Conformations and molecular interactions of poly-γ-glutamic acid as a soluble microbial product in aqueous solutions

Abstract

Soluble microbial products (SMPs) are of significant concern in the natural environment and in engineered systems. In this work, poly-γ-glutamic acid (γ-PGA), which is predominantly produced by Bacillus sp., was investigated in terms of pH-induced conformational changes and molecular interactions in aqueous solutions; accordingly, its sedimentation coefficient distribution and viscosity were also elucidated. Experimental results indicate that pH has a significant impact on the structure and molecular interactions of γ-PGA. The conformation of the γ-PGA acid form (γ-PGA-H) is rod-like while that of the γ-PGA sodium form (γ-PGA-Na) is sphere-like. The transformation from α-helix to random coil in the γ-PGA secondary structure is primarily responsible for this shape variation. The intramolecular hydrogen bonds in the γ-PGA-H structure decrease and intramolecular electrostatic repulsion increases as pH increases; however, the sedimentation coefficient distributions of γ-PGA are dependent on intermolecular interactions rather than intramolecular interactions. Concentration has a more substantial effect on intermolecular electrostatic repulsion and chain entanglement at higher pH values. Consequently, the sedimentation coefficient distributions of γ-PGA shift significantly at pH 8.9 from 0.1 to 1.0 g/L, and the viscosity of γ-PGA (5% w/v) significantly increases as pH increases from 2.3 to 6.0.

Properties of Poly-γ-Glutamic Acid Producing-Bacillus Species Isolated From Ogi Liquor and Lemon-Ogi Liquor

“…There are few reports on the antimicrobial functions of pure γPGA. Although it is reported that higher molecular weight of polymer may diffuse slowly, the factors that are considered for antimicrobial activity are surface charge, hydrophobicity, hydrophilicity, and intramolecular hydrogen bonding (Khalil et al, 2017; Wang et al, 2017), and these differ between producing strains.…”

Complexation and conformation of lead ion with poly-γ-glutamic acid in soluble state

“…The p K a of γ-PGA was determined to be 4.86 [21] in our previous study and Fig 1(A) illustrates the effect of pH on γ-PGA species. The acid form γ-PGA-H rapidly ionized to form the mono-anionic γ-PGA - specie as the pH increased from pH 3.0 to 7.0.…”

“…The purity and glutamic acid content of γ-PGA were determined by amino acid analysis using thin layer chromatography (TLC) and colorimetric analysis after hydrolysis as described by Wang et al [21]. Metal ions were analyzed using an inductively coupled plasma mass spectrometer (ICP-MS, 7700X, Agilent Inc., USA).…”

“…CD measurements were performed based on our previously study [21] on a Chirascan spectrometer (Applied Photophysics Co., UK) using a quartz cell of 1 cm path length. CD spectra of 10 mg/L γ-PGA solutions were collected from 190 to 260 nm.…”

“…While these studies have focused on flocculation ability of γ-PGA in relation to the wastewater treatments, there is no complexation of heavy metals with γ-PGA in diluted solutions, where the polymer-metal complex remains soluble and could enhance the mobility of heavy metals in the environment, has not been investigated in detail. Especially, the conformations and molecular interactions of complexation of heavy metals with γ -PGA in a soluble state may be different from that in a flocculent state [21].…”