Thermal aggregation of bovine serum albumin (BSA) has been studied using dynamic light scattering, asymmetric flow field-flow fractionation and analytical ultracentrifugation. The studies were carried out at fixed temperatures (60°C, 65°C, 70°C and 80°C) in 0.1 M phosphate buffer, pH 7.0, at BSA concentration of 1 mg/ml. Thermal denaturation of the protein was studied by differential scanning calorimetry. Analysis of the experimental data shows that at 65°C the stage of protein unfolding and individual stages of protein aggregation are markedly separated in time. This circumstance allowed us to propose the following mechanism of thermal aggregation of BSA. Protein unfolding results in the formation of two forms of the non-native protein with different propensity to aggregation. One of the forms (highly reactive unfolded form, Uhr) is characterized by a high rate of aggregation. Aggregation of Uhr leads to the formation of primary aggregates with the hydrodynamic radius (Rh,1) of 10.3 nm. The second form (low reactive unfolded form, Ulr) participates in the aggregation process by its attachment to the primary aggregates produced by the Uhr form and possesses ability for self-aggregation with formation of stable small-sized aggregates (Ast). At complete exhaustion of Ulr, secondary aggregates with the hydrodynamic radius (Rh,2) of 12.8 nm are formed. At 60°C the rates of unfolding and aggregation are commensurate, at 70°C the rates of formation of the primary and secondary aggregates are commensurate, at 80°C the registration of the initial stages of aggregation is complicated by formation of large-sized aggregates.

Structural properties of the isolated extrinsic regulatory 33 kDa protein of the water-oxidizing complex were analyzed at different pH values. It was found that (a) titrations of the buffer capacity reveal a characteristic hysteresis effect that is unique for the 33 kDa subunit and is not observed for the other extrinsic proteins, (b) changes of the emission from the fluorescence probe 1,8-ANS are indicative of an increased accessibility of the hydrophobic core of the 33 kDa protein to the dye at lower pH, (c) the near-UV circular dichroism spectrum of the polypeptide is altered owing to a pH decrease from 6.8 to 3.8 and becomes drastically changed at pH 2.8, and (d) the content of secondary structure elements remains virtually constant in the range 3.8 < pH < 6.8, with the following values gathered from far-UV CD spectra: approximately 8% alpha-helix, approximately 33% beta-strand, approximately 15% turns, and approximately 44% random coil. Further acidification down to pH 2.8 gives rise to a decreased alpha-helix and increased beta-strand and random coil content. A theoretical model [Ptitsyn, O., & Finkelstein, A. (1983) Biopolymers 2, 15-22] was used to predict the probability and location of secondary structure elements within the protein sequence. On the basis of these calculations, an extended hydrophobic beta-sheet domain could exist in the center of the protein and an alpha-helix in the C-terminal region. From these data, the 33 kDa protein is inferred to change its tertiary structure in vitro upon acidification of the aqueous environment. Possible implications of these features are discussed.

Colloidal two-dimensional cadmium chalcogenides nanoplatelets have recently emerged as a class of semiconductor nanoparticles with the narrowest emission and absorption excitonic bands that are of interest for optical applications. Here, we have developed a synthesis protocol for 2.5-monolayer-(ML) thick CdSe nanosheets as a single population. We found that a two-step synthesis in the presence of water promoted the growth of atomically thin nanosheets with high structural and morphological perfection. Using a seeded-growth technique, we extended the lateral size of nanosheets up to 400 nm, which led to the formation of multiwall rolled-up nanostructures. Ligand exchange of native oleic acid, attached to Cd-rich (001) planes, with achiral thioglycolic acid and chiral N-acetylcysteine retains a scroll-like morphology of nanosheets, in contrast to a thicker 3.5 ML population. A reorientation from the [110] to [100] folding direction was found during the change from an achiral to a chiral ligand. In the case of ligand exchange with chiral N-acetyl-l- or d-cysteine, we demonstrated that 2.5 ML CdSe nanosheets with 400 nm lateral size have circular dichroism with a dissymmetry g factor up to 3 × 10–3. Strong circular dichroism found for colloidal CdSe nanosheets makes them a promising candidate for polarization-enabled applications, while the growth protocol of the thinnest CdSe nanosheets enriches the known synthesis methods of a set of CdSe nanoplatelet populations.

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Oligomeric and monomeric forms of chlorophyll-protein complexes of photosystem I (PSI) have been isolated from the mesophilic cyanobacterium Spirulina [(1992) FEBS Lett. 309, 340-342]. Electron microscopic analysis of the complexes showed that the oligomeric form is a trimer of the shape and dimensions similar to those of the trimer from thermophilic cyanobacteria. The chlorophyl ratio in the isolated trimer and monomer was found to be 7:3. The trimeric form of PSI complex in contrast to the monomeric one contains the chlorophyll emitting at 760 nm (77K), which is also found in Spirulina membranes and therefore could be used as an intrinsic probe for the trimeric complex. The 77K circular dichroism spectrum of the trimeric form is much more similar to that of Spirulina membranes than the spectrum of the monomer. Thus, the trimeric PSI complexes exist and dominate in the Spirulina membranes.

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Core antenna and reaction centre of photosystem I (PS I) complexes from the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus have been characterized by steady-state polarized absorption spectroscopy, including linear dichroism (LD) and circular dichroism (CD). CD spectra and the second derivatives of measured 77 K CD spectra reveal the spectral components found in the polarized absorption spectra indicating the excitonic origin of the spectral forms of chlorophyll in the PS I complexes. The CD bands at 669-670(+), 673(+), 680(-), 683-685(-), 696-697(-), and 711(-) nm are a common feature of used PSI complexes. The 77 K CD spectra of the trimeric PS I complexes exhibit also low amplitude components around 736 nm for A. platensis and 720 nm for T. elongatus attributed to red-most chlorophylls. The LD measurements indicate that the transition dipole moments of the red-most states are oriented parallel to the membrane plane. The formation of P700(+)A(1)(-) or (3)P700 was monitored by time-resolved difference absorbance and LD spectroscopy to elucidate the spectral properties of the PS I reaction centre. The difference spectra give strong evidence for the delocalization of the excited singlet states in the reaction centre. Therefore, P700 cannot be considered as a dimer but should be regarded as a multimer of the six nearly equally coupled reaction centre chlorophylls in accordance with structure-based calculations. On the basis of the results presented in this work and earlier work in the literature it is concluded that the triplet state is localized most likely on P(A), whereas the cation is localized most likely on P(B).