Circular Dichroism Spectroscopy with the SAC-CI Methodology: A ChiraSac Study

Abstract

Circular dichroism (CD) spectroscopy reflects sensitively the various chemistries involved in chiral molecules and molecular systems. The CD spectra are very sensitive to the conformational changes of molecules: the rotation around the single bond including a chiral atom. It is also sensitive to the changes in the stacking interactions in the chiral DNA and RNA. Since these changes are low-energy processes, we expect that the CD spectra include a lot of information of the chiral molecular systems. On the other hand, the SAC-CI method is a highly reliable excited-state theory and gives very reliable theoretical CD spectra. Therefore, by comparing the experimental CD spectra with the theoretical SAC-CI spectra calculated for various chemical situations, one can study various chemistries such as the nature of the weak interactions involved in chiral molecular systems and biology. Based on these facts, we are developing a new molecular technology called ChiraSac, a term combining chirality and SAC-CI, to study chiral molecular systems and the chemistry involved thereof. We utilize highly reliable SAC-CI method together with many useful quantum chemical methods involved in Gaussian suite of programs. In this chapter, we review our ChiraSac studies carried out to clarify the chemistries of some chiral molecules and molecular systems: large dependences of the CD spectra on the conformations of several chiral molecules in solutions and the effects of the stacking interactions of the nuclear-acid bases in DNA and RNA on the shapes of their CD spectra. The results of our several studies show that the ChiraSac is a useful tool for studying the detailed chemistry involved in chiral molecular and biological systems.