Freeze-thaw is one of those subjects where the details matter more than the headlines. This page pulls together the background, the mechanisms, and the practical points readers ask about most.
Last reviewed on 2025-07-31. Where a claim depends on a specific study, the study is described rather than over-claimed.
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.
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 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 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.
| Property | Value | Notes |
|---|---|---|
| UV absorption maximum | 259–260 nm | Aqueous solution; pH-dependent |
| Common salt form | Disodium salt | Improves aqueous solubility |
| Typical storage temperature | -20 °C or lower | Desiccated and protected from light |
| Common analytical method | HPLC with UV detection | Often paired with mass spectrometry |
| Aqueous stability | pH and temperature dependent | Degrades faster at alkaline pH and high heat |
Nicotinamide adenine dinucleotide, abbreviated NAD+, is a dinucleotide built from adenine, nicotinamide, two ribose sugars, and two phosphate groups. The oxidized form carries a positive charge on the nicotinamide ring, while the reduced form, NADH, is neutral. This pair acts as a reversible electron carrier in cells. NAD+ is present in bacteria, plants, animals, and fungi. Its structure allows it to accept and donate electrons without being consumed in the reactions it supports.
In redox reactions, NAD+ accepts a hydride ion, which consists of two electrons and one proton. The hydride adds to the nicotinamide ring at a specific carbon, converting NAD+ into NADH. Dehydrogenase enzymes use this step in glycolysis, the citric acid cycle, and fatty acid oxidation. NADH later donates electrons to the mitochondrial electron transport chain, helping to drive ATP synthesis. The balance between NAD+ and NADH reflects the metabolic state of a cell, and shifts in that balance can alter how pathways operate.
Beyond electron transfer, NAD+ serves as a substrate for enzymes that cleave it and attach its ADP-ribose portion to other molecules. This group includes poly(ADP-ribose) polymerases, CD38, and sirtuins. Such reactions consume NAD+ and can influence its availability for metabolism. Cells replenish NAD+ through a salvage pathway that recycles nicotinamide and through routes starting from tryptophan or vitamin B3 forms. How these synthesis and consumption routes are coordinated across tissues remains an active area of study, and compartment-specific concentrations are difficult to measure directly.
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.
Most bacterial species are either spherical, called cocci (singular coccus, from Greek kókkos, grain, seed), or rod-shaped, called bacilli (sing. bacillus, from Latin baculus, stick). Some bacteria, called vibrio, are shaped like slightly curved rods or comma-shaped; others can be spiral-shaped, called spirilla, or tightly coiled, called spirochaetes. A small number of other unusual shapes have been described, such as star-shaped bacteria. This wide variety of shapes is determined by the bacterial cell wall and cytoskeleton and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators.
Repotrectinib, sold under the brand name Augtyro, is an anti-cancer medication used for the treatment of non-small cell lung cancer. It is taken by mouth. Repotrectinib is an inhibitor of proto-oncogene tyrosine-protein kinase ROS1 (ROS1) and of the tropomyosin receptor tyrosine kinases (TRKs) TRKA, TRKB, and TRKC. The most common adverse reactions include dizziness, dysgeusia, peripheral neuropathy, constipation, dyspnea, ataxia, fatigue, cognitive disorders, and muscular weakness. Repotrectinib was approved for medical use in the United States in November 2023, and in the European Union in January 2025.
Carey-Ann Burnham is a clinical microbiologist, and a professor of Pathology and Immunology, Molecular Microbiology, Pediatrics and Medicine in Washington University School of Medicine. She is an elected fellow of the American Society for Microbiology.
== Mating == Researchers examined whether female beetles preferred males infected by the tapeworm Hymenolepis diminuta as mates, or noninfected males. Virgin females were found to spend more time near — and to copulate more often with — noninfected males in the experiment. Mating behavior was also found to be influenced by the mass of the male beetle. A larger male was preferred and mated with first compared to a smaller male mate. Overall, female beetles showed mating preferences for noninfected males who were larger in size. One explanation provided by researchers for this finding is that larger, noninfected males may allow females to gain genetic or material benefits.
Sources: en.wikipedia.org
=== 11 October === At least 60 people were killed by RSF shelling on the Dar al-Arqam displacement centre in El Fasher. At least 20 people were killed in an SAF drone strike on Al-Kuma, North Darfur. An unspecified number of people were killed by a joint RSF and SPLM-N drone strike in Dalami, South Kordofan.
A pathologists' assistant (PA or PathA) is a physician extender whose expertise lies in gross examination of surgical specimens as well as performing hospital, medicolegal, and forensic autopsies. Their education is analogous to physician assistants, consisting of a didactic and a clinical component and concluding in a master's degree. Certification of pathologists' assistants is through a board exam by the American Society for Clinical Pathology (ASCP). In the United States, the profession is represented by the American Association of Pathologists' Assistants (AAPA). Pathologists' Assistants in the United States are currently only required to be licensed in three states: Nevada, West Virginia, and New York. In most other states, the scope of PAs falls under CLIA high complexity testing which requires an associate degree.
==== Azapirones (serotonin 5-HT1A receptor agonists) ==== Buspirone (Buspar) – serotonin 5-HT1A receptor partial agonist and other actions Tandospirone (metanopirone; Sediel; SM-3997) – serotonin 5-HT1A receptor partial agonist [42]
Sources: en.wikipedia.org
Chlorarachniophytes, which belong to the phylum Cercozoa, contain a small nucleomorph, which is a relict of the algae's nucleus. Euglenophytes, which belong to the phylum Euglenozoa, live primarily in fresh water and have chloroplasts with only three membranes. The endosymbiotic green algae may have been acquired through myzocytosis rather than phagocytosis. Another group with green algae endosymbionts is the dinoflagellate genus Lepidodinium, which has replaced its original endosymbiont of red algal origin with one of green algal origin. A nucleomorph is present, and the host genome still have several red algal genes acquired through endosymbiotic gene transfer. Also, the euglenid and chlorarachniophyte genome contain genes of apparent red algal ancestry. Other groups have "red" chloroplasts containing chlorophylls a and c, and phycobilins. The shape can vary; they may be of discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon shaped. They have one or more pyrenoids to preserve protein and starch. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with red algae suggest a relationship there. In some of these groups, the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the heterokonts, Haptophyta, and cryptomonads are in fact more closely related to each other than to other groups.
Competitive lateral-flow assays are commonly used for quick drug tests using urine or saliva. In workplace tests, a negative result is generally interpreted as negative, where as a "positive" result is interpreted as needing further confirmatory lab testing to rule out false positives.
Caelius Aurelianus apparently assigned a special chapter for the discussion of diabetes but this chapter seems to have been lost. I have looked for it in the 1529 edition edited by Johnnes Sichart without success. I have attempted to reconstruct this passage on diabetes from later authors noted for their ability to copy, but have not been able to find it. The early printers discarded the manuscripts after their publications were printed; therefore it is unlikely that this missing section will be found. The chief fact is that the concept of diabetes goes back to Demetrius, who lived in the first century B.C.
Sources: en.wikipedia.org
NAD+ and NADH can interconvert quickly after a sample is collected, which can alter the measured ratio. Rapid quenching and cold handling limit enzymatic and chemical changes.
Purity is often checked by HPLC with UV detection, sometimes paired with mass spectrometry for identity. An assay against a standard can quantify the active cofactor content.
Solid NAD+ is usually kept dry, cold, and protected from light. Aqueous working solutions are best prepared fresh because degradation depends on pH, temperature, and time.
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.