chemistry

S-Nitrosothiol

S-Nitrosothiol is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand S-Nitrosothiol rather than just read about it. In short: In organic chemistry, S-nitrosothiols, also known as thionitrites, are organic compounds or functional groups containing a nitroso group attached to the sulfur atom of a thiol. S-Nitrosothiols have the general formula R−S−N=O, where R denotes an organic group.

Key takeaways

S-Nitrosothiol belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect S-Nitrosothiol to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of S-Nitrosothiol from memory before moving on to harder problems.

Reference excerpt

In organic chemistry, S-nitrosothiols, also known as thionitrites, are organic compounds or functional groups containing a nitroso group attached to the sulfur atom of a thiol. S-Nitrosothiols have the general formula R−S−N=O, where R denotes an organic group. S-Nitrosothiols have received much attention in biochemistry because they serve as donors of both the nitrosonium ion NO+ and of nitric oxide and thus best rationalize the chemistry of NO-based signaling in living systems, especially related to vasodilation. Red blood cells, for instance, carry an essential reservoir of S-nitrosohemoglobin and release S-nitrosothiols into the bloodstream under low-oxygen conditions, causing the blood vessels to dilate.

In biochemistry

Originally suggested by Ignarro to serve as intermediates in the action of organic nitrates, endogenous S-nitrosothiols were discovered by Stamler and colleagues (S-nitrosoalbumin in plasma and S-nitrosoglutathione in airway lining fluid) and shown to represent a main source of NO bioactivity in vivo. More recently, S-nitrosothiols have been implicated as primary mediators of protein S-nitrosylation, the oxidative modification of cysteine thiol that provides ubiquitous regulation of protein function. S-nitrosothiols are composed of small molecules, peptides and proteins. The addition of a nitroso group to a sulfur atom of an amino acid residue of a protein is known as S-nitrosylation or S-nitrosation. This is a reversible process and a major form of posttranslational modification of proteins. S-Nitrosylated proteins (SNO-proteins) serve to transmit nitric oxide (NO) bioactivity and to regulate protein function through enzymatic mechanisms analogous to phosphorylation and ubiquitinylation: SNO donors target specific amino acids motifs; post-translational modification leads to changes in protein activity, protein interactions, or subcellular location of target proteins; all major classes of proteins can undergo S-nitrosylation, which is mediated by enzymes that add (nitrosylases) and remove (denitrosylases) SNO from proteins, respectively. Accordingly, nitric oxide synthase (NOS) activity does not directly lead to SNO formation, but rather requires an additional class of enzymes (SNO synthases), which catalyze denovo S-nitrosylation. NOSs ultimately target specific Cys residues for S-nitrosylation through conjoint actions of SNO-synthases and transnitrosylases (transnitrosation reactions), which are involved in virtually all forms of cell signaling, ranging from regulation of ion channels and G-protein coupled reactions to receptor stimulation and activation of nuclear regulatory protein.

Structure The prefix "S" indicates that the NO group is attached to sulfur. The S-N-O angle is near 114°, reflecting the influence of the lone pair of electrons on nitrogen. S-Nitrosothiols may arise from condensation from nitrous acid and a thiol:

RSH + HONO → RSNO + H2O Other methods for their synthesis. They can be synthesized from N2O3 and tert-butyl nitrite (tBuONO) are commonly used.

Reactions Most nitrosothiols are thermally unstable with respect to formation of the disulfide and nitric oxide:

2 RSNO → RSSR + 2 NO That equation reverses under irradiation. Nitrosothiols also nitrosate amines in a similarly radical reaction:

4 RSNO + 2R′2NH → 2 R′2NNO + 2 (RS)2 + N2O + H2O S-Nitrosothiols release nitrosonium ions (NO+) upon treatment with acids:

RSNO + H+ → RSH + NO+ and they can transfer nitroso groups to other nucleophiles, including other thiols:

RSNO + R'SH → RSH + R'SNO These reactions are, however, generally slow equilibria and require distillatory removal of the product to proceed. In the Saville reaction, mercury replaces the nitrosonium ion:

2 RSNO + Hg → Hg(SR)2 + 2 NO

Detection S-Nitrosothiols can be detected with UV-vis spectroscopy.

Thionitrite anion The thionitrite anion (S-N-O−) is part of a family of reduced sulfur-nitrogen anions that requiring extremely bulky cations to form a stable salt. The only known synthesis as of 1990 required the reaction of PPN nitrite with octasulfur in acetone, followed by reduction with triphenylphosphine.

Examples S-Nitrosoglutathione (GSNO) S-Nitroso-N-acetylpenicillamine (SNAP) S-Nitrosotriphenylmethanethiol

References

Illustrations

S-Nitrosothiol: Structure of an S–nitrosothiol. R denotes some organic group. — Structure of an S–nitrosothiol. R denotes some organic group.

Worked examples

Example 1 — a first encounter with S-Nitrosothiol

Start with the simplest possible case. Write down what S-Nitrosothiol claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to S-Nitrosothiol before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about S-Nitrosothiol ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of S-Nitrosothiol

In research: S-Nitrosothiol appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses S-Nitrosothiol in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: S-Nitrosothiol is common in secondary-school and first-year university syllabi. It links to neighbouring topics Functional groups, Nitroso compounds, Sulfur compounds, so understanding it makes those chapters shorter.
In everyday life: Look for S-Nitrosothiol outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study S-Nitrosothiol in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what S-Nitrosothiol means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain S-Nitrosothiol out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is S-Nitrosothiol in simple terms?

Why does S-Nitrosothiol matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study S-Nitrosothiol?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on S-Nitrosothiol.