2008). It might also be of use in Stark spectroscopy experiments on isolated and non-randomly aligned complexes, e.g., in oriented lamellar aggregates. (Stark spectroscopy deals with the effects of applied electric fields on the absorption or emission spectrum of a molecule (Boxer 1996).)

The dependency of the so-called electrochromic absorbance changes on the orientation of the molecules arises from the fact that the field-induced frequency shift of a given absorbance band depends on the relative orientation of the field vector and AZD5582 the transition dipole moment vector of the molecule; in molecules possessing permanent dipole moments, it also depends on the difference between the ground- and excited-state polarizability of the field-indicating pigment molecules (Junge 1977). The orientations of the transition dipole moments are functionally very important: they strongly influence the rates and the routes of excitation energy transfer in the pigment system, which depends on the mutual orientation of the transition dipoles of the acceptor and donor molecules (Van Grondelle et al. 1994). With regard to the excitation energy distribution, excitonically coupled molecules, which usually give rise to characteristic CD bands (see below), and influence the absorbance and

fluorescence properties, are of special interest. Since these also depend on the mutual orientation of the corresponding transition dipoles of the interacting molecules, LD data are also of paramount importance in this respect. Circular dichroism Circular dichroism (CD) refers to the phenomenon where the left- and right-handed circularly polarized light are absorbed to a different extent. CD is drug discovery usually defined as the (wavelength-dependent)

difference in absorption of the left- and the right-handed circularly polarized light: CD = A L − A R. CD arises from the intra- or intermolecular asymmetry (helicity) of the molecular structure. The helicity (chirality or handedness) of the structure means that it cannot be superimposed on its mirror image. As the handedness of a structure is the same from any direction, CD can be observed in randomly oriented samples. (In fact, the general theories are given for spatially averaged samples.) CD signals can originate from different molecular systems of different complexity, and they can give rise to different bands of different physical origins: BCKDHB (i) In the basic case, CD arises from intrinsic asymmetry or the asymmetric perturbation of a molecule (Van Holde et al. 1998). For a Dinaciclib supplier single electronic transition, CD has the same band shape as the absorption, and its sign is determined by the handedness of the molecule (often referred to as positive or negative Cotton effect). (ii) In molecular complexes or small aggregates, CD is generally induced by short-range, excitonic coupling between chromophores (Tinoco 1962; DeVoe 1965). Excitonic interactions give rise to a conservative band structure (i.e.