ANALYTICAL SCIENCES MARCH 2009, VOL The Japan Society for Analytical Chemistry

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1 ANALYTICAL SCIENCES MARCH 2009, VOL The Japan Society for Analytical Chemistry Determination of 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide Hydrochloride by Flow-Injection Analysis Based on a Specific Condensation Reaction between Malonic Acid and Ethylenediamine Kunihiko SENO,* Kazuki MATUMURA,* Koji OSHITA,** Mitsuko OSHIMA,* and Shoji MOTOMIZU* * Chemistry and Biochemistry, Graduate School of Natural Science and Technology & Department of Chemistry, Faculty of Science, Okayama University, Tsushimanaka, Okayama , Japan ** Department of International Conservation Studies for Cultural Properties, Faculty of Cultural Properties, Kibi International University, 8 Iga-machi, Takahashi, Okayama , Japan A sensitive and rapid flow-injection analysis was developed for the determination of 1-(3-dimethylaminopropyl)-3- ethylcarbodiimide hydrochloride (EDC HCl), which was used for the formation of amide (peptide) and esters as a dehydration or condensation reagent. The EDC HCl could be determined by the flow-injection analysis based on a specific condensation reaction between malonic acid and ethylenediamine in aquatic media. The reaction was accelerated at 60 C, and the absorbance of the product was detected at 262 nm. The calibration graph of EDC HCl showed good linearity in the range from 0 to 0.1% (0 to M), whose regression equation was y = x (y, peak area; x, % concentration of EDC HCl). The proposed method allowed high-throughput analysis; the sample throughput was 12 samples per hour. The limit of detection (LOD) and the relative standard deviation (RSD) were M and 1.0%, respectively. This reaction is proceeded in aqueous solution and specific for EDC HCl. (Received December 1, 2008; Accepted January 6, 2009; Published March 10, 2009) Introduction Carbodiimide reagents, such as 1-(3-dimethylaminopropyl)-3- ethylcarbodiimide hydrochloride (EDC HCl), N,N -diisopropylcarbodiimide (DIC) and N,N -dicyclohexylcarbodiimide (DCC), accelerate the formation reaction of esters, 1 4 amides, and peptides, 5 8 as condensing and dehydrating agents, which are often used for polynucleotide synthesis, 9,10 anhydroxydation, 11,12 lactonization 13 and esterification. 14 Therefore, these reagents are quite significant in the field of biochemistry research. DIC and DCC were often used for such dehydration and condensation reaction, which produced the corresponding urea after the reaction. The urea, which is a by-product derived from the reaction of DIC or DCC, is hard to dissolve into almost all solvents, and the separation of the by-product from the main product is too difficult and troublesome. On the other hand, one of the condensing and dehydrating agents, EDC HCl, is widely used for polyaniline-carbon nanotube preparation for a cholesterol biosensor, 15 precolumn derivatization of aliphatic amines for HPLC, 16 molecular beacons formation for DNA reseach, 17 sensor preparation for calcium detection, 18 the fluorescent determination of carboxylic acids, 19,20 and solidphase microsequencing of peptides. 21 The by-product generated from the dehydration and condensation reaction with EDC HCl is easy to dissolve in various kinds of solvent, especially water. Since EDC HCl is less toxic than DIC and DCC, a smooth To whom correspondence should be addressed. motomizu@cc.okayama-u.ac.jp powdery crystal, EDC HCl, is readily treated. Therefore, EDC HCl is an indispensable reagent for the dehydration and condensation reaction in biochemistry, medicinal chemistry and medicinal chemistry. For estimating the purity of an EDC HCl reagent, and the analysis of the residual EDC HCl after the dehydration and condensation reaction, trace EDC HCl at the 10 4 % (10 5 M) level must be accurately determined. In general, spectrophotometric analysis coupled with a titration technique has been used for the determination of EDC HCl, whereas this method has a low reproducibility. Also, a spectrophotometric method by flow injection analysis (FIA) has been developed for the determination of EDC HCl in aqueous samples, 22 which is measured in a short time and with a simple procedure. However, its sensitivity was not good: the limit of quantification (LOQ) was 0.1% ( M). Some other spectrophotometric methods have been reported, whereas their analytical performance is not practical, which requires several hours for an EDC HCl measurement It is caused by the stable ring structure of EDC HCl itself in aquatic media, and it takes much time to react. In our previous work, 25 a simple and rapid FIA technique was developed for EDC HCl determination by using an effective reaction with pyridine and ethylenediamine in acidic solution (0.1 M HCl). This method was then applied to the monitoring of the residual EDC HCl concentration, which remained after the dehydration of phthalic acid in aqueous solution and the esterification of acetic acid in methanol. EDC HCl in pyridine and ethylenediamine could form a ring structure, and it was then detected at 400 nm. The absorbance of the residual EDC HCl (ring structure) decreased along with

2 390 ANALYTICAL SCIENCES MARCH 2009, VOL. 25 an increase in the concentration of the main product in the dehydration and etherification. However, the LOD of this method was not very good, 0.1%. In order to overcome such problems, a novel condensation reaction between malonic acid and ethylenediamine with EDC HCl was developed. The reaction product obtained by the condensation reaction with EDC HCl was detected at 262 nm, and the absorbance of the reaction product was proportional to the concentration of the analyte (EDC HCl). A specific reaction was applied to the FIA technique. The proposed method, which was rapid and sensitive, could be applied for estimating the decomposition rate of EDC HCl in a hydrochloric acid solution. Experimental Reagents and chemicals 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC HCl), which was used as a condensation and dehydration agent, was obtained from Osaka Synthetic Chemical Laboratories, Inc. (Hyogo, Japan). Carboxylic acids (malonic acid, oxalic acid, succinic acid, and phthalic acid), diamines (ethylenediamine, iso-amylamine, o-phenylenediamine, 1,3- diaminopropane, 1,4-diaminobutane, and 1,5-diaminopentane), and amino acids (phenylalanine, glycine, arginine, alanine, and serine) were obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). All other reagents were of extrapure-reagent grade. Ultrapure water (18.3 MW cm 1 ), prepared by a Milli-Q System (Nihon Millipore, Tokyo, Japan), was used throughout. Apparatus A flow-injection system consisted of a double plunger-type pump (RX703T, Sanuki Kogyo Co. Ltd., Tokyo), a sample injection valve (7725i, Rheodine Co. Ltd., USA), and a UVdetector (LC-10AD, Shimadzu Corporation, Kyoto) with a 10- mm path length flow cell, as shown in Fig. 1. The flow rates of the reagent solution (RS) and the carrier solution (CS) were 0.15 ml min 1, and the sample solution (100 ml) was then injected into the CS stream. After the sample was mixed with RS, it was flowed into the RC (reaction coil, 10 m), whose temperature was controlled at 60 C with an air bath (U-620, Sugai Chemical Ind. Co. Ltd., Wakayama, Japan). It was then flowed into the detector (D) through a CC (cooling coil, 0.2 m) dipped in a water bath. The product from the condensation reaction between malonic acid and ethylenediamine with EDC HCl was passed through the detector. The absorbance at 262 nm was recorded by an integrator (R), which was a C-R6A (Shimadzu Corporation, Japan). The peak area was then calculated from the absorbance unit (mv) and time (s). Procedure Stock solutions of malonic acid (0.1 M) and ethylenediamine (0.1 M) were prepared by dissolving each reagent in water. A reagent solution (RS) containing 0.05 M malonic acid and 0.05 M ethylenediamine was prepared by mixing equal volumes of the stock solutions. Ultrapure water was used as a carrier solution (CS). The sample solution, 100 ml (contained W g of EDC HCl) was injected into the CS stream by a sample injector, as shown in Fig. 1. The sample was mixed with the RS and heated at 60 C in the RC (10 m), and was then cooled at 25 C. Finally, it was measured at 262 nm. The peak area (S) of the sample solution which contained W g of EDC HCl, was used to prepare a calibration graph and to calculate the EDC HCl concentration in the sample solution. In the case of determining the residual, EDC HCl contents (%) Fig. 1 Schematic FIA diagram for the determination of EDC HCl. CS, Carrier solution (ultrapure water); RS, reagent solution (0.05 M malonic acid M ethylenediamine); S, sample injector (100 ml of EDC HCl aqueous solution); P1 and P2, double plunger-type pump (each flow rate: 0.15 ml min 1 ); RC, reaction coil (60 C, 10 m); CC, cooling coil (25 C, 0.2 m); BPC, back pressure coil (3 m); D, spectrophotometric detector (262 nm); R, recorder; W, waste. in the sample must be calculated while considering the residual EDC HCl and any by-product as an impurity. The standard solution which contained W 1 g of EDC HCl, was measured, and the peak area (S 1) was used to prepare a calibration graph. The residual EDC HCl contents (%) in the sample were calculated by the following equation: EDC HCl contents (%) = {(S 2 W 1)/(S 1 W 2)} 100 where S 1 and W 1 are the peak area and the weight of the standard solution, and S 2 and W 2 are the peak area and the weight of the sample solution, respectively. Results and Discussion Proposed reaction for EDC HCl detection In our previous work, 25 EDC HCl with ethylenediamine and pyridine was detected at 400 nm, whose molar absorptivity (e) and regression equation were L mol 1 cm 1 and y = x (y, peak area; x, % concentration of EDC HCl), respectively. The LOD was 0.1%, which was low sensitivity. In order to improve the sensitivity, a new detection reaction for the determination of EDC HCl was examined by using carboxylic acids (malonic acid, oxalic acid, succinic acid, and phthalic acid) and diamines (ethylenediamine, iso-amylamine, and o- phenylenediamine). Amino acids, such as phenylalanine, glycine, and arginine, were also examined because they are expected that carboxyl groups and amino groups, which share at the near position, might accelerate an effective condensation reaction in inner-molecular with EDC HCl. From these results, the product from the condensation reaction between malonic acid and ethylenediamine showed efficient absorbance at 262 nm, as shown in Fig. 2(a), whereas the other products from the other carboxylic acids, diamines, and amino acids did not show effective absorbance. The molar absorptivity (e), 665 L mol 1 cm 1, of the product at 262 nm was larger than that of EDC HCl, itself (0.014 L mol 1 cm 1 ), previously reported. 25 It was caused by the formation of a ring structure between malonic acid and ethylenediamine from a condensation reaction with EDC HCl. The effective adsorption might be correlated with the resonance structure of the product, as shown in Fig. 3. At around this wavelength (262 nm), a blank solution containing malonic acid and ethylenediamine did not show an absorption band, as shown in Fig. 2(b). Therefore, this proposed reaction was applied to the FIA.

3 ANALYTICAL SCIENCES MARCH 2009, VOL Fig. 4 Effect of the reaction time on the absorption spectra in the condensation reaction between malonic acid and ethylenediamine. Sample: malonic acid ( M), ethylenediamine ( M), and EDC HCl ( M). Fig. 2 Effect of EDC HCl on the absorption spectra in the condensation reaction between malonic acid and ethylenediamine. (a) 0.05 M malonic acid, 0.05 M ethylenediamine and M EDC HCl, (b) 0.05 M malonic acid and 0.05 M ethylenediamine, (c) 0.05 M malonic acid and M EDC HCl, (d) 0.05 M ethylenediamine and M EDC HCl. Fig. 3 Scheme of the predicted reaction between malonic acid and ethylenediamine with EDC HCl at 60 C in water. Fig. 5 Effect of the ph and the reaction temperature on the EDC HCl reaction. Sample: malonic acid ( M), ethylenediamine ( M), and EDC HCl ( M) adjusted the ph from 3.7 to 10.3; the experimental conditions were the same as in Fig. 1. Condensation reaction between malonic acid and ethylenediamine with EDC HCl Figure 2 shows the effect of EDC HCl on the absorption spectra in the condensation reaction between malonic acid and ethylenediamine. The mixture (a), malonic acid, ethylenediamine, and EDC HCl, in Fig. 2 showed absorbance at 262 nm, whereas the other mixtures, such as (b) malonic acid and ethylenediamine, (c) malonic acid and EDC HCl, and (d) ethylenediamine and EDC HCl, did not show absorbance. Figure 4 shows the effect of the reaction time on the absorption spectra in the condensation reaction between malonic acid and ethylenediamine. The maximum adsorption before the reaction (0 h) was 214 nm, which was due to EDC HCl, itself. As the condensation reaction between the malonic acid and ethylenediamine with EDC HCl progressed, the absorbance at 262 nm evidently increased. It might have been caused by the formation of the predicted ring structure, as shown in Fig. 3, which was obtained from a condensation reaction between malonic acid and ethylenediamine with EDC HCl. Optimization of chemical conditions The condensation agent, EDC HCl, might peculiarly promote the condensation reaction between malonic acid and ethylenediamine. The effect of the ph on the absorbance of the product was examined, as shown in Fig. 5. The absorbance at 262 nm was maximum at around ph 7 to 8.5. In this ph range, carboxyl groups in malonic acid and amino groups in ethylenediamine can exist as COO and NH 3+, considering the acid dissociation constants (pk a values): pk a1 = 2.85, pk a2 = 5.66 for malonic acid and pk a1 = 7.30, pk a2 = for ethylenediamine. Therefore, the malonic acid and ethylenediamine could form an ion pair by an electrostatic interaction, and hydrogen bonding before the reaction, which seemed to be suitable for the condensation reaction forming the ring structure. The product which was predicted as shown in Fig. 3, could not be isolated and crystallized. The temperature for the condensation reaction was examined by varying it in the range from 30 to 80 C. Figure 6 shows the absorbance changes of the product in the condensation reaction between malonic acid and ethylenediamine with EDC HCl at various temperatures. The absorbance of the product quickly increased along with an increase in the reaction temperature, which might mean that high temperature accelerates the condensation reaction with EDC HCl. Then, each absorbance at 25, 60, and 80 C remained constant after the condensation reaction was complete. Therefore, the product in the reaction is considered to be stable. However, the absorbance decreased

4 392 ANALYTICAL SCIENCES MARCH 2009, VOL. 25 Fig. 6 Effect of the reaction temperature on the adsorbance of the product in the condensation reaction between malonic acid and ethylenediamine with EDC HCl. Sample: malonic acid ( M), ethylenediamine ( M), and EDC HCl ( M). Fig. 7 Effect of the concentration of reagent solution (RS) on the peak area. The concentration of the reagent solution (RS) was examined from to M. The experimental conditions were the same as in Fig. 1. with an increase in the reaction temperature, which might be caused by the decomposition of EDC HCl, itself, before the condensation reaction. Considering the effective reaction and the sensitivity, 60 C was selected as the reaction temperature. Also, the base line at higher temperature became unstable. The effect of each reagent concentration on the absorbance was examined by varying it from to 0.5 M for malonic acid and ethylenediamine, which were mixed according to the same molar ratio. The peak area of the product obtained from 0.05 M malonic acid and 0.05 M ethylenediamine was largest, as shown in Fig. 7. They were selected as a reagent solution (RS). Optimization of flow conditions The effect of the sample size on the peak profiles was examined by varying it from 10 to 230 ml, as shown in Fig. 8(a). With an increase in the sample size, the peak area also increased, whereas the large size of the sample solution could not be mixed effectively with RS, and the measurement time was lengthened. Therefore, the sample size was determined to be 100 ml. The flow rate (CS and RS) on the peak profiles was examined by varying it from 0.10 to 0.5 ml min 1. The height and area of the signals increased along with a decrease in the flow rate, because the condensation reaction with EDC HCl needed much time. Considering the sensitivity, measurement time, and reagent consumption, each flow rate of CS and RS was fixed at 0.15 ml min 1. The effect of the reaction coil (RC) length on the peak profiles was tested by varying it from 3 to 15 m, as shown in Fig. 8(b). The linearity of the calibration graph with the peak areas was improved along with an increase in the coil length due to the reaction efficiency. The 10 m reaction coil satisfied the analytical performance, and was used throughout. Calibration graph and analytical performance The calibration graph showed good linearity over a range from 0 to 0.1% (0 to M) of EDC HCl, which was calculated with the peak area (mv s), because the linearity was better than the peak height. The regression equation was y = x (y, peak area; x, % concentration of EDC HCl) or y = x (y, peak area; x, M concentration of EDC HCl). The relative standard deviation (RSD) of 8 measurements of % EDC HCl solution was 1.0%, and the limit of detection (LOD) corresponding to a signal-to-noise ratio of three was %. Fig. 8 Effect of the sample volume and the reaction coil length on the peak area. (a) Sample volume from 10 to 230 ml, (b) reaction coil (RC) length from 0.01 to 0.5 ml min 1. The experimental conditions were the same as in Fig. 1. The sensitivity of the proposed method was improved by about times compared with that of our previous method. 25 The analytical throughput was 12 samples h 1. Interference of coexistences The proposed condensation reaction between malonic acid and ethylenediamine with EDC HCl is quite specific. The interference of carboxylic acids (malonic acid, oxalic acid, succinic acid, and phthalic acid), diamines (ethylenediamine, iso-amylamine, o-phenylenediamine, 1,3-diaminopropane, 1,4- diaminobutane, and 1,5-diaminopentane), and amino acids (phenylalanine, glycine, arginine, alanine, and serine), whose concentration was 0.05 M, similar to that of malonic acid and ethylenediamine, was examined in the condensation reaction between malonic acid and ethylenediamine with the EDC HCl, whereas they did not show any additional absorbance. They did not interfere with the determination of EDC HCl. Also, other carbodiimide reagents, such as diisopropylcarbodiimide (DIC) and dicyclohexylcarbodiimide (DCC), are also expected to accelerate the condensation reaction between malonic acid and ethylenediamine. This proposed reaction might be applied to the determination of DIC and DCC. In addition, the proposed method was not interfered with coexisting carboxylic acids, diamines, and amino acids. The product in this condensation reaction showed a new absorbance band at 262 nm. Therefore, aromatic compounds and nucleic acids, which possess an aromatic ring, show absorbance at around 262

5 ANALYTICAL SCIENCES MARCH 2009, VOL LOD was %, and the RSD (8 measurements of % EDC HCl) was 1.0%. The EDC HCl in aqueous media could be measured by the proposed analytical method in a short time (measurement time per sample, less than 5 min). This technique could also be applied to monitoring the rapid decomposition of EDC HCl reagent in diluted hydrochloric acid. Acknowledgements Fig. 9 Analytical results of the concentration and the decomposed ratio of EDC HCl after mixing the reagents in 0.01 M hydrochloric acid. The initial concentration of EDC HCl was 0.02%. The experimental conditions were the same as in Fig. 1. nm. When the sample contains such matrices, the flow system must be improved as follows. For example, another detector was installed at the position of the post-sample injector, and the absorbance of sample was measured before the condensation reaction. Therefore, EDC HCl in such samples containing an aromatic ring might be measured by using the difference of the absorbance before and after the condensation reaction. The proposed technique, which was a rapid and sensitive method, could be applied to a purity test, quality control, and process control for the production of EDC HCl. Monitoring of EDC HCl decomposition EDC HCl can promote a condensation reaction between carboxylic acids and amines in aquatic media, which is useful for the formation of amide bonds (peptide bond) and esters in the field of biochemistry, whereas the decomposition rate of EDC HCl has not been elucidated and reported in detail, because rapid and sensitive determination methods for EDC HCl have not yet been developed. In this work, the decomposition rate of EDC HCl in 0.01 M hydrochloric acid was examined by the proposed FIA system. The EDC HCl is reported to form convertible structures, which can form a chain structure in diluted hydrochloric acid and a ring structure in water. 23,24,26 Therefore, EDC HCl in water and methanol/water (1:1) was stable due to forming the ring structure by itself. On the other hand, EDC HCl in 0.01 M hydrochloric acid was abruptly decomposed. Figure 9 shows the concentration and the decomposed ratio of EDC HCl in 0.01 M hydrochloric acid obtained by the proposed FIA. Almost EDC HCl in 0.01 M hydrochloric acid was decomposed within 50 min, which might have been caused by the formation of an unstable chain structure by EDC HCl itself. Conclusions The proposed FIA technique based on a specific condensation reaction between the malonic acid and ethylenediamine with EDC HCl was greatly improved for the sensitivity and reproducibility of EDC HCl determination in water samples. The The present study was partially supported by a Grant-in-Aid for Scientific Research (B) (No ) from Japan Society for Promotion of Science (JSPS). References 1. H. G. Khorana, Chem. Rev., 1953, 53, H. G. Khorana and R. A. Todd, J. Chem. Soc., 1953, C. A. Dekker and H. G. Khorana, J. Am. Chem. Soc., 1954, 76, H. G. Khorana and J. P. Vizsolyi, J. Am. Chem. Soc., 1961, 83, J. C. Sheehan and G. P. Hess, J. Am. Chem. Soc., 1955, 77, E. Wünsch and F. Deimer, Z. Physiol. Chem., 1972, 353, E. Wünsch and F. Deimer, Chem. Ber., 1966, 99, W. König and R. Geiger, Chem. Ber., 1970, 103, G. M. Tener, J. Am. Chem. Soc., 1961, 83, E. Ohtsuka, M. Ubasawa, and M. Ikehara, J. Am. Chem. Soc., 1970, 92, E. Vowinkel and C. Walft, Chem. Ber., 1974, 107, E. Vowinkel and I. Büthe, Chem. Ber., 1974, 107, W. S. Johnson, V. J. Bauer, M. A. Frisch, L. H. Dreger, and W. H. Hubbard, J. Am. Chem. Soc., 1961, 83, B. Neises and W. Steglech, Angew. Chem., Int. Ed., 1978, 17, C. Dhand, S. K. Arya, M. Datta, and B. D. Malhotra, Anal. Biochem., 2008, 383, X. Zhao, Y. Li, J. You, Y. Liu, and Y. Suo, Chin. J. Anal. Chem., 2007, 35, C. Situma, A. J. Moehring, M. A. F. Noor, and S. A. Soper, Anal. Biochem., 2007, 363, B. Qi and X. Yang, Chin. J. Anal. Chem., 2007, 35, M. Kobayashi and Y. Chiba, Anal. Biochem., 2007, 219, M. Kobayashi and E. Ichishima, Anal. Biochem., 1990, 189, J. Salnikow, A. Lehmann, and B. Wittmann-Liebold, Anal. Biochem., 1981, 117, B. S. Jacobson and K. R. Fairman, Anal. Biochem., 1980, 106, M. Wilchek, T. Miron, and J. Kohn, Anal. Biochem., 1981, 114, S. H. Chen, Anal. Biochem., 1983, 132, K. Seno, K. Matumura, M. Oshima, and S. Motomizu, Anal. Sci., 2008, 24, T. Ibrahim and A. Williams, J. Am. Chem. Soc., 1978, 100, 7420.