# Bisphenol S

Bisphenol S (BPS, dioxydiphenylsulfone) is an organic compound with the formula (HOC$_6$H$_4$)$_2$SO$_2$. It has two phenol functional groups on either side of a sulfonyl group. It is commonly used in curing fast-drying epoxy resin adhesives. It is classified as a bisphenol, and a close molecular analog of bisphenol A (BPA). BPS differs from BPA in possessing a sulfone group (SO$_2$) as the central linker in the molecule instead of the dimethylmethylene group (C(CH$_3$)$_2$) of bisphenol A.

## History

German chemist Ludwig Glutz (1844–1873) first prepared the compound from phenol and hot sulfuric acid in 1867, designating it oxysulphobenzide. It was used starting in 1869 as a dye.
BPS received the modern name in the late 1950s and is currently common in everyday consumer products. BPS is an analog of BPA that has replaced BPA in a variety of ways, being present in thermal receipt paper, plastics, and indoor dust. After health concerns associated with bisphenol A grew in 2012, BPS began to be used as a replacement.

## Use

BPS is used in curing fast-drying epoxy glues and as a corrosion inhibitor. It is also commonly used as a reactant in polymer reactions.
BPS has become increasingly common as a building block in polyethersulfone and some epoxies, following the public awareness that BPA has estrogen-mimicking properties, and widespread-belief that enough of it remains in the products to be dangerous. However, BPS may have comparable estrogenic effects to BPA. BPS is now used to a variety of common consumer products. In some cases, BPS is used where the legal prohibition on BPA allows products (esp. plastic containers) containing BPS to be labelled "BPA free". BPS also has the advantage of being more stable to heat and light than BPA.
To comply with restrictions and regulations on BPA due to its confirmed toxicity, manufacturers are gradually replacing BPA with other related compounds, mainly bisphenol S, as substitutes in industrial applications.
BPS is also used as an anticorrosive agent in epoxy glues. Chemically, BPS is being used as a reagent in polymer reactions. BPS has also been reported to occur in canned foodstuffs, such as tin cans.
In a 2015 study analyzing BPS in a variety of paper products worldwide, BPS was found in 100% of tickets, mailing envelopes, airplane boarding passes, and airplane luggage tags. In this study, very high concentrations of BPS were detected in thermal receipt samples collected from cities in the United States, Japan, Korea, and Vietnam. The BPS concentrations were large but varied greatly, from a few tens of nanograms per gram to several milligrams per gram. Nevertheless, concentrations of BPS used in thermal paper are usually lower compared to those of BPA. Finally, BPS can get into the human body through dermal absorption from handling banknotes.

## Health effects
### Cardiac effects

Although there is no direct link established between BPS and cardiac disease, it is thought that BPS may operate by a similar mechanism to BPA and could cause cardiac toxicity. In animal studies, BPS has been shown to hinder MI recovery, induce cardiac arrhythmias and cause cardiac developmental deformities. Rats exposed to high doses of BPS were reported to have increased risk of atherosclerosis (a significant risk factor in cardiac disease) due to BPS inducing synthesis of cholesterol in peripheral tissues.

### Neurobehavioral effects

BPS has the potential to have an effect on a wide range of neurological functions. A recent study showed that exposure to BPS during pregnancy may disrupt thyroid hormone levels. These are important in foetal neurodevelopment and prenatal exposure to BPS has been linked to impaired psychomotor development in children. In a study using human embryonic stem cells, BPS was shown to cause a reduction in length of neurites in neuron-like cells. This disruption could lead to neurobehavioral problems such as ASD.
The mechanism of the neurological impact of BPS is thought to be related to its oestrogenic effect which can interfere in the levels and action of thyroid hormone, which is essential for normal development of the nervous system; it regulates migration and differentiation of neural cells, synaptogenesis and myelination.

### Effects on obesity

It has been proposed that BPS has the potential to affect body weight, and several studies have found a correlation between exposure to bisphenols and increased body weight. This is thought to be due to an accumulation of lipids in adipocytes i.e. a build-up of fat in fat cells. It has also been suggested that BPS leads to the formation of new adipocytes as exposure to it increases the expression of related markers. A correlation between exposure to BPS before birth and being overweight has been found in mice, although this was only found when they were also fed a high fat diet.
The pathway through which BPS acts on cells to increase body weight is suggested to be different to the pathway through which BPA acts, even though they have very similar chemical structures.
Only one study has demonstrated a decrease in body weight after BPS exposure, and the affected mice quickly regained the weight they had lost.

### Other metabolic effects

BPS levels in the human body can be measured in the urine. In one study of children, there was a significant correlation between urinary levels of BPS and insulin resistance, abnormal kidney function and abnormal vascular function.
It has been suggested that there is a link between gestational diabetes mellitus and urinary BPS. Therefore, exposure to BPS may be a risk factor for developing the condition.

### Effects on skeletal development

The effect of long term exposure to BPS is an enrichment of osteoclast differentiation and enhanced development of the embryonic skeletal system.

### Effects on early development

BPS, like BPA, can cross the placenta in sheep and alter the endocrine functionality of the placenta. It does this by reducing the maternal serum concentration of trophoblastic proteins. BPS shows almost identical effects on the placenta as BPA, with both BPS and BPS altering almost identical sets of genes.
Fetal exposure to BPS through the placenta, during a critical period, can have negative effects on the developmental programming of the fetus. BPS exposure in the zebrafish model affected development of the hypothalamus and resulted in hyperactive behaviour.
Studies in the Mouse model have shown that exposure to BPS significantly reduced the secretion of testosterone within the mouse fetal testes, with exposure to BPS in female mice also causing a significant fall in egg number, whilst also negatively affecting the quality of oocytes.
Zebrafish and humans share 70% of the same genes that are expressed during development therefore, they are a useful model organism to understand the effects of BPS. Studies in the Zebrafish model have shown that parental exposure to BPS causes disrupted thyroid hormone levels in both the parental generation and F1 generation. Fetal exposure to BPS through the placenta, during a critical period, can have negative effects on the developmental programming of the fetus. Additionally, there is evidence to suggest that embryos with high levels of BPS exhibit teratogenic effects of vital organs such as the heart and liver. Additionally, BPS inhibits the expression of genes within the liver used for the metabolism thus leading to increased stress of the liver through the zebrafish life. Adult zebrafish that are exposed to low levels of BPS during development display hyperactivity due to an exponential increase in neural activity within the hypothalamus.
It is not clear the mechanism of BPS's effect on thyroid hormone levels after human exposure.

### Effects on reproductive health

The endocrine disrupting nature of BPS has encouraged investigations into its affinity to estrogenic receptors, showing BPS to be a weak agonist; similar in potency to BPA, which it has come to substitute. Select studies show BPS to be capable of mimicking estradiol, and sometimes being more effective. The estrogenic activity of BPS has been demonstrated through in vivo rodent studies, inducing growth of the womb, with a range of dosages.
These are pathways necessary for cell function, cell cycle regulation, and neuroendocrine induced behaviours which are important for reproduction. BPS has shown to both disrupt signalling and damage DNA. Androgenic and antiandrogenic activity have also been confirmed by BPS disrupting function of the androgen receptors. Studies on zebrafish have shown decreased egg quality, reduced sperm count, an increased frequency of embryo abnormalities, as well as changes in the mass of gonads; suggesting that BPS is a reproductive toxin for both sexes.
The use of Bisphenol-A in manufacturing of household products has been reduced due to its effects as an endocrine disruptor, with research suggesting a disposition to greater deleterious effects to women as compared to men. Research has suggested that BPA and its cousins (BPS,BPF, etc.) have sex dependent effects on development. Male zebrafish exposed to BPS indicated a significant increase in estrogen levels and a decrease in testosterone levels. The decrease in testosterone when exposed to BPS was found to be 200 times more than the decrease in testosterone by BPA. There are increased levels of mRNA transcription of the aromatase gene and increased levels of mRNA transcription of GnRH genes with decreased levels of follicle stimulating hormone and luteinizing hormone.

## Bisphenol-S concentrations within populations

A relationship to higher BPS concentrations is linked to individuals within certain socio-economic classes hence placing those individuals at greater risk of possible deleterious effects. Individuals with an annual income of less than $20,000 were found to have the highest concentrations of bisphenol and individuals with an annual income of $75,000 or more to have the lowest concentrations, suggesting a linear relationship between bodily concentrations of BPS and income. Black women had the highest concentrations of BPS with levels 93% higher than those of white women.

## Environmental considerations

Recent work suggests that, like BPA, BPS also has endocrine disrupting properties. What makes BPS, and BPA endocrine disruptors is the presence of the hydroxy group on the benzene ring. This phenol moiety allows BPA and BPS to mimic estradiol. In a study of human urine, BPS was found in 81% of the samples tested. This percentage is comparable to BPA which was found in 95% of urine samples. Another study done on thermal receipt paper shows that 88% of human exposure to BPS is through receipts.
The recycling of thermal paper can introduce BPS into the cycle of paper production and cause BPS contamination of other types of paper products. A recent study showed presence of BPS in more than 70% of the household waste paper samples, potentially indicating spreading of BPS contamination through paper recycling.
BPS is more resistant to environmental degradation than BPA, and although not persistent cannot be characterised as readily biodegradable.

## Regulation

In the US it is difficult for consumers to determine if a product contains BPS due to limited labelling regulations. In January 2023 the European Chemicals Agency added bisphenol S to the candidate list for substance of very high concern designation, while it is investigated for reproductive toxicity and endocrine disruption.

## Synthesis

Bisphenol S is prepared by the reaction of two equivalents of phenol with one equivalent of sulfuric acid or oleum.
    2 C$_6$H$_5$OH + H$_2$SO$_4$ → (C$_6$H$_4$OH)$_2$SO$_2$ + 2 H$_2$O
    2 C$_6$H$_5$OH + SO$_3$ → (C$_6$H$_4$OH)$_2$SO$_2$ + H$_2$O
This reaction can also produce 2,4'-sulfonyldiphenol, a common isomeric complication in electrophilic aromatic substitution reactions.
    Chemical reaction to bisphenol S