If you have been reading about Freeze-thaw and want a single page that covers the useful parts, this is it: definitions, context, how it is studied, and the questions that come up repeatedly.
Last reviewed on 2026-05-23. Where a claim depends on a specific study, the study is described rather than over-claimed.
In cells, NAD+ functions primarily as an electron carrier. Dehydrogenase enzymes in glycolysis and the citric acid cycle transfer hydride from substrates to NAD+, producing NADH. NADH then delivers electrons to the mitochondrial respiratory chain, supporting ATP synthesis. In fermentation, NADH is reoxidized to NAD+ so that glycolysis can continue. The balance between NAD+ and NADH helps set metabolic flux. Beyond redox, NAD+ serves as a substrate for enzymes that cleave it, including sirtuins, poly(ADP-ribose) polymerases, and CD38. These reactions consume NAD+ and release nicotinamide and ADP-ribose products.
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.
Biosynthesis of NAD+ starts from nicotinamide, nicotinic acid, or nicotinamide riboside through salvage pathways. A rate-limiting enzyme, nicotinamide phosphoribosyltransferase, converts nicotinamide to nicotinamide mononucleotide. Further coupling with ATP yields NAD+. In mammals, the liver and muscle can synthesize NAD+ from dietary precursors, but tissue levels vary widely. Researchers study these pathways to understand age-related changes, metabolic disorders, and neurodegeneration. Direct causal links between NAD+ decline and disease remain an active area of investigation.
NAD+ is a dinucleotide composed of adenine, ribose, and nicotinamide groups joined by phosphate linkages. It serves as a coenzyme in oxidoreductase reactions, cycling between oxidized NAD+ and reduced NADH. The molecule is water-soluble and occurs in all living cells. Its nicotinamide ring accepts hydride ions during catabolic reactions, linking substrate oxidation to electron transport. This redox couple supports ATP production and helps maintain cytosolic and mitochondrial redox balance in many cell types.
| 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. |
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.
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.
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.
Commercial NAD+ is supplied as a solid, often as the free acid or a salt, and purity is verified by chromatographic methods. Laboratories typically store it desiccated at minus 20 degrees Celsius or below. Working solutions are prepared fresh because even sterile aqueous solutions can lose activity over hours to days depending on pH and temperature. Documentation may include a certificate of analysis, an assay value, and a recommended retest date. Researchers should verify identity and purity when results depend on precise cofactor concentrations.
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.
Quantification of NAD+ in biological samples usually relies on separation techniques coupled to sensitive detection. High-performance liquid chromatography with ultraviolet detection can measure the oxidized form by its absorbance near 260 nm, while mass spectrometry provides greater specificity and can distinguish NAD+ from close analogs. Enzymatic cycling assays use coupled dehydrogenase reactions to amplify signal and estimate NAD+ concentrations in cell or tissue extracts. Because NAD+ and NADH interconvert rapidly, sample preparation must quench metabolism quickly and preserve the redox state before analysis.
The stability of NAD+ depends on pH, temperature, light exposure, and the presence of degradative enzymes. Aqueous solutions are generally more stable under mildly acidic to neutral conditions and degrade faster under alkaline conditions or prolonged heat. The solid is hygroscopic and should be stored desiccated, often frozen, and protected from repeated freeze-thaw cycles. In laboratory handling, aliquots reduce repeated temperature changes, and chelating agents may limit metal-catalyzed hydrolysis in some buffers. These practices matter because even small amounts of NADH or hydrolysis products can interfere with quantitative assays.
Quality control for NAD+ materials typically combines identity, purity, and water content checks. Identity may be confirmed by ultraviolet spectrum, retention time in chromatography, or mass accuracy, while purity is assessed by HPLC peak area or quantitative nuclear magnetic resonance. Residual water and solvents can affect molar calculations and enzyme assays, so Karl Fischer titration or thermogravimetric analysis may be used. Commercial materials vary in grade and counterion form, and published methods should specify the exact salt or hydrate when reporting concentrations. Regulatory status depends on intended use, with research reagents, dietary ingredients, and clinical products treated under different frameworks.
Cave of Elijah is the name used for two grottoes on Mount Carmel, in Haifa, Israel, associated with Biblical prophet Elijah. The main shrine known as the "Cave of Elijah" is located on Haifa's Allenby Road, on Mount Carmel, approximately 40 m above sea level. For centuries, it has been a destination for Jewish, Christian, Muslim and Druze pilgrims. The Cave of Elijah in Allenby Road is divided into twos sections for praying, one for men and one for women; the cave is behind a velvet curtain. The Cave is also known as el-Khader in Arabic. The Druze regard it as holy, and many among them identify Elijah as "el-Khidr", the green prophet who symbolizes water and life. The cave has been considered by some as miracle-working. Sick people are said to be brought to the Cave in hope that they will be cured. A second grotto, also associated with Elijah, is located nearby, under the altar of the main church of the Stella Maris Monastery, also on Mount Carmel.
Some have raised ethical objections specifically with respect to pharmaceutical patents and the high prices for drugs that they enable their proprietors to charge, which poor people around the world, cannot afford. Critics also question the rationale that exclusive patent rights and the resulting high prices are required for pharmaceutical companies to recoup the large investments needed for research and development. One study concluded that marketing expenditures for new drugs often doubled the amount that was allocated for research and development. Other critics claim that patent settlements would be costly for consumers, the health care system, and state and federal governments because it would result in delaying access to lower cost generic medicines. Novartis fought a protracted battle with the government of India over the patenting of its drug, Gleevec, in India, which ended up in a Supreme Court in a case known as Novartis v. Union of India & Others. The Supreme Court ruled narrowly against Novartis, but opponents of patenting drugs claimed it as a major victory.
== Early life and education == Xing received Chinese traditional private education in his childhood. In 1933, he graduated from Fu Jen Catholic University with a diploma in Chemistry. Xing did his postgraduate work at University of Illinois at Urbana–Champaign under Roger Adams's guidance and obtained a doctorate degree in 1936. Later he went to the Ludwig-Maximilians-Universität München (LMU), conducting research on bufotoxins at Wieland's laboratory.
Different bat species have different diets, including insects, nectar, pollen, fruit and even vertebrates. Megabats are mostly fruit, nectar and pollen eaters. Due to their small size, high metabolism and rapid burning of energy through flight, bats must consume large amounts of food for their size. Insectivorous bats may eat over 120 percent of their body weight per day, while frugivorous bats may eat over twice their weight. They can travel significant distances each night, exceptionally as much as 38.5 km (24 mi) in the spotted bat (Euderma maculatum), in search of food. Bats may acquire water from the food they eat or drink from sources like lakes and streams, flying over the surface and dipping their tongues into the water. Bats as a group appear to be losing vitamin C synthesis. Such a loss was recorded in 34 bat species from six major families, both insect- and fruit-eating, with the cause being a single mutation inherited from a common ancestor. Vitamin C synthesis has been recorded in at least two species of bat, Leschenault's rousette (Rousettus leschenaultii) and the great roundleaf bat (Hipposideros armiger).
Another perception of the vagina was that the release of vaginal fluids would cure or remedy a number of ailments; various methods were used over the centuries to release "female seed" (via vaginal lubrication or female ejaculation) as a treatment for suffocatio ex semine retento (suffocation of the womb, lit. 'suffocation from retained seed'), green sickness, and possibly for female hysteria. Reported methods for treatment included a midwife rubbing the walls of the vagina or insertion of the penis or penis-shaped objects into the vagina. Symptoms of the female hysteria diagnosis – a concept that is no longer recognized by medical authorities as a medical disorder – included faintness, nervousness, insomnia, fluid retention, heaviness in abdomen, muscle spasm, shortness of breath, irritability, loss of appetite for food or sex, and a propensity for causing trouble. It may be that women who were considered suffering from female hysteria condition would sometimes undergo "pelvic massage" – stimulation of the genitals by the doctor until the woman experienced "hysterical paroxysm" (i.e., orgasm). In this case, paroxysm was regarded as a medical treatment, and not a sexual release. The vagina has been given many vulgar names, three of which are pussy, twat, and cunt. Cunt is also used as a derogatory epithet referring to people of either sex. This usage is relatively recent, dating from the late nineteenth century.
Sources: en.wikipedia.org
244Cm(12C,xn)256−xNo (x=4,5) This reaction was studied in 1958 by scientists at the LBNL using a 95% 244Cm curium target. They were able to measure 7.43 MeV decays from250Fm, associated with a 3 s 254No parent activity, resulting from the reaction (246Cm,4n). The 3 s activity was later reassigned to252No, resulting from reaction (244Cm,4n). It could however not be proved that it was not due to the contaminant 250mFm, unknown at the time. Later work in 1959 produced 8.3 MeV alpha particles with a half-life of 3 s and a 30% SF branch. This was initially assigned to 254No and later reassigned to 252No, resulting from reaction with the 244Cm component in the target. The reaction was restudied in 1967 at the LBNL and a new activity assigned to 251No was measured.
== Description == Eupolyphaga sinensis is often described as a wingless cockroach, this however is not entirely true. E. sinensis is a sexually dimorphic species, with the females being wingless, while the males are winged. Females are generally larger in size and darker in color than males. Within their range E. sinensis display significant size differences based on temperature and latitude. Body size of both males and females increase at high and low latitudes, with those found at an intermediate range being smaller. This body size difference is a result of genetic differentiation between populations, being adapted to their native climate.
In September 1939, the Nationalist government promulgated the New County System (县各级组织纲要) to promote local self-government and popular political participation. Liu implemented this system in Xikang in order to increase the province's self-reliance and to curb the influence of the tusi. Liu identified the training of an administrative cadre as one of the key priorities in building Xikang. To this end, training institutes were established at Kangding (1938) and Hanyuan (1939), training around 2,000 officials. Liu's rhetoric at the time was anti-corruption, stating that only through thoroughly cleansing the officialdom would the counties west of the Jinsha River (then under Tibetan rule) be recovered for Xikang. In June 1941, he established the Xikang Provincial Training Corps, which ran until December 1946 and trained around 4,500 students in all. Liu personally interviewed each applicant and graded their exam papers, ensuring that all new graduates would be loyal to him. This administrative cadre, personally loyal to Liu, was one of the factors that helped him resist central government encroachment.
==== Other optical imaging ==== For in vivo fluorescence-based optical imaging, dyes that emit near infrared (NIR) wavelengths >700 nm are most effective, such as indocyanine green, but encounter limitations with reduced circulation time and nonspecific interactions with other biological factors that affect the fluorescence. pH-sensitive nanogels with functionalized surface receptors to target cancer cells were loaded with a fluorescent dye that was only released upon endocytosis. These nanogels successfully generated a fluorescent signal from within the cancer cells and many other groups have developed similar technologies.
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.
NAD+ is the oxidized form and NADH is the reduced form of the same coenzyme. NAD+ accepts electrons during oxidation reactions, becoming NADH, which can donate electrons in other reactions. The ratio between them helps describe a cell's redox state.