Spectral Diffusion Experiment with a Denatured Protein

Parameters of the protein energy landscapes of several light-harvesting complexes probed via spectral hole growth kinetics measurements

The parameters of barrier distributions on the protein energy landscape in the excited electronic state of the pigment/protein system have been determined by means of spectral hole burning for the lowest-energy pigments of CP43 core antenna complex and CP29 minor antenna complex of spinach Photosystem II (PS II) as well as of trimeric and monomeric LHCII complexes transiently associated with the pea Photosystem I (PS I) pool. All of these complexes exhibit sixty to several hundred times lower spectral hole burning yields as compared with molecular glassy solids previously probed by means of the hole growth kinetics measurements. Therefore, the entities (groups of atoms), which participate in conformational changes in protein, appear to be significantly larger and heavier than those in molecular glasses. No evidence of a small (∼1 cm(-1)) spectral shift tier of the spectral diffusion dynamics has been observed. Therefore, our data most likely reflect the true barrier distributions of the intact protein and not those related to the interface or surrounding host. Possible applications of the barrier distributions as well as the assignments of low-energy states of CP29 and LHCII are discussed in light of the above results.

Myoglobin with chlorophyllous chromophores: influence on protein stability

The stabilities of myoglobin, apo-myoglobin, and of two myoglobins with chlorophyllous chromophores (Zn-pheophorbide a and Zn-bacteriopheophorbide a), have been studied by thermal and chemical denaturation. With guanidinium chloride, the stability order is myoglobin>Zn-pheophorbide-myoglobin>Zn-bacteriopheophorbide-myoglobin approximately apo-myoglobin. The thermal behavior is more complex. The transition temperature of thermal unfolding of the apoprotein (62.4 degrees C) is increased by Zn-pheophorbide a (83.9 degrees C) and Zn-bacteriopheophorbide a (82.6 degrees C) to a similar degree as by the native chromophore, heme (83.5 degrees C). The recovery with Zn-pheophorbide (92-98%) is even higher than with heme (74-76%), while with Zn-bacteriopheophorbide (40%) it is as low as with the apoprotein (42%). Recovery also depends on the rates of heating, and in particular the time spent at high temperatures. It is concluded that irreversibility of unfolding is related to loss of the chromophores, which are required for proper re-folding.

Investigation of spectral diffusion in ribonuclease by photolabeling of intrinsic aromatic amino acids

Spectral diffusion dynamics in ribonuclease A was observed via the broadening of photochemical holes burned into the absorption spectrum of intrinsic tyrosine residues. Unlike previous results based on hole burning of chromophores in the pockets of heme proteins, where spectral diffusion develops according to a power law in time, the dynamics in ribonuclease follow a logarithmic law. The results suggest that the experiment preferentially labels the tyrosines located on the surface of the protein where the two-level system dynamics of the glass host matrix exert a strong influence.

Myoglobin with modified tetrapyrrole chromophores: binding specificity and photochemistry

Complexes were prepared of horse heart myoglobin with derivatives of (bacterio)chlorophylls and the linear tetrapyrrole, phycocyanobilin. Structural factors important for binding are (i) the presence of a central metal with open ligation site, which even induces binding of phycocyanobilin, and (ii) the absence of the hydrophobic esterifying alcohol, phytol. Binding is further modulated by the stereochemistry at the isocyclic ring. The binding pocket can act as a reaction chamber: with enolizable substrates, apo-myoglobin acts as a 13(2)-epimerase converting, e.g., Zn-pheophorbide a' (13(2)S) to a (13(2)R). Light-induced reduction and oxidation of the bound pigments are accelerated as compared to solution. Some flexibility of the myoglobin is required for these reactions to occur; a nucleophile is required near the chromophores for photoreduction (Krasnovskii reaction), and oxygen for photooxidation. Oxidation of the bacteriochlorin in the complex and in aqueous solution continues in the dark.