2,6-Pyridinedicarbothioic acid (PDTC) is an organosulfur compound that is produced by some bacteria. It functions as a siderophore

Siderophore

Siderophores are small, high-affinity iron chelating compounds secreted by grasses and microorganisms such as bacteria and fungi...

, a small chelating agent with a high affinity for iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

. Siderophores are deployed as ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 scavengers for microbes. Siderophores solubilize compounds by forming strong complexes. PDTC is secreted by the soil

Soil

Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 Pseudomonas stutzeri

Pseudomonas stutzeri

Pseudomonas stutzeri is a Gram-negative, rod-shaped, motile, single polar-flagellated, soil bacterium first isolated from human spinal fluid.. It is a denitrifying bacterium, and strain KC of P. stutzeri may be used for bioremediation as it is able to degrade carbon tetrachloride. It is also an...

and Pseudomonas putida

Pseudomonas putida

Pseudomonas putida is a gram-negative rod-shaped saprotrophic soil bacterium. Based on 16S rRNA analysis, P. putida has been placed in the P. putida group, to which it lends its name....

.

Synthesis and biosynthesis

PDTC can be synthesized by treating the pyridine-2,6-dicarboxylic acid (or its diacid dichloride) with H₂S in dry pyridine

Pyridine

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

:

NC₅H₃(COOH)₂ + 2 H₂S → NC₅H₃(COSH)₂ + 2 H₂O

This produces an orange 1:1 pyridinium

Pyridinium

Pyridinium refers to the cationic form of pyridine. This can either be due to protonation of the ring nitrogen or because of addition of a substituent to the ring nitrogen, typically via alkylation. The lone pair of electrons on the nitrogen atom of pyridine is not delocalized, and thus pyridine...

 salt of 2,6-pyridinedicarbothioate. Treatment of this salt with acid give PDTC, which can then be extracted with dichloromethane

Dichloromethane

Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...

.

The biosynthesis of PDTC remains unclear although some insights can be deduced from the genetics. It is suggested that Pseudomonas stutzeri may have acquired at least one of the genes by lateral transfer from mycobacteria. In a proposed biosynthetic sequence pyridine-2,6-dicarboxylic acid, a known bacterial metabolite, is activated as its bis-adenosine monophosphate (AMP) derivative. The sulfur donor and its activation remain uncertain.

Coordination chemistry

PDTC binds to both Fe²⁺ and Fe³⁺. The ferric complex is brown, whereas the ferrous complex is blue. In the presence of air, the ferrous complex oxidizes to the ferric compound. It is iron selective as only the Fe complex is soluble in water. PDTC is produced mainly during the exponential phase of bacterial growth. The conditions at which Pseudomonas produces PDTC is 25 °C, pH=8 and sufficient aeration.