DETERMINING STABILITY CONSTANTS OF METAL-PROLINE 2′DEOXYMUGINEIC ACID (PDMA) CHELATES VIA POTENTIOMETRIC AND SPECTROPHOTOMETRIC TITRATION

Trace metal limitation is a widespread ecologic and agricultural issue with far-reaching implications for managing healthy and productive soils worldwide (Kraemer et al., 2006). Several essential metal nutrients primarily exist outside of the bioavailable pool due to their slow dissolution kinetics or low solubilities (Kraemer et al., 2015). To overcome this problem, organisms have developed the ability to exude metal chelating compounds called siderophores that promote biologic uptake of essential metal nutrients (Harrington et al., 2015). These exudates display widely variable binding affinities for specific metals, contributing to large discrepancies in nutrient uptake rates in competitive soil ecosystems (Treeby et al., 1989). Our study seeks to quantify the binding affinities and stability constants of Mn, Co, Cu, Ni, and Zn with PDMA, a recently developed analog of the well-studied but notoriously expensive 2′- deoxymugineic acid (DMA) (Suzuki et al., 2021). Stability constants of each metal-siderophore complex were determined via simultaneous potentiometric and spectrophotometric titration of 1 mM of metal and 1 mM of siderophore in a 0.1 M electrolyte background using a standardized 0.1 M NaOH titrant. A UV-VIS-NIR Deuterium-Halogen Light Source (OceanOptics, FL, USA) was used to collect our spectral measurements following each dose of titrant. We fit our data in hypspec to determine stability constants and relative binding affinities for specific metal-siderophore combinations. Our results are critical for furthering our understanding of PDMA’s effectiveness at mobilizing trace metals, particularly in nutrient-poor, alkaline soils that comprise approximately one third of Earth’s land area (Kraemer et al., 2006).