Characterization of Different Molecular Size Fractions of Glomalin-Related Soil Protein From Forest Soil and Their Interaction With Phenanthrene

Abstract

As a natural organic compound secreted by arbuscular mycorrhizal fungi (AMF), glomalin-related soil protein (GRSP) is an important part in soil, affecting the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in it. Previous research have demonstrated that GRSP could enhance the availability of PAHs in the soil and favor their accumulation in plant roots. However, a scarcity of research exists on the different molecular weights of GRSP interacting with PAHs due to their complexation and heterogeneity. In this research, the extracted GRSP in soil was divided into three molecular weight (Mw) fractions of GRSP (<3,000, 3,000-10,000, and >10,000 Da), whose characteristics and binding capacity of PAHs were conducted by using UV-visible absorption, quenching fluorometry and, Fourier transform infrared spectroscopy. The results showed that the GRSP was composed of abundant compounds, it has a wide distribution of molecular weight, and the >10,000 Da Mw fraction was dominant. For three Mw fractions of GRSP, they have some difference in spectral features, for example, the >10,000 Da fraction showed higher dissolved organic carbon (DOC) contents, more phenolic hydroxyl groups, and stronger UV adsorption capacity than the low and middle Mw fractions. In addition, the interaction between GRSP and phenanthrene is related to the characteristics of the Mw fractions, especially the phenolic hydroxyl group, which has a significantly positive correlation with a binding coefficient of K _A (k = 0.992, p < 0.01). Simultaneously, hydrophobic, NH-π, and H-bound also played roles in the complexation of phenanthrene with GRSP. These findings suggested that different GRSP _Mw fractions could influence the fate, availability, and toxicity of PAHs in soil by their interaction.

Keywords: association coefficient; glomalin-related soil protein (GRSP); molecular weight; phenanthrene; spectroscopic characterization.

FIGURE 1

Sequential extraction of glomalin-related soil protein with a different molecular weight in soil.

FIGURE 2

UV–visible spectra of glomalin-related soil protein (GRSP)_MW solution. The SUVA₂₅₄ of GRSP_MW solution (A), SUVA₂₆₀ (B), and SUVA₂₈₀ (C) express the degree of aromatization and hydrophobicity of GRSP_MW, respectively; E4/E6 means the ratio of UV being 465 and 665 nm (D), which characterized the degree of decay and molecular weight of humus; ΔlogK is the tonal coefficient, the logarithm difference between the absorbance at 400 and 600 nm (E). (F) Absorbance of GRSP_MW from 220 to 800 nm. FU, F1–F3 represents different molecular fractions from GRSP_bulk: <3,000, 3,000–10,000 and 10,000 Da, respectively.

FIGURE 3

Phenanthrene-glomalin-related soil protein (GRSP) binding probed by GRSP-caused fluorescence quenching. The quenching constant (K_sv), binding constant (K_A), and number of binding sites (n) for the binding of phenanthrene with GRSP were calculated from the Stern–Volmer plot and site-binding plot, respectively.

FIGURE 4

The FTIR analysis of interactions between phenanthrene and molecular weight glomalin-related soil protein (GRSP). (A1–D1) means the functional groups of GRSP _{< 3,000 Da}, GRSP_{3,000–10,000 Da}, GRSP _{> 10,000 Da}, and GRSP_bulk. (A2–D2) means the changes after the interaction between GRSP and phenanthrene.