X‐Ray Structure of a Self‐Assembled Gelating Fiber

Abstract

Almost seventy years ago, chemist Dorothy Jordan noted that "gels are easier to recognize than to define"."] We will attempt a definition nonetheless : Gels are semi-rigid colloidal systems rich in liquid. Protoplasm ranks as the most widespread example of this peculiar state of matter. A gel is formed by a molecular network, often fibrous in nature, that entrains the dispersing medium within its capillary spaces. Most organic gelators are polymeric (for example, gelatin, ['] poIy(acry1ic acid) ,I3] and hyaluronic acidr4]), but small molecules (for example. cholesteryl 4-(2-anthryloxy)b~tanoate, [~~ 5-hexadecyl-2,4,6-triaminopyrimidine,l6] N-octylglucona~nide,~'~ and arborolsr8]) can induce gelation as well.We have long been interested in a little-known but remarkable gelator. dibenzoyl-L-cystine (DBC).19] Although most gels require 1 -1 0 O i O gelator, DBC in water rigidifies at concentrationsas low as 2 mM (0.1 YO). While attempting to learn how such miniscule quantities of DBC manage to self-assemble into a macroscopic network, we performed an X-ray analysis of a gel "fiber". Structural data at the atomic level is virtually unavailable for gel networks, and it is only through good fortune that these data became accessible. But before giving the details, let us summarize the properties of acylcystines in water.DBC forms a firm, translucent gel in a thermally reversible process at a concentration of 4 mM in ethanol (5%)/water. Ethanol was used because the DBC tends to crystallize from pure water. There is, clearly. a delicate balance between gelation and crystallization. The gelating properties are destroyed when the S-S linkage is replaced by CH,CH, or CH=CH groups."'] This implies that the CH,-S-S-CH, dihedral angle["] of about 90 'C plays a key role in the formation of the molecular network. Gels d o not form from esterified DBC or at basic p H values, which suggests the presence of either a favorable -COOH/-COOH attraction or a deleterious -COY /-CO; repulsion. Addition of NaCl (0.1 M) softens the gels, whereas more than 10 YO dioxane induces crystallization.Diacetyl-L-cystine fails to gelate in ethanol ( 5 %)/water. Di-noctanoyl-L-cystine (4 mM) forms only a soft and cloudy gel. The superior quality of the DBC gel leads us to suspect that aromatic K -~T stacking stabilizes the creation of fibers. This conclusion is appealing. because intermolecular hydrogen bonding between DBC molecules in water would seem too unfavorable to serve as the sole adhesive force. Assembly of DBC molecules appears to be a cooperative event much like the micellization of surfactants. This was affirmed by a primitive yet informative experiment in which 2.0 x 2.0 cm2 squares of Parafilm were placed on the surface of aged DBC gels. Known weights were then positioned carefully at the center of a film until the film submerged. A plot of "supportable weight" vs. DBC concentration (not shown) gave a sharp break at 1.6 KIM, a value that might be qualitatively regarded as the "critical gelation concentration".Spin-lattice relaxation stud...

p‐Quaterphenylene as an Aggregation‐Induced Emission Fluorogen in Supramolecular Organogels and Fluorescent Sensors

“…Systematic characterizations of the gels with UV/Vis, FTIR, 1 HNMR, and X-ray diffraction (XRD) spectra were performed to identify the intermolecular interactions and the driving force for the formation of organogels. [45][46][47][48][49][50] Compared with TP-1,i n the dilute toluenes olution, the UV/Vis absorption peak of QP-1,a ssigned to the p-p*t ransition, shifted to longerw avelengths (ca. 317 nm for TP-1 and 324 nm for QP-1,F igure S4 in the Supporting Information) owing to the elongation of the conjugated core.…”

Supramolecular Nanofibers by Self‐Organization of Bola‐amphiphiles through a Combination of Hydrogen Bonding and π–π Stacking Interactions

“…Enhancement of the stability and robustness of these self-organized structures may rely on a combination of different intermolecular interactions. [10,11] Inspired by the early work of Fuhrhop et al and Ringsdorf and co-workers on the self-organization of bola-amphiphiles, [12,13] we have introduced rigid mesogenic groups or strong p-p-stacking moieties into bolaamphiphiles, resulting in significantly enhanced stability of the self-assembled structures. [14,15] In this Communication, we describe the design and synthesis of a new bola-amphiphile with two carboxylic acid end groups and a diketopyrrolopyrrole (DPP) chromophore in the central part.…”

Low Molecular Weight Gelators for Organic Solvents

“…Recent investigations showed that only the pure enantiomer forms gels whereas the racemate crystallizes, and that both hydrogen bonding and π-stacking interactions are important for gelation [27]. In addition, D-glucose, D-galactose and D-mannose-benzylidene derivatives have recently been reported as potent organogelators [28], as well as a number of other compounds which gelate organic solvents via intermolecular hydrogen bonds [29,30,31,32,33,34]. …”