The Importance of Reaction Conditions on the Chemical Structure of N,O-Acylated Chitosan Derivatives

Abstract

The structure of acylated chitosan derivatives strongly determines the properties of obtained products, influencing their hydrodynamic properties and thereby their solubility or self-assembly susceptibility. In the present work, the significance of slight changes in acylation conditions on the structure and properties of the products is discussed. A series of chitosan-acylated derivatives was synthesized by varying reaction conditions in a two-step process. As reaction media, two diluted acid solutions—i.e., acetic acid and hydrochloric acid)—and two coupling systems—i.e., 1-ethyl-3-(3-dimethyl-aminopropyl)-1-carbodiimide hydrochloride (EDC) and N–hydroxysulfosuccinimide (EDC/NHS)—were used. The chemical structure of the derivatives was studied in detail by means of two spectroscopic methods, namely infrared and nuclear magnetic resonance spectroscopy, in order to analyze the preference of the systems towards N- or O-acylation reactions, depending on the synthesis conditions used. The results obtained from advanced 1H-13C HMQC spectra emphasized the challenge of achieving a selective acylation reaction path. Additionally, the study of the molecular weight and solution behavior of the derivatives revealed that even slight changes in their chemical structure have an important influence on their final properties. Therefore, an exact knowledge of the obtained structure of derivatives is essential to achieve reaction reproducibility and to target the application.

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“…This belonged to O–H and N–H stretching vibration and was the characteristic adsorption peak by the overlapping of intramolecular and intermolecular hydrogen bonds of CS compound. Around 1647 cm −1 was the absorption peak of C=O stretching vibration, 1559 cm −1 was the absorption peak of N–H in-plane bending vibration and 1378 cm −1 was the absorption peak of N–H out of plane bending vibration [ 20 ]. About 1152 cm −1 was the asymmetrical stretching vibration peak of C–O–C oxygen bridge, whereas 1068 and 1028 cm −1 peak represented the stretching vibration of sugar ring.…”

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“…Synthesis of chitosan derivative was successfully performed according to our earlier procedure [ 27 ]. Conducting the reaction between chitosan and linoleic acid in water adjusted to pH = 4.0 using HCl with equimolar amount of EDC as a coupling agent, leads to N , O -acylated chitosan derivative with altered solubility comparing to the unmodified chitosan.…”

“…The other bands observed are: newly formed 1200 cm −1 and 1250 cm −1 bands characteristic for ester stretching mode of C=O and C−O; band at 1150 cm −1 related to asymmetric bridge oxygen stretching; band at 1425 cm −1 (CH)/1435 cm −1 (CH_LA) assigned to CH 2 and CH 3 vibration. Detailed structure description can be found in our earlier work [ 27 ].…”

“…Chitosan derivative was synthesized and characterized as described previously [ 27 ]. Briefly, reaction was conducted at pH = 4.0 (aqueous HCl solution), at room temperature.…”

“…In the next stage, solution of activated LA was added dropwise to the chitosan solution and stirred for 24 h. In order to purify the product, the reaction mixture was dialyzed against water for 48 h at 25 °C (Spectra/Por TM 3, MWCO: 3.5 kDa, Gardena, CA, USA) and then freeze-dried (freeze dryer, Christ, Alpha 1-2, Martin Christ, GmbH, Osterode am Harz, Germany ) for 48 h. The lyophilized CH_LA was in yellowish, solid, sponge-like form. A weight average molecular weight of 65.8 kDa was obtained by GFC [ 27 ].…”