Graduate Thesis Or Dissertation
 

Interpreting the circular dichroism of cyclic dimers of amino acids

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  • Independent systems theory has been used to interpret the circular dichroism (CD) spectra in the vacuum uv of three cyclic dimers of amino acids. First, the theory for rotational strengths is developed in terms of degenerate and nondegenerate group functions. All terms to first order in perturbation theory are kept. The theory is then used to calculate the CD due to the nπ₋, π0π₋, nσ*, π0σ* and π₊π₋ transitions on the amide groups. The nπ₋ and π0π₋ are assigned to the three long wavelength bands in L-prolyl-L-proline diketopiperazine (PPDKP). In the L-alanyl-glycine diketopiperazine (AGDKP) and L-alanyl-L-alanine diketopiperazine (AADKP), the first two bands are assigned to π0π₋.The intense bands below 180 nm in all three molecules cannot be explained by the transitions on the amide groups. Thus, a study of σσ* transitions is carried out using alkane compounds as models for the diketopiperazines. Using degenerate perturbation theory, the energies and rotational strengths for these alkane compounds are calculated. The resulting CD curves allow the assignment of the 140 nm and 160 nm bands in PPDKP and the 140 nm band in AGDKP and AADKP to σσ* transitions. Through the interaction of the π0π₋ transition with the 180 nm σσ* transitions in PPDKP the intense band at 180 nm is assigned to σσ*. The explicit interaction of the various σσ* transitions with the π0π₋ transition provides a nonconservative part to the π0π₋ CD curve of correct sign and magnitude for the three molecules. The polarizability approximation gives the wrong sign to this part in the case of AGDKP and AADEP.
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