Pigment spectra and intermolecular interaction potentials in glasses and proteins

Spectral shift mechanisms of chlorophylls in liquids and proteins

Origins of non-excitonic spectral shifts of chlorophylls that can reach -1,000 cm(-1) in pigment-protein complexes are actively debated in literature. We investigate possible shift mechanisms, basing on absorption and fluorescence measurements in large number of liquids. Transition wavelength in solvent-free state was estimated (±2 nm) for chlorophyll a (Chl a, 647 nm), Chl b (624 nm), bacteriochlorophyll a (BChl a, 752 nm), and pheophytines. The dispersive-repulsive shift is a predominating mechanism. It depends on polarizability difference between the ground and the excited state Δα and the Lorenz-Lorentz function of refractive index of solvent (n). The approximate (± 2Å(3)) increase of polarizability Δα is close to 15Å(3) for S(1) bands of Chl a, BChl a, and BPheo a, slightly larger for Chl b (18Å(3)), and less for Pheo a (11Å(3)). The effect of solvent polarity, expressed in terms of static dielectric permittivity (ε) is relatively minor, but characteristic for different pigments and transitions. Remarkably, maximum influence of ε on S(1) band of BChl a is less (-20 ± 10 cm(-1)) than that for Chl a (-50 ± 10 cm(-1)), and not correlated with dipole moment changes on excitation Δμ (∼2D and 0.1 ± 0.1D, respectively). Hydrogen bonding in protic solvents produces red shifts in Chl a (-60 cm(-1)) and BChl a (-100 cm(-1)), but not in Chl b. Second axial ligand of BChl a has no influence on the S(1) band, whereas the S(2) transition suffers a -400 to -600 cm(-1) down shift. Aromatic character of solvent is responsible for a ∼-100 cm(-1) red shift of both Q transitions in BChl a. The S(1) bands in chlorophylls are relatively insensitive with respect to dielectric properties and specific solvation. Therefore, nontrivial mechanisms, yielding large site-energy shifts are expected in photosynthetic chlorophyll-proteins.

Hole-burning spectroscopy as a probe of nano-environments and processes in biomolecules: a review

Hole-burning spectroscopy, a high-resolution spectroscopic technique, allows details of heterogeneous nano-environments in biological systems to be obtained from broad absorption bands. Recently, this technique has been applied to proteins, nucleic acids, cells, and substructures of water to probe the electrostatic conditions created by macromolecules and the surrounding solvent. Starting with the factors that obscure the homogeneous linewidth of a chromophore within an inhomogeneously broadened absorption or emission band, we describe properties and processes in biological systems that are reflected in the measured hole spectra. The technique also lends itself to the resolution of perturbation experiments, such as temperature cycling to elucidate energy landscape barriers, applied external electric fields (Stark effect) to measure net internal electric fields, and applied hydrostatic pressure to find the volume compressibility of proteins.