Blends of Plasticized Corn Gluten Meal and Poly(ε ... - ACS Publications

This study attempts to explore the value-added applications of corn gluten meal (CGM), the inexpensive byproduct from corn based ethanol industries...
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Ind. Eng. Chem. Res. 2006, 45, 6147-6152

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MATERIALS AND INTERFACES Value-Added New Materials from Byproduct of Corn Based Ethanol Industries: Blends of Plasticized Corn Gluten Meal and Poly(E-caprolactone) Dinesh Aithani and Amar K. Mohanty* School of Packaging, Michigan State UniVersity, 130 Packaging Building, East Lansing, Michigan 48824

This study attempts to explore the value-added applications of corn gluten meal (CGM), the inexpensive byproduct from corn based ethanol industries. The byproducts, CGM, was plasticized using glycerol/ethanol mixture, denatured by the addition of guanidine hydrochloride (GHCl), and then blended with poly(caprolactone), PCL, a synthetic commercial biodegradable polymer. Extrusion followed by injection molding was adopted to fabricate the newly blended green materials. The processing conditions affected the performance of the blends. The GHCl modified corn gluten meal was characterized by IR spectroscopy. The developed materials were characterized through their thermomechanical, tensile, and Izod impact strength measurements. The effects of processing conditions on properties of blends were investigated. One of the promising outcomes of this research was that the GHCl modified corn gluten meal based bioplastic, on blending with PCL had a substantially higher percent elongation. Scanning electron microscopy (SEM) analysis revealed better compatibility of PCL with GHCl modified plasticized CGM. The percent elongation and impact strength of the blended material was found to be higher than high-density polyethylene (HDPE). 1. Introduction Biodegradable polymers are getting increasing attention recently due to their possible environmental benefits and societal attractions. The utilization of agricultural products in plastic applications is considered an interesting way to reduce surplus farm production and to develop nonfood applications.1 Plastics account for 11.3 wt % of the total waste generation in United States,2 and the major portion of this waste is from the packaging stream. Biodegradable polymers can provide environmental benefits through degradation on disposal by living organisms as a contrast to traditional polymers such as polyethylene (PE) and polypropylene (PP), etc., that stick with the environment for many years after disposal.3 The use of some biodegradable materials could make use of renewable resources and can maintain neutrality with respect to the release of carbon dioxide.4,5 Biodegradable plastic can be a very good choice for packaging of items used for a short period of time, and when disposed of, they can be degraded in a suitable composting environment unlike conventional plastics. The use of biodegradable plastic can also avoid the recycling of plastics, which are undesirable due to soiling by food or other biological substances.3 The ethanol production from corn is done by two different processes, i.e., dry milling and wet milling. One of the major byproducts obtained from wet milling industries is corn gluten meal (CGM). Similarly one of the major byproducts obtained from dry milling ethanol industries is distillers’ dried grains with solubles (DDGS). These byproducts from corn based ethanol industries are mostly used for animal feeds. But extensive research can find the value-added applications of such * To whom correspondence should be addressed. Tel.: +1-517-3553603. Fax: +1-517-353-8999. E-mail: [email protected].

byproducts in making biodegradable plastics. The plasticization of corn gluten meal (CGM) is reported6 to exhibit thermoplastic type behavior. Corn gluten meal mainly consists of proteins (60%) and hydrophobic amino acids (10% leucine) with the remaining components mainly being moisture, fiber, and lipids.7 The high demand for the production of bioethanol, a better substitute for the ethanol produced from nonrenewable sources, means increased generation of CGM. The current use of CGM is mainly as animal feed, but extensive research is directed toward finding additional value-added applications for CGM. One of the growing avenues is the design and engineering of CGM-based biodegradable plastics which would compete with the petroleum based products on the basis of cost and performance. The production of CGM from renewable and abundant resources has attracted attention in the field of packaging due to its biodegradability.8 Poly(-caprolactone) (PCL) is an aliphatic polyester. It is one of the important biodegradable polymers gaining interest due to its technological properties and inherent biocompatibility.9-12 It is well-known for its biodegradability and is often used for packaging, plant containers, and medical devices.13,14 It is semicrystalline in nature with a melting point of around 60 °C15 and a glass transition temperature (Tg)16 in the range of -65 to -60 °C. PCL can be blended with a variety of other polymers to improve their properties.17 Blending a natural polymer with polyester is one way to reduce cost and to improve the biodegradability of the resulting polymeric blend.16 Therefore, blending of a petroleum-based biodegradable polymer (PCL) with a compatible natural biodegradable material like CGM may provide an alternative packaging material with lower cost and environmentally friendly properties. Recently, several studies have been made on CGM based biodegradable materials. Wu et al.18 have successfully extruded and compression molded a CGM-wood fiber composite into sheets and injection molded

10.1021/ie0513200 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/27/2006

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the same into pots. CGM, glycerol, urea, and an organic acid were used to prepare compression molded thermoplastic by Nobuhiro et al.19 The interaction of CGM with polar and amphiphilic plasticizers has been studied by Gioia et al.20 The uniqueness of the present investigation includes the plasticization along with the denaturization of CGM that imparted a much improved percent elongation to the blends. This newly developed green plastic shows potential as a substitute for polyethylene on the basis of specific performance with an added advantage of biodegradability. 2. Experimental Details 2.1. Materials. The commercial corn gluten meal, a byproduct of corn based ethanol industries with protein content of ∼60%, was supplied by Cargill Inc., Minnesota. A commercial grade PCL (TONE 787) with a melting point of 60 °C was obtained from Union Carbide Corp. Glycerol (99.9% purity), ACS grade from J. T. Baker, and ethanol, 190 proof ACS grade from Pharmco, were used. Guanidine hydrochloride was supplied by Acros Organic, New Jersey. 2.2. Preparation of Blends. A microscale extruder with injection molder, called a microcompounding instrument (DSM Research, The Netherlands), was used for blending. It consists of twin vertical co-rotating screws, with a length of 150 mm, L/D of 18, and a maximum capacity of 15 cm3. The blending was accomplished by plasticizing CGM with glycerol and ethanol (denoted as plasticized CGM or CGMP) followed by chemical modification with guanidine hydrochloride (GHCl). The CGMP was made by weighing the CGM and glycerol/ ethanol separately and then adding glycerol/ethanol slowly into the CGM while mixing to ensure uniformity of distribution. The specified ratio of CGMP and GHCl was weighed and melt blended using the DSM microcompounding instrument. The GHCl modified CGMP was dried at 60 °C for 5 h in a vacuum oven before blending with PCL. Blending of GHCl modified CGMP with PCL was followed by injection molding to prepare samples for dynamic mechanical analysis (DMA) and tensile testing. The blended material was drawn and injection molded on a DSM injection molder at 120 °C and 100 psi pressure. The mold temperature was set at 40 °C. 2.3. Characterization. 2.3.1. Infrared Spectroscopy. The infrared spectrum was measured using a Perkin-Elmer FT-IR spectrophotometer. An ATR attachment was used to measure the IR spectrum for the CGMP before and after chemical modification in order to observe the chemical changes that occurred in the structure. The IR spectrum of GHCl was also obtained. 2.3.2. Dynamic Mechanical Analyzer. The storage modulus of the blend was measured using a Model Q800 dynamic mechanical analyzer (DMA) from TA Instruments, using a single-cantilever clamp and multifrequency strain mode. Measurements were made at 1 Hz and 15 µm amplitude over the temperature range of 30-60 °C at the heating rate of 2 °C/ min. 2.3.3. Tensile Testing. The tensile testing was done on Universal Test System load frame according to ASTM D638. The tensile samples were made by injection molding immediately after extrusion from the DSM. A load cell of 1000 lb capacity was used for the testing, with a cross-head speed of 2 in./min for tensile tests. 2.3.4. IZOD Impact Testing. The samples were notched using a TMI notching cutter (model TMI 22-05, Testing Machine Inc., Amityville, NY), and the impact strength was measured using a TMI IZOD impact strength tester (model 43-

Table 1. Modulus of PCL Blends with GHCl Modified CGMP Processed at 150 rpm and Temperatures of 120 and 150 °C at the Ratio of PCL:CGMP:GHCl ) 50:37.5:12.5 (wt %) processing temp, °C

at 30 °C

storage modulus (MPa) at 40 °C

at 50 °C

120 150

88 ( 10.77 87 ( 3.17

75 ( 9.43 74 ( 2.57

49 ( 5.52 48 ( 1.73

Table 2. Modulus of PCL Blends with GHCl Modified CGMP Processed at 150 °C and Different Processing Speeds of 75 and 150 rpm at the Ratio of PCL:CGMP:GHCl ) 50:37.5:12.5 (wt %) processing speed, rpm

at 30 °C

storage modulus (MPa) at 40 °C

at 50 °C

75 150

170 ( 31.64 87 ( 3.17

148 ( 29.27 74 ( 2.57

105 ( 22.28 48 ( 1.73

02). A 5 lb pendulum was used for impact measurement. ASTM D 256 was followed for the testing. 2.3.5. Scanning Electron Microscopy. The fractured surfaces were coated with gold thin films and examined in a JEOL 6300 field emission scanning electron microscope (FESEM) at an accelerating voltage of 10 kV. 3. Results and Discussion One of the promising ways to find value-added applications of inexpensive byproduct like CGM is the development of CGM based biodegradable plastics. In this piece of research work we look forward to the use of CGM without any purification in order to make it more cost-effective with an overall target of finding new value-added materials. “Zein”, an alcohol soluble protein, can be extracted from CGM through solvent extraction. Polymers derived purely from zein are very expensive and have found limited applications.21 The brittle nature of the corn protein based materials necessitates the use of plasticizers. Glycerol has been extensively studied as a plasticizer.18,22 The plasticization effect of glycerol is attributed to its small size which helps in its insertion and positioning within the protein network23,24 and thereby reducing the intermolecular forces and increasing the mobility of protein chains.25 The use of ethanol aids the processing of CGM.18 The glycerol/ethanol (3:1) mixture was used for better plasticization of CGM system. The GHCl was used to break down the protein structure in order to make it more flexible and compatible with the other components of the polymer system. Various studies have been done on the interaction of GHCl with proteins, which revealed the induction of denaturation and unfolding of the protein structure.26-28 Denaturation is the alteration of secondary, tertiary, or quaternary structures in the protein molecules. The GHCl forms cross-link at different locations in the side chains and on the backbone of the protein

Figure 1. Percentage elongation of PCL blends with modified CGMP processed at 150 °C and different processing speeds of 75 and 150 rpm.

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Figure 2. IR spectroscopy of CGMP before and after chemical modification with GHCl: A ) GHCl; B ) GHCl:CGMP ) 25:75 (wt %); C ) CGMP.

Figure 3. Effect of GHCl on tensile strength of PCL blends (ratio PCL:CGMP:GHCl): (A) 100:0:0; (B) 50:37.5:12.5; (C) 50:42.5:7.5; (D) 50:45:5; (E) 50:50:0.

molecule by means of hydrogen bonding and van der Waals interactions.28 This phenomenon is likely responsible for the improvement of mechanical properties of the protein blends. In melt processing technique it is important to optimize the processing conditions in order to get the best performing materials. The processing conditions, i.e., the temperature and processing speed, play important roles in determining the final properties of the blend. To analyze the effect of processing conditions, a blend containing PCL:CGMP:GHCl at the ratio of 50:37.5:12.5 on a weight basis were processed at 120 and 150 °C, keeping the processing speed constant. The mechanical properties (modulus) of the resulting blends were evaluated using DMA. The blend processed at 150 °C shows more consistency in results than the blend processed at 120 °C, which is evident from the standard deviation results (Table 1). The superior consistency of modulus data obtained at the higher temperature as contrasted with that obtained from lower temperature is taken as the basis for the more optimized condition of processing used under the present investigations. On the basis of these results, the processing temperature of 150 °C was used versus 120 °C in the subsequent studies. To study the effect of extruder screw speeds on blend properties, the blends were prepared at 150 °C using two different screw speeds. Speeds of 75 and 150 rpm were used

Figure 4. Effect of GHCl on percentage elongation of PCL blends (ratio PCL:CGMP:GHCl): (A) 100:0:0; (B) 50:37.5:12.5; (C) 50:42.5:7.5; (D) 50:45:5; (E) 50:50:0.

during processing. The storage modulus of the blend was reduced by about 50% with an increase in processing speed (Table 2). The material blended at 75 rpm had large average standard deviations in its storage modulus. The material blended at 150 rpm exhibited a much improved percent elongation and reduced storage modulus (Figure 1) in contrast to their counterparts blended at 75 rpm. On the basis of these results, e.g., considering the low standard deviation in modulus data and much superior elongation data at 150 rpm in contrast to those at 75 rpm, the processing speed of 150 rpm was chosen in contrast to 75 rpm for further studies. Such observations were attributed to inconsistent mixing at lower screw speeds, or higher orientation is generated at higher speed due to higher shear rate. The GHCl modified corn gluten meal based bioplastics were characterized by using IR spectroscopy. A peak shift was observed in GHCl modified CGMP bioplastic at 1651 cm-1 (Figure 2) in contrast to CGMP bioplastic (e.g. unmodified CGMP) at 1624 cm-1. This peak shift is attributed to hydrogen bonding.29 Researched by Dunbar et al.28 as well supports the formation of a hydrogen bond during the interaction of GHCl with protein. Blends of PCL and zein have shown poor mechanical properties in previous research.16 These results were attributed to incompatibility between the components of the system. In this present study, we used plasticized CGM as well as GHCl

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Figure 5. Stress-strain curves for PCL blends with and without GHCl (ratio PCL:CGMP:GHCl): (A) 50:50:0; (B) 50:42.5:7.5.

Figure 6. Comparison of percent elongation and impact strength of PCL, PCL blend, and HDPE (HDPE data from ref 30): (A) PCL:CGMP:GHCl, 100:0:0; (B) PCL:CGMP:GHCl, 50:42.5:7.5; (C) HDPE.

modified CGMP as the blending partners for PCL in order to study the effectiveness of GHCl in designing new biodegradable plastics. Such developed biodegradable plastic blends exhibit some useful properties that can be targeted in certain plastic applications as a substitute for high-density polyethylene (HDPE). The tensile strength of PCL was reduced upon blending with CGMP (Figure 3) as expected. The effect of GHCL on the percent elongation of the blends is represented in Figure 4. The percent elongation of PCL decreased when blended with CGMP but showed significant improvement when blended with GHCl modified CGMP. As apparent from Figure 4 the percent elongation was not proportional to the percentage of added GHCl. The elongation of the blend first increased with increasing GHCl content from 5 to 7.5% and then decreased with

Figure 7. Storage modulus of PCL and its blends.

further addition of GHCl content up to 12.5%. Therefore, 7.5% GHCl was considered the optimum amount under the present experimental conditions investigated. The higher elongation of the blend from PCL and GHCL modified CGMP is attributed to optimum hydrogen bonding at a specific concentration of GHCL. Higher concentration of GHCl beyond this optimum amount might have caused phase separation under the present experimental conditions. The stress-strain curve in Figure 5 shows the effect of GHCl in the blend. The higher area under the curve for the blend that was modified with GHCl indicates higher plastic character in contrast to its counterpart not modified with GHCl. The PCL-CGMP blended with 7.5% GHCl was found to be better in comparison to the other composition. This composition was compared to one of the injection molded grade HDPE.30 The percent elongation of this blend was 450%, while HDPE has a percent elongation of 320% (Figure 6). This blend was also found to have higher impact strength in contrast to HDPE (Figure 6). The storage modulii of PCL and its blends with CGMP as well as with GHCl modified CGMP (with varying content of GHCl) were measured using DMA at 30, 40, and 50 °C (Figure 7). The PCL as well as all of the blends exhibited decreasing modulus values with an increase of temperature from 30 to 50 °C. As expected, the modulus of the PCL decreased on blending with CGMP as well as GHCl modified CGMP bioplastics. In the comparative study to observe the effect of GHCl proportion in the GHCl modified CGMP, it was observed that 7.5% of GHCl resulted in an optimized modulus and percent elongation values (Figures 3, 4, and 7) of the resulting biodegradable blends. The blend morphology is closely related to its mechanical properties. In our investigations of blend compositions and their mechanical property evaluations, we found that GHCl modified CGMP was superior to the unmodified counterpart. Blends with PCL:CGMP:GHCl in the ratio of 50:42.5:7.5 and 50:50:0 were selected and the fractured surfaces of these compositions were examined using scanning electron microscopy (SEM) in order to reveal the morphology of the blend with and without GHCl. The morphological analysis revealed that the blend without GHCl contained cavities, which must have created in the fracture region during the spontaneous separation of the PCL and CGMP regions due to the weak interfacial adhesion between PCL and CGMP. The presence of GHCl in the blend enhanced the interfacial adhesion by means of the secondary forces and

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Figure 8. SEM photomicrographs of the blends (ratio PCL:CGMP:GHCl): (A, B) 50:42.5:7.5 (scale bars are 50 and 10 µm, respectively); (C, D) 50:50:0 (scale bars are 50 and 10 µm, respectively).

Figure 9. Storage modulus of PCL and its blends.

exhibited quite a high degree of homogeneity, which was a sign of increased compatibility between PCL and CGMP phases (Figure 8). In the current research, using a unique chemical modification of CGMP with GHCl followed by blending with PCL has resulted in a new biodegradable plastic composition having a tensile strength of ∼10 MPa and a percent elongation of 462% (Table 3). The newly developed biodegradable plastic shows immense potential for plastic film applications. In a separate investigation, the content of PCL was increased by 10% in the blend system. A comparison of some properties of the two blends is shown in Table 3. The blend with 7.5% GHCl was found to be the best in the overall properties under the present experimental conditions.

Table 3. Comparison of Properties between 50 and 60% PCL Based Blend blend composition

tensile strength (MPa)

% elongation

50:50 PCL:(GHCl-modified CGMP) 60:40 PCL:(GHCl-modified CGMP)

10 ( 1.61 17 ( 1.13

462 ( 33.3 598 ( 15.6

To further improve the properties of this blend so that it can be compared with some other synthetic plastics, the percentage of PCL was increased from 50 to 60%. The resulting blend with 60% PCL showed improvement in its mechanical properties as shown in Table 3 and Figure 9. The tensile strength, percent elongation, and modulus improved by 70, 29, and 28%, respectively.

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4. Conclusion The main motivation of the present investigations was to design novel green biodegradable plastics with the maximum permissible content of CGM, thereby finding value-added application of the byproducts of corn based ethanol. In the current research we developed injection molded biodegradable plastic from the blends of corn gluten meal based bioplastic and PCL. The choice of the ratio of components, the melt processing temperature, and the screw speed of the extruder affected the overall properties of the resulting biodegradable polymer blends. The uniqueness of the present investigation is the chemical modification with GHCl, which has resulted in a biodegradable plastic having a high percent elongation, which shows potential applications for biodegradable packaging. Among the different blends tested, the blend with 7.5% GHCl produced the best results under present experimental conditions. Further increase in the PCL content, i.e., from 50 to 60% in the final blend, improved the overall mechanical properties of the blend. The morphological analysis revealed increased homogeneity between PCL and CGM phases in the presence of GHCl. Acknowledgment We are thankful to Mr. Tom Guinan, Cargill Inc., MN, for donating corn gluten meal samples. The authors are also thankful to Michigan State University for start-up funding to A.K.M. Literature Cited (1) Averous, L.; Moro, L.; Dole, P.; Fringant, C. Properties of thermoplastic blends: Starch-polycaprolactone. Polymer 2000, 41, 4157. (2) Basic Facts: Municipal Solid Waste (MSW), http://www.epa.gov/ msw/facts.htm (accessed July 7, 2006). (3) Gross, R. A.; Kalra, B. Biodegradable polymers for the environment. Science 2002, 297, 803. (4) Saimon, J.; Muller, H.; Koch, R.; Muller, V. Thermoplastic and biodegradable polymers of cellulose. Polym. Degrad. Stab. 1998, 59, 107. (5) Mohanty, A. K.; Misra, M.; Hinrichsen G. Biofibers, biodegradable polymers and biocomposites: An overview. Macromol. Mater. Eng. 2000, 276/277, 1. (6) di Gioia, L.; Cuq, B.; Guilbert, S. Effect of hydrophilic plasticizer on thermomechanical properties of corn gluten meal. Cereal Chem. 1998, 75, 514. (7) Watson, S.; Ramstad, P. Corn: Chemistry and Technology; American Association of Cereal Chemists; St. Paul., MN, 1987. (8) Cuq, B.; Gontard, N.; Guilbert, S. Protein as agricultural polymers for packaging production. Cereal Chem. 1998, 75(1), 1. (9) Fields, R. D.; Rodriguez, F.; Finn, R. K. Microbial degradation of polyesters: Polycaprolactone degraded by P. pullulans. J. Appl. Polym. Sci. 1974, 18, 3571. (10) Benedict, C. V.; Cook, W. J.; Jarrett, P.; Cameron, J. A.; Huang, S. J.; Bell, J. P. Fungal degradation of polycaprolactones. J. Appl. Polym. Sci. 1983, 28, 327. (11) Benedict, C. V.; Cook, W. J.; Huang S. J. Polycaprolactone degradation by mixed and pure cultures of bacteria and a yeast. J. Appl. Polym. Sci. 1983, 28, 335.

(12) Doi, Y.; Kanesawa, Y.; Kunioka, M.; Saito, T. Biodegradation of microbial copolyesters: Poly(3-hydroxybutyrate-co-3-hydroxyvaleratea) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate). Macromolecules 1990, 23, 26. (13) Potts, J. E. In Aspects of the Degradation and Stability of Polymers; Jelinek, H. H. G., Ed.; Elsevier: Amsterdam, 1978; p 617. (14) Yasin, M.; Tigh, B. J. Polymers for biodegradable medical devices. VIII. Hydroxybutyrate-hydroxyvalerate copolymers: Physical and degradative properties of blends with polycaprolactone. Biomaterials 1992, 13 (1), 9. (15) Matzinos, P.; Tserki, V.; Gianikouris, P. E.; Panayiotou, C. Processing and characterization of LDPE/starch/PCL blends. Eur. Polym. J. 2002, 38, 1713. (16) Corradini, E.; Mattoso, L. H. C.; Guedes, C. G. F.; Rosa, D. S. Mechanical, thermal and morphological properties of poly(-caprolactone)/ zein blends. Polym. AdV. Technol. 2004, 15, 340. (17) Iannace, S.; De Luca, N.; Nicolais, L.; Carfagna, C.; Huang, S. J. Physical characterization of incompatible blends of polymethylmethacrylate and polycaprolactone. J. Appl. Polym. Sci. 1990, 41, 2691. (18) Wu, Q.; Sakabe, H.; Isobe, S. Processing and properties of low cost corn gluten meal/wood fiber composite. Ind. Eng. Chem. Res. 2003, 42, 6765. (19) Nobuhiro, H.; Suzuki, K.; Takanori, E.; Mituga, H. Thermoplastic Stuff and Its Processing Method. Japan Patent, 6-192577, 1994. (20) di Gioia, L.; Guilbert, S. Corn protein-based thermoplastic resins. Effect of some polar and amphiphilic plasticizer J. Agric. Food Chem. 1999, 47, 1254. (21) Lawton, J. W. Review: Zein: A history of processing and use. Cereal Chem. 2002, 79, 1. (22) Lawton, J. W. Plasticizers for zein: Their effect on tensile properties and water absorption of zein films. Cereal Chem. 2004, 81, 1. (23) Kalichevsky, M. T.; Jaroszkiewicz, E. M.; Blanshard, J. M. V. Glass-transition of gluten. 1. Gluten and gluten sugar mixtures. Int. J. Biol. Macromol. 1992, 14, 257. (24) Cuq, B.; Gontard, N.; Cuq, J.; Guilbert, S. Functional properties of fish myofibrillar protein-based films as affected by hydrophilic plasticizers. J. Agric. Food Chem. 1997, 45, 622. (25) Guilbert, S.; Biquit, B. In L’emballage des Denrees Alimentaries Grande Consommation; Bureau, G., Mutton, J. L., Eds.; Technique et Documentation: Paris, 1989; p 20. (26) Courtenay, E. S.; Capp, M. W.; Record, M. T., Jr. Thermodynamics of interactions of urea and guanidinium salts with protein surface: Relationship between solute effects on protein processes and changes in water-accessible surface area. Protein Sci. 2001, 10, 2485. (27) Ahmad, B.; Ahmad, M. D.; Haq, S. K.; Khan, R. H. Guanidine hydrochloride denaturation of human serum albumin originates by local unfolding of some stable loops in domain III. Biochim. Biophys. Acta 2005, 93, 1750. (28) Dunbar, J.; Yennawar, H. P.; Banerjee, S.; Luo, J.; Farber, G. K. The effect of denaturants on protein structure. Protein Sci. 1997, 6, 1727. (29) Silverstein, R. M.; Bassler, G. C. Spectrometric Identification of Organic Compounds, 2nd ed.; 1968. (30) Technical Data Sheet of Dow Polyethylene 25455N (Injection molding resin); Dow Plastics: Midland, Michigan, 2001 (Aug).

ReceiVed for reView November 27, 2005 ReVised manuscript receiVed May 26, 2006 Accepted June 8, 2006 IE0513200