Characterization of Dialkyldithiophosphates as Slow Hydrogen Sulfide Releasing Chemicals and Their Effect on the Growth of Maize

Abstract

Hydrogen sulfide is a key gasotransmitter for plants and has been shown to greatly increase their growth and survival in the presence of environmental stressors. Current methods for slowly releasing hydrogen sulfide use chemicals, such as GYY-4137, but these result in the release of chemicals not found in the environment, and chemicals used may lack structures that can be readily tuned to affect the rate of release of hydrogen sulfide. In this article, we describe the synthesis and slow release of hydrogen sulfide from dialkyldithiophosphates, which are a new set of hydrogen sulfide releasing chemicals that can be used in agriculture. The rates of hydrolysis of dibutyldithiophosphate and GYY-4137 were measured in water at 85 °C and compared with each other to investigate their differences. GYY-4137 is widely used as a chemical that slowly releases H 2 S, but its rate of release was not previously quantified. The release of hydrogen sulfide in water at room temperature was measured for a series of dialkyldithiophosphates using a hydrogen sulfide electrode. It was shown that the structure of the dialkyldithiophosphate affected the amount of hydrogen sulfide released. The final degradation products of dibutyldithiophosphate were shown to be phosphoric acid and butanol, which are chemicals found in the environment. This result was notable because it demonstrated that dialkyldithiophosphates degrade to safe, natural chemicals that will not pollute the environment. To demonstrate that dialkyldithiophosphates have potential applications in agriculture, maize was grown for 4.5 weeks after exposure to 1−200 mg of dibutyldithiophosphate, and the weight of corn plants increased by up to 39% at low loadings of dibutyldithiophosphate.

Synthesis, Stability, and Kinetics of Hydrogen Sulfide Release of Dithiophosphates

“…In our prior work, we showed that the hydrolysis of dibutyldithiophosphate followed the mechanism shown in Figure 6 . 11 The first step of the hydrolysis was release of H 2 S, and the oxo intermediate was observed in the 31 P NMR spectra. No other intermediates were observed during this reaction for dibutyldithiophosphate.…”

“…In our prior work, less than 3% of the dithiophosphates synthesized from fatty alcohols hydrolyzed after 35 days, and the results shown in Table 1 were consistent with that observation. 11 Of the 13 chemicals investigated, 8 of the dithiophosphates showed less than 3% hydrolysis, and 3 others were not soluble in water and not investigated. The potassium and triethylamine salts of 5 , 7 , and 13 were synthesized in attempts to improve their solubilities, but these salts were insoluble in water.…”

“…In a recent publication, we reported the synthesis of dithiophosphates synthesized from the C2 to C12 fatty alcohols and P 4 S 10 at an elevated temperature of 85 °C for up to 16 h ( Figure 3 a). 11 The acids slowly decomposed at room temperature, so the salts were isolated and shown to be stable at room temperature under ambient conditions for months. These reaction conditions were used as the starting point for the synthesis of the dialkoxydithiophosphates reported here, and the syntheses were successful to yield the dialkoxydithiophosphates with yields from 66 to 93% ( Figure 3 b).…”

“…The dithiophosphate synthesized from n-butanol (1) hydrolyzed 3.5× slower than five-membered ring dithiophosphate 2, 7.4× slower than the five-membered ring dithiophosphate 3, and 1.4× faster than the six-membered ring dithiophosphate 4. Interestingly, the rates of hydrolysis of similarly structured disulfidedithiophosphates with five- (11) and six-( 12) membered rings containing sulfur atoms were within 2× of the rate of hydrolysis of 10.…”

“…In recent work, we showed that milligram loadings of GYY-4137 and dibutyldithiophosphate increased the harvest yields of radishes ( Raphanus sativus ), peas, and lettuce and increased the growth of corn plants. 11 , 14 The mechanism for how H 2 S has an important effect on plants is poorly understood, although it is likely that it acts on plant enzymes similar to how it has been shown to act on enzymes in human cells. 4 , 7 − 10 The application of exogenous H 2 S in agriculture is a new and potentially useful target to promote the survival of plants in a changing climate and to improve the harvest yields of many crops.…”