl moieties in proteins. Cooling to 140°K or below brings out fine structure CD bands in all of the diketopiperazines examined. Studies with c-Gly-l-Tyr, c-l-Val-l-Tyr, and c-l-Phe-l-Tyr confirm that the tyrosyl 1lb CD spectrum normally has about the same shape as the absorption spectrum. The CD spectra of c-l-Tyr-l-Tyr, however, are somewhat distorted, because of a small exciton CD contribution superimposed on the normal tyrosyl CD spectrum. The shapes of the tryptophanyl CD bands cannot be determined as precisely due to overlap of the 1la and 1lb electronic bands. Nevertheless, the relative heights of the two major 1lb bands are similar in the CD spectra of c-Gly-l-Trp, c-l-Phe-l-Trp, c-l-Val-l-Trp, and c-d-Val-l-Trp. In the c-l-Trp-l-Trp CD spectra, however, a different ratio of 1lb intensities is observed, suggesting that c-l-Trp-l-Trp may also have a small exciton CD contribution. The tyrosyl and tryptophanyl 1lb fine structure CD bands evident at low temperature can be identified in the same diketopiperazine when these bands are much more intense than the 1la tryptophanyl bands, as is the case for c-l-Trp-l-Tyr. The identification is not as feasible when the 1lb intensities are low, as occurs in c-d-Trp-l-Tyr. The CD intensities are enhanced greatly in those diketopiperazines having two aromatic residues with the l configuration. This enhancement seems to result from coupling of the 1lb electronic transition in either a tyrosyl or tryptophanyl moiety with the far ultraviolet transitions in the second aromatic side chain. Conformational equilibria were examined by cooling the diketopiperazines. The increase in CD intensity upon cooling is much less for the diketopiperazines than for the noncyclic, monomeric tyrosine and tryptophan derivatives. This finding confirms that the diketopiperazines have fewer conformers than do the noncyclic derivatives. For diketopiperazines containing only a single aromatic residue, the conformer with the aromatic side chain folded over the diketopiperazine ring is dominant even at room temperature. In diketopiperazines having 2 aromatic residues with the l configuration, the most stable conformer has both aromatic side chains sharing the space over the diketopiperazine ring.