Joseph A. Akkara

Enzymatic synthesis and modification of polymers in nonaqueous solvents

Enzymes catalyse several reactions that are difficult to perform with chemical catalysts and that are important in the synthesis and modification of different polymers in organic solvents. In enzyme-based synthesis, alteration of the reaction medium can have a significant influence on the molecular weight, polydispersity, yield and architecture of the polymers that are produced. Modification of these macromolecules for industrial applications requires an understanding of the different reaction strategies involved.

Mirrorless all-optical bistability in bacteriorhodopsin

We report direct observations of all‐optical mirrorless bistability associated with saturable absorption in three kinds of bacteriorhodopsin (BR) samples: wild‐type BR in water solution and dispersed in thin films of a polymer matrix as well as water solution of the genetically engineered mutant BRD96N. The experiments are carried out with picosecond pulses at 532 nm. The values measured for the saturation intensity are explained in terms of the relaxation of the excited M state population to the B state of the BR photocycle for the three kinds of samples studied.

All-optical logic gates with bacteriorhodopsin films☆

We demonstrate an all-optical switch using molecular states in a bacteriorhodopsin (bR) thin film. All-optical logic gates are implemented with wild-type and chemically stabilized films of bacteriorhodopsin using a two-color backward degenerate four-wave mixing geometry. The films are observed to be stable over a period of four years. The sensitivity of each bR film is markedly different due to the large differences in the saturation intensity which can be controlled by different means. Red light is used to form a grating due to the B to M transition and blue light is used to form a grating due to the fast photochemical transition from M to B. Each of the two wavelengths in the experimental system acts as an input to the all-optical gate and the phase conjugate signal beam bears the output of the gate.

Optical Fourier processing using photoinduced dichroism in a bacteriorhodopsin film.

The photoinduced dichroism in bacteriorhodopsin films has been investigated in view of its application for optical Fourier processing. A simple optical system for Fourier processing is demonstrated that does not require precise alignment and vibration isolation. The basic principle of operation relies on the intensity dependence of photoinduced dichroism in a bacteriorhodopsin film. Experimental results with Fourier processing are shown for edge enhancement.

Solvent effects in horseradish peroxidase-catalyzed polyphenol synthesis

Abstract Enzyme-solvent-monomer molecular interactions and their effect on horseradish peroxidase-catalyzed polymerization of m-cresol in ethanol/water mixtures were studied. A mechanistic approach of the effect of reaction medium composition on the poly (m-cresol) molecular weight and polydispersity was elucidated from the standpoint molecular interactions. Solvent effects on the enzyme activity and structure were studied by reaction kinetics and spectroscopic methods including UV-vis, fluorescence, circular dichroism and electron paramagnetic spectroscopy (EPR). In view of the results from these studies, the observed polymer molecular weight profile could be deduced from the solubility of the polymer as well as the partitioning of the monomer between solvent and the enzyme active site.

All-optical light modulation in bacteriorhodopsin films

We report a convenient method to obtain all‐optical light modulation in bacteriorhodopsin films using a degenerate four‐wave mixing geometry. Chemically stabilized films of bacteriorhodopsin in a polymer matrix for which the lifetime of the excited M state is tens of seconds were used to demonstrate all‐optical light intensity modulation. The films are observed to be stable over a period of 4 years. Due to the sensitivity of the films, small intensities of order microwatt/ cm2 are required in the modulation experiments. Furthermore the fast photochemical transition from M to B permit reasonably fast modulation speeds independent of the slow thermal M and B relaxation time. The experimental system also acts as an all‐optical switch where a low power blue pulse turns on a signal red beam.

Ferrite synthesis in microstructured media: Template effects and magnetic properties

Inverse micelles and organogels provide novel environments to synthesize ferrite particles. The fluid microstructure provides a template for the synthesis. Our experiments with ferrite synthesis in inverse micelles indicate the formation of superparamagnetic nanoparticles. Of interest is the encapsulation of these particles in polymer microspheres. The encapsulation is done using simple polymer precipitation in the micellar nonsolvent. The process results in a polymer-ferrite composite exhibiting supermagnetism. Low temperature spin glass properties of the composite are characterized through SQUID measurements. These composites have a superparamagnetic blocking temperature of 16 K and follow Curie–Weiss law at temperatures above 60 K with the fitted parameters: C=0.941 emu/g K, θ=−287 K, and TIP=0.0001 emu/g. Since the polymer used is polyphenol, a highly functionalizable material, the composite is well suited for applications in magnetic bioseparations and magnetic coatings.

Enzymatic polymerization of amphiphilic alkyl tyrosine derivatives from emulsions

Abstract Horseradish peroxidase catalyzed polymerizations of amphiphilic derivatives of d - and l -isomers of tyrosine have been carried out in micellar solutions. The rate of polymerization has been found to be maximum at a pH of 6.1–6.2 and showed a second-order dependence on the monomer concentration. This enzyme shows stereospecificity towards the d -isomer compared to the l -isomer, which is consistent with the aqueous reaction with underivatized isomers of tyrosine. The resulting polymers, having a molecular weight of 2700, have been characterized by UV-Vis spectroscopy and 1 H NMR. The polymer shows surface activity at the air-water interface unlike the corresponding monomer, which aggregates under identical conditions.

Dispersion of the nonlinear absorption of copper phthalocyanine in a silica xerogel matrix through the visible spectrum

Recently, macrocyclic organic dyes have attracted a lot of attention as possible candidates for applications in optical power limiters and pulse shaping devices. We present results for the effective excited state absorption cross section of copper phthalocyanine tetrasulfonate (CuPcTs) in a CuPcTs/Silica xerogel composite. The sample was prepared by the sol-gel method using ultrasound energy to promote the hydrolysis of tetramethoxysilane which was the silica precursor. The study has been performed in the visible region of the spectrum from 450 to 730 nm. This study represents a complete description of the dispersion of the nonlinear absorption for the composite. Measurements were also obtained for a solution of the CuPc in sulfuric acid for comparison purposes. We observed reverse saturable absorption in the range of 450–540 nm wavelength and saturable absorption in the range 570–730 nm wavelength for the composite.

The morphology of phenolic polymers enzymatically synthesized in surfactant microstructures

Abstract Currently, there is much interest in materials synthesis through biomimetic pathways [1,2]. Biological and biomimetic routes to polymer and particle synthesis lead to materials with novel structural properties that can be replicated with rather high fidelity [3]. We report morphological characteristics of the enzymatic synthesis of phenolic polymers in the microstructured environment of water-in-oil microemulsions or reversed micelles. The enzyme, horseradish peroxidase, is encapsulated in the micelles. The synthesis appears to be an example of interfacial polymerization as the monomer partitions to the micelle oil-water interface. The final polymer has the interesting morphology of interconnected spherical particles. The microstructured environment appears to play an important role in morphology development, and a high surfactant to monomer ratio (> 3:1) allows the formation of spherical morphologies.