Sinikka Parkkinen

Fractionation of natural and model egg-white protein solutions with modified and unmodified polysulfone UF membranes

Mostly, fractionation in laboratory experiments have been carried out with single or binary model protein solutions. The question has arisen whether these experiments can be representative for industrial and natural biological solution fractionations. In this study, a comparison of single, ternary, and natural egg-white solutions is made. Thus, fractionation of ternary mixtures of ovalbumin, conalbumin, and lysozyme and natural egg-white protein solutions at different pH and two ionic strengths was studied with unmodified and UV modified polysulfone ultrafiltration membranes. The modified membranes had an increased initial water flux and their zeta potentials were more negative than those of the unmodified membranes. The UV modified membranes became more hydrophilic due to the formation of carboxylate and sulphonate groups. In ultrafiltration of single protein solutions the highest flux reduction and the lowest protein retention were obtained with ovalbumin at its isoelectric point (pH 4.8). At this pH lysozyme and conalbumin were positively charged and highly retained because of effective size exclusion due to charge repulsion. Also, in fractionation of ternary mixtures and natural egg-white solutions, ovalbumin was the major protein that could permeate the membranes at pH 4.8. Ovalbumin was highly retained due to charge repulsion at all other tested pH values. Retention of ovalbumin and transmission of lysozyme increased in ultrafiltration of egg-white solutions in the presence of salt at pH 4.8. It could be seen clearly that the behaviour of ovalbumin being the most abundant protein, mainly determined the fractionation properties of the mixtures. Comparison of the results from fractionation of solutions of ternary model proteins and natural egg-whites showed that retention was lower and flux reduction smaller in UF of the natural egg-white solutions. This was probably due to interaction of proteins or/and salts presents in the natural egg-white solutions, which could not be modelled by the main protein components. On the whole it seemed as the proteins studied behaved much in the same way in mixtures as separately.

Color sensitive retina based on bacteriorhodopsin.

Bacteriorhodopsin (BR), a membrane protein of a microorganism Halobacterium salinarium has been studied since the 80s as a potential material for information technology. The information processing applications of BR employ either photochromic or photoelectric properties of the protein. In this study we discuss about design principles and describe our study of the use of bacteriorhodopsin as a sensor material for a color sensitive artificial retina. This retina includes low-level processing of input information. The design of a color sensitive matrix element, the self-organizing color adaptation algorithm and a system model for the retina are presented.

The photochromic properties of 4-keto bacteriorhodopsin

The all-trans-retinal of the native chromoprotein of bacteriorhodopsin (BR) purple membrane was replaced with the synthetic 4-keto retinal. Gelatin films were made from the reconstituted BR and the intensity-dependent basic optical properties were investigated. It is shown that at room temperature the M-state is composed of two different absorption bands (maximum at 413 nm and 435 nm) with different relaxation times. Unexpected low-intensity transmission properties which cannot be explained by the two-level model are reported. The M-state of the 4-keto BR is shown to be about 100 times slower than the M-state of the similar wild type BR-film.

Modified bacteriorhodopsins as a basis for new optical devices

The photochemical properties of various bacteriorhodopsin analogs in water suspensions and PVA films have been studied and their promise for technological purposes (especially as a basis for colour-imaging and -recognition devices) is shown.

Transient absorption and photovoltage study of' self-assembled bacteriorhodopsin/polycation multilayer films

A series of organized (PDAC/PM)(n) (poly(diallyldimethylammonium chloride)/purple membrane) multilayer films were prepared by alternate adsorptions of positively charged PDAC polyelectrolyte and negatively charged purple membrane (PM). The kinetics of the photocycle of bacteriorhodopsin (bR) in PM was studied by flash photolysis and transient photovoltage methods. Although the orientation of the adsorbed bR depends on the pH of the PM suspension, the kinetics of the photo-induced reaction cycle in dehydrated films is independent of the deposition pH. In dry (PDAC/PM)(n) films the decay of the M intermediate to the initial bR state is multiexponential and delayed to several minutes for both orientations. A simultaneous two-exponential decay in millisecond time domain was observed at red wavelengths. The source of the red-shifted absorption is suggested to be the C(610) intermediate of the cis photocycle of bR.

Color-sensitive biosensors for imaging

Research on an artificial retina is mainly concentrated on the design of silicon based integrated circuits, where the spatial structure of the circuit and the electrical design of a pixel plays the main role. Color sensitivity has not been an issue, or it is realized by using separate filters in front of the circuit. There are only few studies concerning an artificial retina based on a biomolecular structure. In this study we describe some optically active biomolecules, which are potential materials for construction of an artificial retina. We use bacteriorhodopsin and its variants. This protein is produced by a bacterium, Halobacterium salinarium. The protein is closely related to the light sensitive molecule of human retina. The studied molecules have different wavelength properties with each other and when mounted on a polyvinylalcohol film, they produce an electric signal from optical input. We have produced a set of protein films with different wavelength properties and we report the basic characteristics of these films, referring them as basic elements of an artificial retina. The design principles of a biomolecule based artificial retina is also discussed.

The intensity dependent refractive index change of photochromic proteins

We report the nonlinear refractive index behaviour of bacteriorhodopsin and photoactive yellow protein films. A Michelson interferometer with a probe and excitation beams was used in the measurements. By using this technique instead of the Z-scan we can easily separate the index change caused by photochromism and the thermal expansion. Sigmoidal index changes in the logarithmic intensity scale were observed.

Photoelectrical properties of protein-based optoelectronic sensor

Bacteriorhodopsin (BR) has been studied as a biomaterial for molecular computing applications. Thin film elements based on BR in polyvinylalcohol were prepared to determine the photoelectrical properties of the material for the development of an optoelectronic sensor. The properties were studied by registering the photovoltage in time, measuring the intensity, area and wavelength dependence of the photoelectric response, and evaluating the element quality. The thin film elements produce a stable photovoltage, the intensity and area dependencies are close to linear, and the wavelength dependence is closely related to the absorption spectrum of BR. The homogeneity of the element thin films is good based on the relatively small variance of the photoelectric response, thus it is feasible to continue the development of an artificial retina based on BR.

Nonlinear transmittance of the 4-keto bacteriorhodopsin

The photocycle of the 4-keto bacteriorhodopsin is investigated. We constructed a multilevel theoretical model for the nonlinear transmittance properties of the material. Adjusting the relaxation parameters we are able to fit the theoretical intensity dependent transmittance curves into the experiments and to determine the photocycle from simple optical measurements.

Photochromic properties of photoactive yellow protein

The basic intensity dependent optical properties of photoactive yellow protein were studied and shown to resemble to those of bacteriorhodopsin. Using a set of rate equations and a modified photocycle the transmittance behaviour was explained. Fitting the theoretical intensity dependent transmittance curves to the measurements was done by adjusting the photochemical parameters and the relaxation times. The model explains the experiments.