Evaluation of the effects of phenylalanine and carboxylate on the rheological behaviors of small molecule hydrogelators
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Evaluationoftheeffectsofphenylalanineandcarboxylateonthe rheologicalbehaviorsofsmallmoleculehydrogelators containingnaphthalene Junfeng Shi, Yue Pan, Yuan Gao, and Bing Xu* Department of chemistry, Brandeis University, 415 South Street, MS 015, Waltham, MA 02453, USA ABSTRACT By systematically altering the number and position of phenylalanine and carboxylate groups on a series of hydrogelators containing a naphthalene motif, we evaluated the correlation of molecular structures, self-assembly, and the rheological properties of the hydrogels. The storage moduli of the hydrogels decrease with the increase of the number of phenylalanine or with the insertion of a cysteine residue, and the effect of the carboxylic group on the rheological properties depends on the backbone of the hydrogelators. Transmission electron microscopy shows that these hydrogelators self-assemble in water to form nanofibers and result in threedimensional networks. Circular dichroism experiment indicates the hydrogelators self-assemble to form ȕ-sheet-like structure within the nanofibers. This work suggests that control of the synergy of hydrogen bonding and aromatic-aromatic interactions may offer a feasible way to modulate the rheological properties of molecular hydrogels consisting of small molecules. INTRODUCTION This article reports the correlation of rheological behavior and molecular structure of the molecular hydrogels made of new hydrogelators containing a naphthalene motif. Molecular hydrogels,1-13 as the consequence of self-assembly of small molecules in water to form three dimensional and continuous networks, have found increased applications in biomedicine, for example, as encapsulation matrix for cell cultures in three dimensional environment,14-18 as drug delivery systems,19-23 as a platform for biosensors,24, 25 and as a medium for regenerative tissue engineering.26, 27 The successful application of hydrogels not only requires certain biological functionalities, but also demands proper elasticity of the hydrogels for a variety of applications.28, 29 Although the increase of the volume fraction (i.e., the concentration) of the hydrogelators raises the elasticity of the hydrogels, there are instances that only a limited amount of hydrogelators can dissolve in water. Therefore, it is useful and important to correlate the molecular structures with the rheological behavior of the hydrogelators because a subtle change of the molecular structures may confer the significant changes of intermolecular interactions and affect the rheological behavior of the resulted hydrogels.30, 31 It, however, remains a challenge to establish this apparently simple relationship between the molecular structure of the hydrogelator and the elasticity of the hydrogel. In this work, we study several hydrogelators containing a naphthalene and intend to provide some insights on the correlation between the rheological behavior and molecular structure of this type of hydrogelators. We choose naphthalene as the aromatic moiety to promote hydrogelat
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