Enzymatic Degradation of Biodegradable Polyester Composites of Poly(L‐lactic acid) and Poly(ε‐caprolactone)

Summary: Two different types of biodegradable polyester composites, PLLA fiber‐reinforced PCL and PCL/PLLA blend films were prepared at PCL/PLLA ratio of 88/12 (w/w), together with pure PCL and PLLA films. Their enzymatic degradation was investigated by the use of Rhizopus arrhizus lipase and proteinase K as degradation enzymes for PCL and PLLA chains, respectively. In the FRP film, the presence of PLLA fibers accelerated the lipase‐catalyzed enzymatic degradation of PCL matrix compared with that in the pure PCL film, whereas in the blend film, the presence of PLLA chains dissolved in the continuous PCL‐rich domain retarded the lipase‐catalyzed enzymatic degradation of PCL chains. In contrast, in the FRP film, the proteinase K‐catalyzed enzymatic degradation of PLLA fibers was disturbed compared with that of the pure PLLA film, whereas in the blend film, the proteinase K‐catalyzed enzymatic degradation rate of particulate PLLA‐rich domains was higher than that of pure PLLA film. The reasons for aforementioned enhanced and disturbed enzymatic degradation are discussed.

Normalized PCL weight loss of pure PCL, FRP, and blend films as a function of Rhizopus arrhizus lipase‐catalyzed enzymatic degradation time.     

Intestinal absorption of dietary carotenoids

The assessment of carotenoid bioavailability has long been hampered by the limited knowledge of their absorption mechanisms. However, recent reports have elucidated important aspects of carotenoid digestion and absorption. Disruption of food matrix and increasing amounts of fat seem to enhance the absorption of carotenes to a larger extent than that of xanthophylls. Comparing different carotenoid species, xanthophylls seem to be more easily released from the food matrix and more efficiently micellized than the carotenes. On the other hand, carotenes are more efficiently taken up by the enterocytes. However, carotenoid emulsification and micellization steps are largely affected by the food matrix and dietary components, being the main determinant of carotenoid bioavailability from foodstuffs. Although the intestinal uptake of carotenoids has been thought to occur by simple diffusion, recent studies reported the existence of receptor-mediated transport of carotenoids in enterocytes. Comparisons between the intestinal absorption of a wide array of carotenoids would be useful to elucidate the absorption mechanism of each carotenoid species, in view of the recent indications that intestinal carotenoid uptake may involve the scavenger receptor class B type I and possibly other epithelial transporters. The unraveling of the whole mechanism underlying the absorption of carotenoids will be the challenge for future studies.     

Less Carcinogenic Chlorinated Estrogens Applicable to Hormone Replacement Therapy

Human estrogens prescribed for hormone replacement therapy (HRT) are known to be potent carcinogens. To find safer estrogens, several chlorinated estrogens were synthesized and their carcinogenic potential were determined. A pellet containing either 2-chloro-17β-estradiol (2-ClE₂) or 4-chloro-17β-estradiol (4-ClE₂) was implanted subcutaneously for 52 weeks into August Copenhagen Irish (ACI) rats, a preferred animal model for human breast cancer. 17β-Estradiol (E₂) frequently induced mammary tumors while both 2-ClE₂ and 4-ClE₂ did not. Their 17α-ethinyl forms, thought to be orally active estrogens, were also synthesized. Neither 2-chloro-17α-ethinylestradiol (2-ClEE₂) nor 4-chloro-17α-ethinylestradiol (4-ClEE₂) induced tumors. The less carcinogenic effects were supported by histological examination of mammary glands of ACI rats treated with the chlorinated estrogens. A chlorine atom positioned at the 2- or 4-position of E₂ may prevent the metabolic activation, resulting in reducing the carcinogenicity. 2-ClE₂ and 4-ClE₂ administered subcutaneously and 2-ClEE₂ and 4-ClEE₂ given orally to ovariectomized rats all showed uterotrophic potency, albeit slightly weaker than that of E₂. Our results indicate that less carcinogenic chlorinated estrogens retaining estrogenic potential could be safer alternatives to the carcinogenic estrogens now in use for HRT.     

Hetero-stereocomplex formation of stereoblock copolymer of substituted and non-substituted poly(lactide)s

The crystallization behavior of the stereoblock copolymer of substituted and non-substituted poly(lactide)s, i.e., poly(d-2-hydroxybutyrate) and poly(l-lactide) chains having the opposite configurations [P(D-2HB)-b-PLLA] and the reference block copolymer of poly(d-2-hydroxybutyrate) and poly(d-lactide) chains with the identical configurations [P(D-2HB)-b-PDLA] was investigated. At the crystallizable temperature range of 60-160 °C, the crystallized P(D-2HB)-b-PLLA contained solely the hetero-stereocomplex crystallites as a crystalline species, without formation of poly(d-2-hydroxybutyrate) or poly(l-lactide) homo-crystallites, in contrast with their polymer blends. On the other hand, at the crystallizable temperature range of 60-140 °C, the crystallized P(D-2HB)-b-PDLA had only PDLA homo-crystallites as crystalline species, reflecting no co-crystallites formation between poly(d-2-hydroxybutyrate) and poly(d-lactide) chains having the same configurations. The equilibrium melting temperature of hetero-stereocomplex crystallites in P(D-2HB)-b-PLLA was 189.0 °C, which was higher than 171.3 °C of PDLA homo-crystallites in P(D-2HB)-b-PDLA. Although the final crystallinity of P(D-2HB)-b-PLLA was higher than those of P(D-2HB)-b-PDLA, the spherulite growth rate of P(D-2HB)-b-PLLA was lower.The regime analysis indicated unusual nucleation mechanism of P(D-2HB)-b-PLLA.     

Inhibition effect of CO on hydrogen permeability of Pd–Ag membrane applied in a microchannel module configuration

The surface adsorption effect of CO on the hydrogen permeability of a 12.5 micron-thick Pd₇₇Ag₂₃ membrane has been evaluated quantitatively under experimental conditions close to the operating conditions of the highly-efficient membrane reformer (MRF) system developed by Tokyo Gas. The permeability of the membrane was measured in the conditions of CO concentration between 1 and 5 vol.% at a temperature and pressure of up to 500 °C and 0.6 MPa, respectively. High feed flow rates and a microchannel module configuration were applied in the flux measurements to ensure that the results are obtained with limited influence of concentration polarization adjacent to the membrane surface and hydrogen depletion along the microchannel length. While the CO inhibition effect was close to negligible at 500 °C, it was significant at lower temperatures. At a feed pressure of 0.2 MPa, the CO inhibition effect was only 0.2% at a CO concentration of 1 vol.% and the effect was 3.6% at a CO concentration of 5 vol.% at 500 °C. The CO inhibition effect were 3.4% for 1 vol.% CO and 14.1% for 5 vol.% CO at 400 °C. Measurements were also carried out at a high feed pressure of 0.6 MPa to evaluate the pressure dependence of the CO inhibition effect. The CO inhibition effect decreased to 0.7% at a CO feed concentration of 5 vol.% at 500 °C. Lower CO inhibition effect were also observed at 450 and 400 °C compared to the data obtained with the feed pressure of 0.2 MPa, while the inhibition levels were almost the same at 350 °C. Though the CO inhibition effect is larger at a lower feed pressure of 0.2 MPa, the effect was only 0.2% at 1 vol.% CO at 500 °C, which is close to the operating conditions of the MRF system. This study quantitatively revealed that the CO inhibition effect on hydrogen flux is extremely small when the membrane is operated at temperatures equal to or higher than 500 °C, even for state-of-the-art thin membranes. The performance of the Tokyo Gas MRF seems thus mainly limited by concentration polarization effects.     

Poly(L‐lactide). X. Enhanced surface hydrophilicity and chain‐scission mechanisms of poly(L‐lactide) film in enzymatic,alkaline, and phosphate‐buffered solutions

Attempts were carried out to enhance the surface hydrophilicity of poly(L ‐lactide), that is, poly(L ‐lactic acid) (PLLA) film, utilizing enzymatic, alkaline, and autocatalytic hydrolyses in a proteinase K/Tris–HCL buffered solution system (37°C), in a 0.01N NaOH solution (37°C), and in a phosphate‐buffered solution (100°C), respectively. Moreover, its chain‐scission mechanisms in these different media were studied. The advancing contact‐angle (θ_a) value of the amorphous‐made PLLA film decreased monotonically with the hydrolysis time from 100° to 75° and 80° without a significant molecular weight decrease, when enzymatic and alkaline hydrolyses were continued for 60 min and 8 h, respectively. In contrast, a negligible change in the θ_a value was observed for the PLLA films even after the autocatalytic hydrolysis was continured for 16 h, when their bulk M_n decreased from 1.2 × 10⁵ to 2.2 × 10⁴ g mol^?1 or the number of hydrophilic terminal groups per unit weight increased from 1.7 × 10^?5 to 9.1 × 10^?5 mol g^?1. These findings, together with the result of gravimetry, revealed that the enzymatic and alkaline hydrolyses are powerful enough to enhance the practical surface hydrophilicity of the PLLA films because of their surface‐erosion mechanisms and that its practical surface hydrophilicity is controllable by varying the hydrolysis time. Moreover, autocatalytic hydrolysis is inappropriate to enhance the surface hydrophilicity, because of its bulk‐erosion mechanism. Alkaline hydrolysis is the best to enhance the hydrophilicity of the PLLA films without hydrolysis of the film cores, while the enzymatic hydrolysis is appropriate and inappropriate to enhance the surface hydrophilicity of bulky and thin PLLA materials, respectively, because a significant weight loss occurs before saturation of θ_a value. The changes in the weight loss and θ_a values during hydrolysis showed that exo chain scission as well as endo chain scission occurs in the presence of proteinase K, while in the alkaline and phosphate‐buffered solutions, hydrolysis proceeds via endo chain scission. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1628–1633, 2003     

Stereocomplex crystallization and spherulite growth behavior of poly(l-lactide)-b-poly(d-lactide) stereodiblock copolymers

The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (M_n) of 3.9 × 10³, 9.3 × 10³, and 1.1 × 10⁴ g mol⁻¹, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (X_c) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle M_n of 9.3 × 10³ g mol⁻¹. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in M_n, but seems to be caused by the upper critical M_n values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.     

Structure determination of tubular crystals of membrane proteins. IV. Distortion correction and its combined application with real-space averaging and solvent flattening

A method for correction of three-dimensional distortions has been developed for helical assemblies and applied to tubular crystals of Ca2+-ATPase. This method approximates distorted helical particles with short straight segments of different orientation parameters, which are determined by fitting them to the reference data in reciprocal space. Thus, the method follows Beroukhim and Unwin [Ultramicroscopy 70 (1997) 57], but is more extended to achieve better alignment and to cope with images of poor S/N ratio. Substantial improvements were achieved by dividing the reference image into the segments of optimal lengths in exactly the same way as the test, and treating the distortions in the near and far sides of a helical particle separately. The improvement was further enhanced when combined with real-space averaging [Yonekura, Toyoshima, Ultramicroscopy 84 (2000) 15] and solvent flattening [Yonekura, Toyoshima, Ultramicroscopy 84 (2000) 29], and most pronounced when all these three were applied iteratively.     

Hysteresis of critical currents of superconducting bridges in low perpendicular magnetic fields

Hysteresis of critcal currentsI _c of superconducting bridges with In, Nb, and NbN has been studied in low perpendicular magnetic fields. Influences of bridge geometry, small field sweep, trapped flux, and bombardment of argon ions on the hysteresis were made clear. The experimental results suggest that the edge pinning and trapped flux in the bank of bridges are associated with the hysteresis. The peak value ofI _c of NbN bridges, as well as granular Al and In bridges reported before, in decreasing fields agrees with the calculated pair-breaking current. The origin of the hysteresis is discussed.     

High recovery system in seawater reverse osmosis plants

Seawater desalination by the reverse osmosis (RO) method is an energy-saving system compared with the evaporating method, and can perform seawater desalination efficiently. Seawater RO desalination technology has been established and become a reliable system. Seawater desalination plants using RO technology have spread and the scale of the plants has increased significantly. More economical and efficient RO method seawater desalination systems have come to be required. A high recovery system, which offers reduction of plant construction cost and running cost was devised. Towards realization of this high recovery system, simulation and the field tests were done to confirm the practicality. Furthermore, a high recovery system was adopted for the biggest desalination plant in Japan, and it is performing favorably. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008