freeze-thaw cycle comes up often in conversation and rarely with the context attached. Here we lay out the basics in order, then work through the practical considerations.
Last reviewed on 2025-11-16. Where a claim depends on a specific study, the study is described rather than over-claimed.
NAD+ is relatively unstable in aqueous solution, especially at neutral or alkaline pH and at elevated temperatures. It is typically stored dry, protected from light and moisture, and kept cold or frozen for long-term use. Solutions are often prepared fresh or buffered to mildly acidic pH to slow hydrolysis. Repeated freeze-thaw cycles can reduce integrity. Laboratories may verify concentration using ultraviolet absorbance at 259 nm or by enzymatic assay. These handling practices are general laboratory conventions rather than universal rules.
Research on NAD+ often examines changes with age, diet, exercise, and disease states, but causal relationships are difficult to establish. Some studies measure NAD+ levels, while others assess enzyme activity or downstream markers. In the literature, terms such as "NAD+ decline" and "NAD+ boosting" appear in both scientific and commercial contexts, sometimes without precise definitions. Whether changes in measured NAD+ directly produce health effects remains an open question. Results from cells, animals, and humans cannot be assumed to translate directly.
Measuring NAD+ in biological samples requires care because the molecule is chemically reactive and present at low concentrations in some tissues. Common approaches include enzymatic cycling assays, high-performance liquid chromatography, and liquid chromatography coupled to mass spectrometry. Each method has different sensitivity and specificity, and sample preparation can affect results. Acidic or alkaline extraction steps are used in some protocols, but the choice depends on the analyte and matrix. No single method is universally optimal for every tissue or fluid.
Solid NAD+ is relatively stable when kept dry, cold, and protected from light. Aqueous solutions are more vulnerable to hydrolysis and can lose activity during repeated freeze-thaw cycles or prolonged storage at ambient temperature. Stability depends on pH, ionic strength, and the presence of degrading enzymes or metal ions. For many laboratory uses, aliquots are stored frozen and thawed only once. Exact degradation rates vary by matrix, so stability should be checked for each application rather than assumed.
Laboratory handling of NAD+ follows standard practices for hygroscopic fine chemicals. Personnel typically avoid inhalation and skin contact, use gloves and eye protection, and work in a ventilated area. Quality control may include ultraviolet absorbance at the nicotinamide maximum, chromatographic purity, water content, and identity confirmation by mass spectrometry. Because commercial preparations can contain counterions, residual solvents, or related nucleotides, a certificate of analysis helps verify the material. Researchers should confirm that the form supplied matches the intended assay.
| Property | Value | Notes |
|---|---|---|
| Appearance | White to off-white powder | Typical solid form; varies with purity |
| Storage temperature | -20 °C or lower | Common for long-term dry storage |
| Solubility class | Water-soluble | Also dissolves in aqueous buffers |
| Typical analytical method | HPLC or LC-MS | Used for quantification in complex samples |
| UV absorbance maximum | About 259 nm | In neutral aqueous solution |
NAD+ is a dinucleotide composed of two nucleotides joined by a pyrophosphate linkage. One nucleotide contains adenine, and the other contains nicotinamide. The oxidized form carries a positive charge on the nicotinamide ring and is abbreviated NAD+. It functions as a cofactor in hydride-transfer reactions, accepting electrons in catabolic pathways. In cells, it interconverts with reduced NADH, forming a redox couple central to energy metabolism. The molecule is water-soluble and does not cross cell membranes freely without specific transport or precursor pathways.
The nicotinamide ring undergoes reversible reduction at the para position, converting NAD+ to NADH. This reaction transfers a hydride equivalent, not a free hydrogen atom or electron alone. Because the redox pair has a defined reduction potential, it links oxidation of fuels to respiratory chain activity. Many dehydrogenases use NAD+ as a co-substrate and produce NADH. The ratio of NAD+ to NADH reflects metabolic state and influences flux through several pathways.
NAD+ also serves as a substrate for enzymes that cleave it, including sirtuins, PARPs, and CD38. These enzymes consume NAD+ and release nicotinamide and ADP-ribose or related products. The dual roles as redox cofactor and signaling substrate connect NAD+ to DNA repair, circadian regulation, and calcium signaling. Cellular NAD+ concentrations vary by tissue, time of day, and stress exposure. How these consumption pathways interact with redox balance remains an active area of research.
Solid NAD+ is usually supplied as a white to off-white powder or lyophilized preparation. It is hygroscopic and should be kept desiccated at low temperature, commonly -20 °C or below for long-term storage. Aqueous solutions are less stable than dry material and are often prepared fresh or stored frozen in aliquots. Light exposure and repeated freeze-thaw cycles can promote degradation, so amber containers and single-use aliquots are preferred. Buffered solutions near neutral pH are generally less stable than acidic or frozen preparations.
Quantification of NAD+ in biological samples typically uses liquid chromatography coupled to mass spectrometry. Enzymatic cycling assays offer higher throughput and rely on NAD+ dependent dehydrogenases to amplify signal. Both approaches require careful sample quenching because NAD+ can be rapidly consumed or converted after collection. Acidic extraction is common for NAD+, while alkaline conditions favor NADH in some protocols. Isotopically labeled internal standards help correct for losses during extraction and ionization.
=== 1997–2016: Valve === In July 1997, Laidlaw joined the video game company Valve while they were developing their first game, the first-person shooter (FPS) Half-Life (1998). He was hired to work on another game, Prospero, but switched when it was cancelled and the Half-Life project expanded. Laidlaw said his contribution was to add "old storytelling tricks" to Valve's ambitious designs. Rather than dictate narrative elements, he worked with the team to improvise ideas and was inspired by their experiments. He contributed to the "visual grammar" of the level design and focused on "doing storytelling with the architecture ... The narrative had to be baked into the corridors." For Half-Life 2 (2004), Valve developed the characterization. Laidlaw created family relationships between the characters, saying it was a "basic dramatic unit everyone understands" that was rarely used in games. He also worked on Half-Life 2: Episode One (2006) and Half-Life 2: Episode Two (2007), plus several canceled Half-Life projects, including Half-Life 2: Episode Three and a virtual reality (VR) game set on a time-travelling ship. Laidlaw said he had intended Episode Three to end the Half-Life 2 story arc, at which point he would "step away from it and leave it to the next generation". In 2012, Laidlaw started a Twitter account to tell a story about the Half-Life 2 character Dr Breen. He described the story as "fan fiction", and wrote: "I personally cannot give the world a Half-Life game.
=== Whole genome sequencing (WGS) === Whole genome sequencing and genomics applications can be used for large-scale alignment and comparative analysis with both bacteria and fungi. WGS can be used to diagnose, identify, or characterize an organism down to the individual base pairs by sequencing the entire genome. WGS can also be used to compare the genomes or average nucleotide identity (ANI) of the shared genes between two strains and can be a robust way to compare genetic relatedness and if often used for investigating organisms involved in foodborne illness and other outbreaks.
Gastric acid or stomach acid is the acidic component – hydrochloric acid – of gastric juice, produced by parietal cells in the gastric glands of the stomach lining. In humans, the pH is between one and three, much lower than most other animals, but is very similar to that of carrion-eating carnivores that need protection from ingesting pathogens. With this higher acidity, gastric acid plays a key protective role against pathogens. It is also key in the digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids. Gastric acid is regulated in feedback systems to increase production when needed, such as after a meal. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring a regulated pH. These cells also produce mucus – a viscous barrier to prevent gastric acid from damaging the stomach. The pancreas further produces large amounts of bicarbonate, secreting this through the pancreatic duct to the duodenum to neutralize gastric acid passing into the digestive tract. The secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the hydrochloric acid is secreted into the lumen of the stomach. The pH level is maintained by the proton pump H+/K+ ATPase. The parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of pH in the blood, known as an alkaline tide.
Sources: en.wikipedia.org
Illicit drug use in Mexico is low compared to the United States, but it is on the rise, with the availability of narcotics gradually increasing since the 1980s. The export rate of cocaine to the US decreased following stricter border control measures in response to the September 11 attacks. Drug shipments are often delayed in Mexican border towns before delivery to the U.S., which has forced drug traffickers to increase prices to account for transportation costs. These delays have contributed to the increased rates of local drug consumption. As a result of "spillover" along the U.S.-bound drug trafficking routes and more stringent border enforcement, Mexico's northern border states have seen increased levels of drug consumption and abuse, including elevated rates of drug injection, up to 10 to 15 times the national average. These rates are accompanied by mounting rates of HIV and STIs among injection drug users (IDUs) and sex workers, reaching a 5.5% prevalence in cities such as Tijuana and Ciudad Juárez, which also report STI rates of 64% and 83%, respectively. Violence and extortion of IDUs and sex workers directly and indirectly elevate the levels of risk behavior and poor health outcomes among members of these groups. Marginalization of these vulnerable groups by way of physical and sexual violence and extortion by police threatens the cross-over of infection from high-prevalence groups to the general population.
The protecting group is abbreviated Cbz or, in honour of Zervas, simply Z. The typical route for deprotection involves hydrogenolysis under mild conditions e.g. with hydrogen gas and a catalyst such as palladium on charcoal. The discovery of the Bergmann-Zervas synthesis has been characterised as "epoch-making" as it allowed the advent of controlled synthetic peptide chemistry, completing the work started in the early 20th century by Bergmann's mentor Emil Fischer. Previously impossible to synthesise oligopeptides with a highly specific sequence and reactive side chains were consequently produced in the 1930s by Bergmann and Zervas. The ability of Z-protection to prevent racemization of activated derivatives of the protected amino acids and the importance thereof were also noted by the two chemists. Indeed, their method became the standard in the field for the following two decades until further developments in the early 1950s with the introduction of mixed anhydrides (e.g. the Boc group).
Hydrocodone/paracetamol (also known as hydrocodone/acetaminophen) is the combination of the pain medications hydrocodone (an opioid) and paracetamol (acetaminophen). It is used to treat moderate to severe pain. It is taken by mouth. Recreational use is common in the United States. Common side effects include dizziness, sleepiness, constipation, and vomiting. Serious side effects include addiction, decreased rate of breathing, low blood pressure, severe allergic reactions, and liver failure. Use during pregnancy may harm the fetus. Use with alcohol is not recommended. Hydrocodone works by binding to the mu-opioid receptor. How paracetamol works is unclear but may involve blocking the creation of prostaglandins. Hydrocodone/paracetamol was approved for medical use in the United States in 1982. In the United States, it is a schedule II controlled substance. In 2023, it was the 25th most commonly prescribed medication in the United States, with more than 21 million prescriptions. It is not available in the United Kingdom, though the combination codeine/paracetamol (co-codamol) is. It is sold under the brand names Vicodin and Norco among others.
Sources: en.wikipedia.org
== History == In the 17th century, copper miners in Saxony, Germany, began to experience irritation caused by a "dark red ore". Since the substance, which would later be called nickel, led to many ailments, they believed it to be protected by "goblins", and called it "Goblin's Copper". Josef Jadassohn described the first case of metal contact dermatitis in 1895, to a mercury-based therapeutic cream, and confirmed the cause by epicutaneous patch testing. In the next century nickel began to be mass-produced for jewelry worldwide due to its cheap cost, resistance to corrosion and high supply. American Metal Market predicted in 2001 that concerns about the health effects of nickel and the use of nickel-free Nordic gold in the Euro would impact the nickel market, estimating 50,000 to 70,000 tonnes of nickel would be scrapped from obsolete coinage in 2002. A large comprehensive study of healthy US volunteers in 1979 found that 9% had been unknowingly sensitized to nickel. As of 2008, that number has tripled. Most importantly, nickel allergy among children is increasing, with an estimated 250,000 children sensitized to nickel. Published literature shows an exponential increase in reported nickel allergy cases. The North American Contact Dermatitis Group (NACDG) patch tested 5,085 adults, presenting with eczema-like symptoms, showing 19.5% had a positive reaction to nickel. Nickel allergy is also more prevalent in women (17.1%) than men (3%), possibly due to cultural norms related to jewelry and ear piercings and therefore increased exposure to nickel.
=== Class II === Class II proteins are dominant in β-sheets and the catalytic sites are localized in the core region. The peptide regions required to drive fusion are formed from the turns between the β-sheets. They usually start as dimers, becoming a trimer as fusion happens.
All of the lower respiratory tract including the trachea, bronchi, and bronchioles is lined with respiratory epithelium. This is a ciliated epithelium interspersed with goblet cells which produce mucin the main component of mucus, ciliated cells, basal cells, and in the terminal bronchioles–club cells with actions similar to basal cells, and macrophages. The epithelial cells, and the submucosal glands throughout the respiratory tract secrete airway surface liquid (ASL), the composition of which is tightly regulated and determines how well mucociliary clearance works. Pulmonary neuroendocrine cells are found throughout the respiratory epithelium including the alveolar epithelium, though they only account for around 0.5 percent of the total epithelial population. PNECs are innervated airway epithelial cells that are particularly focused at airway junction points. These cells can produce serotonin, dopamine, and norepinephrine, as well as polypeptide products. Cytoplasmic processes from the pulmonary neuroendocrine cells extend into the airway lumen where they may sense the composition of inspired gas.
Sources: en.wikipedia.org
Researchers often use enzymatic cycling assays, liquid chromatography, or mass spectrometry. The choice depends on sample size, sensitivity needs, and available equipment. Because NAD+ can degrade quickly, rapid extraction and careful handling are important.
Differences can arise from sample type, extraction method, normalization strategy, and analytical platform. Time of day, diet, and physiological state may also matter. These factors make direct comparisons across studies difficult.
NAD+ is generally more stable when stored dry and cold, and it can degrade in aqueous solutions over time. Heat, light, and alkaline conditions can accelerate loss. Laboratory protocols therefore often recommend frozen storage and protection from light.
Common laboratory methods include enzymatic cycling, high-performance liquid chromatography, and liquid chromatography with mass spectrometry. The choice depends on sample type, expected concentration, and available equipment.