The two neutral His that coordinate the BChls appeared to have the NMR signatures of a double-protonated, i.e. positively charged His (Alia et al. 2001, 2004). This was explained by a charge selleck chemicals llc transfer in the ground state between the His and their coordinating BChl, resulting in a partial positive charge on the His imidazoles. In density functional theory (DFT) modeling, the effect would disappear if the BChl-His geometry was optimized beforehand, but was clearly present when the coordinates were taken directly from the X-ray structure (Wawrzyniak et al. 2008). Running a geometry optimization would increase the distance between the His and the BChl from 2.12 to 2.31 Å.
One could argue that this moderate change falls within the error of the X-ray spatial resolution, and indeed the sensitivity of the NMR chemical shifts to electronic effects, which might be induced by small spatial PD0332991 solubility dmso re-arrangements, exceeds the resolution LY2109761 clinical trial of the X-ray crystallographic structures. The LH2 His model explained the electronic effects of charge transfer by mechanical stress, induced by
the protein conformational constraints in the LH2 oligomer packing. It was speculated that the His-BChl charge transfer could have an effect on the light-harvesting properties. A more clear example how a coordinating His may control the chromophore function was found for the special pair of photosystem II. Here, the inverted electronic charge of the Chl nitrogens in the special pair was explained by a hinge model, in which the coordinating His imidazole ring hangs over the Chl macrocycle, altering its electronic structure in the ground state and its oxidation state compared to PSI (Diller et
al. 2007). The light-harvesting complex 2 as an NMR model; the BChl pigments In addition to the protein chemical shifts, NMR assignments were obtained for the BChl-conjugated macrocycles of the three types of BChl in LH2: the α- and β-bound BChls that build a ring of BChl dimers, called the B850 GPCR & G Protein inhibitor band, and the so-called B800 BChls that form a ring of monomers (van Gammeren et al. 2005a). To discriminate between the B850 and B800 signals, a sample was prepared from unlabeled LH2 of which the B800 BChls were extracted and substituted with uniformly labeled BChls. The three types of BChls have a distinctive set of chemical shifts, reflecting their conformational structures and variation in the local protein environment. The differences between the NMR signals in the protein-bound BChls and free BChl in organic solvent Δσ determine the electronic structures in the ground state. Recently, the data set was expanded with the BChl assignments of the acidophila LH1 complex, the core antenna that forms a ring-shaped oligomer of dimer αβ subunits surrounding the photosynthetic reaction center (RC) (Pandit et al. 2010a).