ON THE ROLE OF FORBIDDEN LOW-LYING EXCITED STATES OF LIGHT-HARVESTING CAROTENOIDS IN ENERGY TRANSFER IN PHOTOSYNTHESIS*

Abstract—It is proposed that the low-lying. electric-dipole forbidden A_g excited electronic state recently discovered in β-carotene acts as the energy donor in Förster-type excitation transfer to antenna Chl molecules folbwing β-carotene's light-harvesting function in photosynthesis. The mechanism and conclusion should apply as well to all carotenoid accessory pigments.     

Incorporating water-release and lateral protein interactions in modeling equilibrium adsorption for ion-exchange chromatography

The equilibrium adsorption of two albumin proteins on a commercial ion exchanger has been studied using a colloidal model. The model accounts for electrostatic and van der Waals forces between proteins and the ion exchanger surface, the energy of interaction between adsorbed proteins, and the contribution of entropy from water-release accompanying protein adsorption. Protein-surface interactions were calculated using methods previously reported in the literature. Lateral interactions between adsorbed proteins were experimentally measured with microcalorimetry. Water-release was estimated by applying the preferential interaction approach to chromatographic retention data. The adsorption of ovalbumin and bovine serum albumin on an anion exchanger at solution pH>pI of protein was measured. The experimental isotherms have been modeled from the linear region to saturation, and the influence of three modulating alkali chlorides on capacity has been evaluated. The heat of adsorption is endothermic for all cases studied, despite the fact that the net charge on the protein is opposite that of the adsorbing surface. Strong repulsive forces between adsorbed proteins underlie the endothermic heat of adsorption, and these forces intensify with protein loading. It was found that the driving force for adsorption is the entropy increase due to the release of water from the protein and adsorbent surfaces. It is shown that the colloidal model predicts protein adsorption capacity in both the linear and non-linear isotherm regions, and can account for the effects of modulating salt.     

Flow microcalorimetric measurements for bovine serum albumin on reversed-phase and anion-exchange supports under overloaded conditions

Heat of adsorption data using flow microcalorimetry is reported for the adsorption of bovine serum albumin (BSA) on C18 and C4 chromatographic supports. Exothermic heats were obtained in all cases. Data for the effect of salt indicate that conformational changes in adsorbed protein appear to be greatest in the absence of salt. Also, the specific surface area of the support was found to influence behavior more strongly than the length of the carbon ligand. Heats of adsorption of BSA on an ion-exchange support were also measured. Endothermic heats were obtained in all cases. The data indicate that the observed heat effects may be strongly influenced by the release of water from the surface.     

Zn(II), Ni(II), Cu(II), and Fe(III) complexes of potentially bimetalating tris(pyridine- and imidazole-appended) picket-fence naphthylporphyrins with benzyl ether spacers: implications for cytochrome c oxidase active-site modeling

Two new unsymmetrical picket-fence naphthylporphyrin ligands, 1 and 2, and several of their metalated porphyrinato complexes have been synthesized as precursor model compounds for the binuclear (Fe/Cu) cytochrome c oxidase (CcO) active site. 1 and 2 have a naphthylporphyrin superstructure that has been specifically incorporated to confer long-term configurational stability to the atropisomeric products. The two picket-fence porphyrin ligands also bear covalently linked, axially offset tris(heterocycle) coordination sites for a copper ion, much like that found in the native enzyme. Monometallic porphyrin complexes [M = Zn(II), Ni(II), Cu(II), and Fe(III)] of the pyridine-appended ligand 1 have been prepared and spectroscopically and magnetically characterized. An unusual monomeric iron(III) hydroxo porphyrin complex was isolated upon workup of the compound formed under ferrous sulfate/acetic acid reflux conditions. There is general difficulty in forming binuclear complexes of 1, which is attributed to the conformational flexibility of the benzyl ether type picket spacers. The potential of ligands such as 1 and 2 for future CcO active-site modeling studies is considered.