In-Situ Atomic Force Microscopy Observation of Enzymatic Degradation in Poly(hydroxyalkanoic acid) Thin Films: Normal and Constrained Conditions

Synthesis, properties and applications of biodegradable polymers derived from diols and dicarboxylic acids: from polyesters to poly(ester amide)s

Poly(alkylene dicarboxylate)s constitute a family of biodegradable polymers with increasing interest for both commodity and speciality applications. Most of these polymers can be prepared from biobased diols and dicarboxylic acids such as 1,4-butanediol, succinic acid and carbohydrates. This review provides a current status report concerning synthesis, biodegradation and applications of a series of polymers that cover a wide range of properties, namely, materials from elastomeric to rigid characteristics that are suitable for applications such as hydrogels, soft tissue engineering, drug delivery systems and liquid crystals. Finally, the incorporation of aromatic units and α-amino acids is considered since stiffness of molecular chains and intermolecular interactions can be drastically changed. In fact, poly(ester amide)s derived from naturally occurring amino acids offer great possibilities as biodegradable materials for biomedical applications which are also extensively discussed.

Enzymatic processes for biodegradation of poly(hydroxyalkanoate)s crystals

Poly(hydroxyalkanoate)s (PHAs) have attracted much attention as environmentally compatible polymeric materials that can be produced from renewable carbon resources. Biodegradation of PHA materials occurs by the function of extracellular PHA depolymerase secreted from microorganisms. Thus, elucidation of the enzymatic degradation mechanism for PHA materials is important to design PHA materials with desirable properties and controlled biodegradability. The solid PHA polymer is a water-insoluble substrate but PHA depolymerases are soluble in water. Therefore, the enzymatic degradation of PHA materials is a heterogeneous reaction on the material’s surface. Two distinct processes are involved during the degradation, namely, adsorption of the enzyme on the surface of PHA material and the subsequent hydrolysis of polymer chains. Atomic force microscopy (AFM) is a powerful tool that has been used for the quantitative analysis of PHA crystal degradation. AFM enables the characterization of the crystal surface nanostructure in a buffer solution. By using in-situ (real-time) AFM observations, we recently succeeded in observing the degradation processes of PHA crystals. Subsequently, we were also able to investigate the degradation rates of PHA crystals using the same technique. In this review, we have attempted to give an overview concerning the direct visualization of the adsorption, as well as the hydrolysis reactions of PHA depolymerases at the nanometer scale. In addition, we present other analytical techniques besides AFM as a complimentary approach to analyze the effect of enzyme adsorption on PHA crystals.Key words: poly(hydroxyalkanoate) (PHA), enzymatic degradation, lamellar crystal, PHA depolymerase.

Dynamic Adsorption Behavior of Poly(3-hydroxybutyrate) Depolymerase onto Polyester Surface Investigated by QCM and AFM

Time-dependent adsorption behavior of poly(3-hydroxybutyrate) (PHB) depolymerase from Ralstonia pickettiiT1 on a polyester surface was studied by complementary techniques of quarts crystal microbalance (QCM) and atomic force microscopy (AFM). Amorphous poly(l-lactide) (PLLA) thin films were used as adsorption substrates. Effects of enzyme concentration on adsorption onto the PLLA surface were determined time-dependently by QCM. Adsorption of PHB depolymerase took place immediately after replacement of the buffer solutions with the enzyme solutions in the cell, followed by a gradual increase in the amount over 30 min. The amount of PHB depolymerase molecules adsorbed on the surface of amorphous PLLA thin films increased with an increase in the enzyme concentration. Time-dependent AFM observation of enzyme molecules was performed during the adsorption of PHB depolymerase. The phase response of the AFM signal revealed that the nature of the PLLA surface around the PHB depolymerase molecule was changed due to the adsorption function of the enzyme and that PHB depolymerase adsorbed onto the PLLA surface as a monolayer at a lower enzyme concentration. The number of PHB depolymerase molecules on the PLLA surface depended on the enzyme concentration and adsorption time. In addition, the height of the adsorbed enzyme was found to increase with time when the PLLA surface was crowded with the enzymes. In the case of higher enzyme concentrations, multilayered PHB depolymerases were observed on the PLLA thin film. These QCM and AFM results indicate that two-step adsorption of PHB depolymerase occurs on the amorphous PLLA thin film. First, adsorption of PHB depolymerase molecules takes place through the characteristic interaction between the binding domain of PHB depolymerase and the free surface of an amorphous PLLA thin film. As the adsorption proceeded, the surface region of the thin film was almost covered with the enzyme, which was accompanied by morphological changes. Second, the hydrophobic interactions among the enzymes in the adlayer and the solution become more dominant to stack as a second layer.

Enzymatic degradation processes of lamellar crystals in thin films for poly[(R)-3-hydroxybutyric acid] and its copolymers revealed by real-time atomic force microscopy

Enzymatic degradation processes of flat-on lamellar crystals in melt-crystallized thin films of poly[(R)-3-hydroxybutyric acid] (P(3HB)) and its copolymers were characterized by real-time atomic force microscopy (AFM) in a phosphate buffer solution containing PHB depolymerase from Ralstonia pickettii T1. Fiberlike crystals with regular intervals were generated along the crystallographic a axis at the end of lamellar crystals during the enzymatic degradation. The morphologies and sizes of the fiberlike crystals were markedly dependent on the compositions of comonomer units in the polyesters. Length, width, interval, and thickness of the fiberlike crystals after the enzymatic degradation for 2 h were measured by AFM, and the dimensions were related to the solid-state structures of P(3HB) and its copolymers. The width and thickness decreased at the tip of fiberlike crystals, indicating that the enzymatic degradation of crystals takes place not only along the a axis but also along the b and c axes. These results from AFM measurement were compared with the data on crystal size by wide-angle X-ray diffraction, and on lamellar thickness and long period by small-angle X-ray scattering. In addition, the enzymatic erosion rate of flat-on lamellar crystals along the a axis was measured from real-time AFM height images. A schematic glacier model for the enzymatic degradation of flat-on lamellar crystals of P(3HB) by PHB depolymerase has been proposed on the basis of the AFM observations.