Measurement of Forces between Galactomannan Polymer Chains:  Effect of Hydrogen Bonding

Abstract

Packing free energies and structural transitions of concentrated arrays of guar galactomannan macromolecules and of guars modified by hydroxypropyl substitution (HPG) have been studied using the osmotic stress method combined with X-ray scattering. All show a liquid crystalline structure with packing free energies that are very similar for guar and HPG and well described by the model of Selinger and Bruinsma for entropic steric repulsion between chains. In addition, a transition from the liquid crystalline form to a crystalline structure is observed as native guar becomes more densely packed. This transition is related to the propensity of guar to form intermolecular hydrogen bonds in solutions. Hydroxypropyl substitution of galactomannan hydroxyl groups causes steric interference that decreases the stability of this hydrogen-bonded crystalline structure. Even for moderately hydroxypropyl-substituted guar (∼0.3 HP/sugar residue), the transition occurs at a much higher osmotic pressure than for native guar. The extra work needed to crystallize this HPG compared with guar is calculated to be 3 kT/mannose unit or 6-7 kT per hydroxypropyl group. No transition was found for more highly substituted guars. Urea increased the osmotic pressure necessary for the transition of guar but also resulted in new crystalline packing structure.

Guar gum: processing, properties and food applications—A Review

“…With slight addition of guar gum, there is alteration in electrokinetic properties of any system markedly (Glicksman 1969). Substitution of hydroxyl groups in guar gum with hydroxypropyl causes steric hindrance that decreases the stability of hydrogen bonds (Cheng and Prud'homme 2002).…”

Low viscosity hydrogel of guar gum: Preparation and physicochemical characterization

“…It is a galactomannan, which consists of a (1→4) linked ␤-mannopyranosyl backbone partially substituted at O-6 with ␣-d-galactopyranosyl side groups, with the ratio of mannose to galactose ∼1.6-1.8:1 [8]. It has been extensively used in a range of applications because of its unique ability to alter rheological properties [9].…”

An aerogel obtained from chemo-enzymatically oxidized fenugreek galactomannans as a versatile delivery system

“…(Crescenzi et al, 2004;Daas, Schols, & de Jongh, 2000;Daas, Grolle, van Vliet, Schols, & de Jongh, 2002;Daniel, Whistler, Voragen, & Pilnik, 1994;Liyanage, Abidia, Auldb, & Moussa, 2015;McCleary, Clark, Dea, & Rees, 1985;Merlini, Boccia, Mendichi, & Galante, 2015;Sittikijyothin, Torres, & Gonç alves, 2005;Wei et al, 2015). This peculiar structure makes them fairly soluble in water at different temperatures, flexible in application and chemically/biochemically quite reactive (Cheng, Prud'homme, Chick, & Rau, 2002).…”