Freeze-thaw 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 2026-03-24. Where a claim depends on a specific study, the study is described rather than over-claimed.
Biosynthesis occurs through salvage, Preiss-Handler, and de novo pathways. In mammals, the salvage pathway from nicotinamide predominates, and NAMPT is often described as rate-limiting. Nicotinamide riboside and nicotinic acid enter related routes that converge on NAD+ production. Tissue NAD+ concentrations vary widely and are maintained by a balance of synthesis and consumption. Some studies report age-related declines in certain tissues, but whether these changes cause disease or can be reversed to improve human health remains an open question.
NAD+ stands for nicotinamide adenine dinucleotide, the oxidized form of a coenzyme found in all living cells. The molecule consists of two nucleotides, adenine and nicotinamide ribose, joined through phosphate groups. Its chemical formula is C21H27N7O14P2, and the free acid has a molar mass near 663.43 grams per mole. In redox reactions, NAD+ accepts a hydride ion and becomes NADH. The pair NAD+ and NADH participates in hundreds of metabolic reactions, including steps in glycolysis, the citric acid cycle, and oxidative phosphorylation.
Beyond redox chemistry, NAD+ serves as a substrate for enzymes that transfer ADP-ribose or remove acetyl groups. Sirtuins, PARPs, and CD38-family enzymes consume NAD+ and produce nicotinamide and ADP-ribose-related products. These reactions link NAD+ availability to DNA repair, chromatin modification, and cellular signaling. Because the molecule is central to energy metabolism and regulation, changes in its concentration are studied in aging, immunity, and metabolic research. The balance between synthesis and consumption varies by tissue, developmental stage, and physiological state.
In humans, NAD+ can be synthesized from nicotinic acid, nicotinamide, nicotinamide riboside, and tryptophan through overlapping pathways. The salvage pathway recycles nicotinamide back to NAD+ and is often considered a major route in many tissues. Dietary precursors and intracellular recycling both contribute to the pool, but the quantitative importance of each source remains an active research question. NAD+ levels are not uniform across organs or cell compartments. Measurements in blood do not necessarily reflect concentrations inside tissues.
| Property | Value | Notes |
|---|---|---|
| Molecular formula | C21H27N7O14P2 | Oxidized form; NADH adds a hydride equivalent. |
| Molar mass | 663.43 g/mol | Free acid form; salts have different values. |
| CAS Registry Number | 53-84-9 | Common identifier for beta-NAD. |
| Appearance | White to off-white powder | Hygroscopic; may absorb moisture from air. |
| Solubility | Freely soluble in water | Poorly soluble in most organic solvents. |
NAD+ is commonly measured by high-performance liquid chromatography with ultraviolet detection, often at 254 or 260 nm. Enzymatic cycling assays provide higher sensitivity by coupling NAD+ to a reporter reaction. Mass spectrometry can distinguish NAD+ from close analogues and confirm isotope labeling. Sample preparation usually involves rapid quenching of metabolism to prevent interconversion with NADH. Because NAD+ and NADH differ by one hydride, extraction conditions strongly affect the measured ratio.
In aqueous solution, NAD+ is most stable under mildly acidic to neutral conditions and degrades faster at high pH or elevated temperature. The molecule can hydrolyze at the pyrophosphate bond or undergo nonenzymatic cyclization. Buffers, chelating agents, and cold temperatures slow these losses during analysis. Repeated freeze-thaw cycles are generally avoided because they can promote degradation and concentration changes. Light exposure is also controlled, though NAD+ is less photolabile than some related nucleotides.
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.
Commercial NAD+ is available at research grade, often with purity specifications determined by high-performance liquid chromatography. Certificates of analysis may report water content, residual solvents, and counterion identity. Identity can be confirmed by ultraviolet absorbance near 260 nm, mass spectrometry, or enzymatic activity. Because different salt forms and hydration states exist, researchers should verify that the product matches the intended molecular form. Lot-to-lot variation in purity can affect quantitative assays and should be documented.
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.
NAD+ is the oxidized form of nicotinamide adenine dinucleotide, a coenzyme built from two nucleotides joined by a phosphate linkage. One nucleotide carries adenine, and the other carries nicotinamide; the plus sign denotes a formal positive charge on the nicotinamide ring, not a free proton. In cells, NAD+ and its reduced partner NADH form a reversible redox pair. That pair participates in electron transfer reactions throughout metabolism. The abbreviation NAD+ is common in biochemistry, while NAD(H) sometimes denotes the combined pool.
The molecule was first described in the early twentieth century as a factor that promoted fermentation in yeast extracts. Later work linked it to hydrogen transfer and to the oxidation of nutrients in living tissues. Its structure was resolved as a dinucleotide, which explained why it could accept and donate electrons at specific enzyme sites. Today, NAD+ is recognized as a central substrate and signaling precursor, not merely a metabolic cofactor. Whether all observed NAD+ changes reflect causal signaling remains an open question.
Related compounds include NADH, the reduced form, and NADP+, which carries an additional phosphate group. NADP+ and NADPH often serve in biosynthetic and antioxidant reactions, while NAD+ and NADH are more associated with energy-yielding catabolism. Nicotinamide, nicotinic acid, and nicotinamide riboside are precursors that can enter salvage pathways. The exact contribution of dietary precursors to tissue NAD+ pools is an area of active investigation. Some studies measure labeled precursors to trace those routes.
Measuring NAD+ in biological samples requires rapid processing because the compound can degrade or interconvert after collection. Common approaches include enzymatic cycling assays, high-performance liquid chromatography, and mass spectrometry. Each method has different sensitivity, specificity, and susceptibility to interference from related nucleotides. Sample type matters: cultured cells, animal tissues, and human blood present distinct challenges. Reported values can vary widely across laboratories because of differences in extraction, normalization, and analytical platform. Standardization remains an open issue in the field.
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.
He worked on other methods of analytical separation of biological materials, notably electrophoresis and displacement chromatography, but also was influential in developing biochemical engineering within the Chemical Engineering Department at Yale. He published about 300 papers and held 9 patents. He was a member of the United States National Academy of Engineering. He received many other honors and awards and is remembered in the Horváth Laboratory of Separation Science at Innsbruck.
the person responsible under § 115 StrlSchG (in German) paragraph 2 or § 153 StrlSchG (in German) paragraph 1. The SSR numbers must then be available for further use as part of normal communication with monitoring stations or radiation pass authorities. The SSR number is derived from the social security number and personal data using non-traceable encryption. The transmission takes place online. Approximately 420,00 persons are monitored for radiation protection in Germany (as of 2019). Emergency responders (including volunteers) who are not occupationally exposed persons within the meaning of the Radiation Protection Act also require an SSR number retrospectively, i.e. after an operation in which they were exposed to radiation above the limits specified in the Radiation Protection Ordinance, as all relevant exposures must be recorded in the Radiation Protection Register.
The first MP, installed at Yale in 1963, operated consistently at 10–11 megavolts; a later installation at Strasbourg reached 18. At these energies, electron stripping becomes highly efficient. A uranium ion passing through the terminal can lose more than 20 electrons, enabling heavy-ion fusion experiments impossible with earlier machines. Advances in gamma-ray spectroscopy combined with MP tandems enabled precision measurements of nuclear structure. HVEC manufactured 10 MP units between 1965 and 1973 for institutions including the University of Minnesota, Chalk River, and the Max Planck Institute in Heidelberg. The XTU ("Holy Roman Emperor") was designed for superheavy element synthesis. Theoretical models predicted an "island of stability" beyond element 110 where nuclei would resist rapid decay. The XTU's 20-megavolt rating would accelerate uranium ions to nearly one billion electron volts—enough to overcome Coulomb barriers in heavy-element fusion. Two prototypes operated solely on a test basis at Burlington before the project was cancelled. One later sold to Italy's national laboratory at Legnaro in 1979.
=== Pipelines === South Africa's pipeline freight sector is the smallest of the country's freight modes by network size. However, pipelines are of strategic importance, as they continuously transport energy products between refineries, ports, storage depots, and major inland markets. These products include natural gas, crude oil, and refined petroleum products such as gasoline, diesel, paraffin, and jet fuel. Fuel pipelines in SA are predominantly managed by Transnet Pipelines (TPL), a division of state-owned company Transnet. Two other players in the sector are the Republic of Mozambique Pipeline Investments Company (ROMPCO), which owns and operates the 865km Mozambique to South Africa natural gas pipeline, and Sasol, which owns and operates portions of the downstream gas transmission network that distribute gas after it reaches Secunda via ROMPCO. At Secunda, Sasol operates the world's largest coal-to-liquids plant. The pipelines exists in large part to supply inland fuel demand, especially in Gauteng, without relying solely on road tankers or rail tank wagons. Major metros such as Cape Town and Durban are able to acquire their fuel by sea. The network of pipelines also supplies natural gas to KwaZulu-Natal. They span the provinces of KwaZulu-Natal, Free State, Mpumalanga, Gauteng, and North West. As of 2026, the network consists of:
Sources: en.wikipedia.org
== Perpendicular bisectors of sides == These inequalities deal with the lengths pa etc. of the triangle-interior portions of the perpendicular bisectors of sides of the triangle. Denoting the sides so that
=== In relation with experience === Thomas Nagel has said that "There are elements which, if added to one's experience, make life better; there are other elements which if added to one's experience, make life worse. But what remains when these are set aside is not merely neutral: it is emphatically positive." Experiences are central to a proposed dimension of well-being called psychological richness. This additional dimension of well-being was proposed as an empirically supported expansion to the hedonic vs. eudaimonic well-being dichotomy. Whereas hedonic well-being can be measured via life satisfaction, and eudaimonic well-being can be measured via one's perceptions of the meaning of their life, psychological richness is measured via characteristic experiences. Psychological richness is cultivated through having psychologically rich experiences, which are characterized as varying, interesting, novel, challenging, and perspective-changing, as subjectively measured by the experiencer. One line of evidence for this comes from studies conducted with college students, where students who went on trips (new and unusual experiences), whether they be short excursions or semester-length study abroad programs, reported increased psychological richness, but not increases in happiness or meaning (Oishi et al., 2021). In contrast to hedonic well-being, which is thought to result in personal satisfaction, and eudaimonic well-being, which is thought to result in societal contribution, psychological richness is thought to result in wisdom.
=== From mechanical blood processing during surgery === In some surgical procedures (especially some heart operations) where substantial blood loss is expected, machinery is used for intraoperative blood salvage. A centrifuge process takes blood from the patient, washes the red blood cells with normal saline, and returns them to the patient's blood circulation. Hemolysis may occur if the centrifuge rotates too quickly (generally greater than 500 rpm)—essentially this is hemolysis occurring outside of the body. Increased hemolysis occurs with massive amounts of sudden blood loss, because the process of returning a patient's cells must be done at a correspondingly higher speed to prevent hypotension, pH imbalance, and a number of other hemodynamic and blood level factors. Modeling of fluid flows to predict the likelihood of red cell membrane rupture in response to stress is an active area of research.
Sources: en.wikipedia.org
Nicotinamide adenine dinucleotide, with the plus sign indicating the oxidized form. It is a coenzyme present in all living cells. The reduced form is NADH.
No. NAD+ is oxidized and accepts electrons, while NADH is reduced and carries them. Together they form a redox pair central to energy metabolism.
NAD+ itself is not a common dietary component in significant amounts. Precursors such as nicotinamide, nicotinic acid, and nicotinamide riboside can be converted through biosynthetic pathways. Direct absorption of intact NAD+ is limited.
The plus sign indicates the oxidized form of nicotinamide adenine dinucleotide, which can accept electrons. When it accepts electrons, it becomes NADH. The two forms together support redox reactions in cells.