کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
1231489 1495266 2013 13 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Spectral shift mechanisms of chlorophylls in liquids and proteins
موضوعات مرتبط
مهندسی و علوم پایه شیمی شیمی آنالیزی یا شیمی تجزیه
پیش نمایش صفحه اول مقاله
Spectral shift mechanisms of chlorophylls in liquids and proteins
چکیده انگلیسی

Origins of non-excitonic spectral shifts of chlorophylls that can reach −1000 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 (±2 Å3) increase of polarizability Δα is close to 15 Å3 for S1 bands of Chl a, BChl a, and BPheo a, slightly larger for Chl b (18 Å3), and less for Pheo a (11 Å3). 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 S1 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 (∼2 D and 0.1 ± 0.1 D, 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 S1 band, whereas the S2 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 S1 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.

Figure optionsDownload as PowerPoint slideHighlights
► Solvent shift mechanisms were studied in spectra of chlorophylls.
► Specific and non-specific effects are distinguished.
► Transition energies are obtained for pigments in solvent-free state.
► Polarizability increases upon excitation.
► Bacteriochlorophyll is sensitive to aromatic character of solvent.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy - Volume 102, February 2013, Pages 301–313
نویسندگان
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