This is a working overview of freeze-thaw cycle, written for readers who want more than a one-paragraph summary but less than a textbook.
Reviewed 2025-09-08. Anything still debated is marked as such rather than presented as settled.
Quantification in biological matrices relies on liquid chromatography coupled to tandem mass spectrometry with stable-isotope internal standards. Low plasma concentrations and adsorption to container surfaces both complicate measurement. Solid-phase extraction is often needed to reduce matrix interference before injection. Reported limits of quantification differ widely between laboratories, which makes direct comparison of pharmacokinetic results difficult and limits meta-analysis.
Peptide bonds are vulnerable to protease attack, and Selank is no exception. Measured half-life in serum is short, on the order of minutes in several reports, which explains why intranasal administration is the common route described in the literature. Absorption across the nasal mucosa partially bypasses first-pass hepatic metabolism. Quantitative data on human bioavailability remain limited and are difficult to compare across studies.
Purity assessment relies mainly on reverse-phase high-performance liquid chromatography with ultraviolet detection. Because the peptide lacks a strong chromophore, detection often uses backbone absorbance near 214 nm. Identity is confirmed by mass spectrometry, typically electrospray ionization or matrix-assisted laser desorption, comparing the measured mass against the expected value. Amino acid analysis can verify composition after acid hydrolysis. Diastereomer content and residual counterions are reported less often, although both can influence biological assays.
Lyophilized material is generally stable for extended periods when kept dry at or below minus twenty degrees Celsius. Working solutions are less stable, and common practice is to aliquot and freeze them so that repeated freeze-thaw cycles are avoided. Aqueous solutions are sensitive to pH extremes and to microbial growth, so short-term storage at refrigerator temperature is typical. Oxidation and hydrolysis are the principal degradation routes. Reconstitution with sterile water or a mild buffer is standard, and solutions should be protected from light.
Regulatory treatment varies by jurisdiction. In Russia the compound is a registered prescription product, while in the European Union and the United States it is generally handled as a research chemical without a marketing authorization. Suppliers therefore operate outside pharmaceutical oversight, and buyers rely on supplier documentation for purity and identity claims. Chain of custody and third-party testing are the main verification tools. Analysts note that the absence of a pharmacopoeial monograph for research-grade material limits standardization across vendors.
| Property | Value | Notes |
|---|---|---|
| Dry storage temperature | -20 C or below | Sealed, desiccated, protected from light |
| Solution stability | Hours to days at 2-8 C | Depends on buffer and microbial load |
| Preferred pH range | Near neutral to slightly acidic | Alkaline pH accelerates hydrolysis |
| Main degradation routes | Hydrolysis and deamidation | No cysteine or methionine in sequence |
| Identity confirmation | Electrospray mass spectrometry | Observed mass compared with 751.9 Da |
Development took place at the Institute of Molecular Genetics of the Russian Academy of Sciences, where a series of short peptides were designed in the 1980s and 1990s. Selank was selected from variants of tuftsin that showed resistance to plasma peptidases. Russian regulatory approval covers it as an anxiolytic agent given intranasally. Outside that market the compound is normally handled as a research chemical rather than a medicine, and no widely recognised international pharmacopoeial monograph exists. The name Selank is a coined trade designation rather than a systematic chemical name.
Enzymatic stability motivates the extra three residues at the carboxyl end. Native tuftsin is cleaved quickly by circulating aminopeptidases and carboxypeptidases, which limits its duration of action and its usefulness as a tool compound. Extending the chain with proline-rich segments is a common design tactic because proline constrains the backbone and slows proteolysis. The same Pro-Gly-Pro motif appears in other Russian-developed peptides of the era. Whether the full seven-residue chain is required for activity, or whether it acts mainly as a prodrug releasing tuftsin, remains unresolved.
Selank is a synthetic heptapeptide with the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro. Its four N-terminal residues reproduce tuftsin, a tetrapeptide fragment of the immunoglobulin heavy chain, while the C-terminal Pro-Gly-Pro extension is a synthetic addition. The peptide has a molecular mass near 752 daltons and carries a net positive charge at physiological pH because of the arginine and lysine side chains. Published indexes list it under the name Selank and the sequence abbreviation TKPRPGP. Solid-phase peptide synthesis is the standard production route for research quantities.
Identity and purity of selank are established with reversed-phase high-performance liquid chromatography coupled to mass spectrometry. The peptide elutes from C18 columns with acetonitrile gradients in water containing trifluoroacetic acid or formic acid, and detection is usually performed by ultraviolet absorbance near 214 nm. Electrospray ionization in positive mode gives a doubly protonated ion near m/z 377, consistent with a mass of about 752 Da. Amino acid analysis or tandem mass spectrometry of fragment ions confirms the sequence. Because the molecule has no aromatic residues, it lacks a usable 280 nm chromophore, so low-wavelength detection or mass spectrometry is required.
Peptide bonds in selank are susceptible to hydrolysis under strongly acidic or basic conditions, and the terminal proline residues are vulnerable to exopeptidase activity in biological samples. Lyophilized powder stored dry at -20 °C typically remains stable for extended periods, whereas aqueous solutions degrade faster and may lose measurable purity within days to weeks depending on pH, temperature, and microbial load. Repeated freeze-thaw cycles promote aggregation and adsorption to container surfaces. For analytical work, solutions are usually prepared fresh, kept cold, and used within a single working day.
Handling follows standard practice for research peptides. Material is weighed in a low-humidity environment because the powder absorbs atmospheric moisture. Purity is reported as the percentage area of the main peak in a chromatogram, with specifications commonly set at 95 percent or higher; values below that threshold indicate the presence of truncated or modified species. Residual trifluoroacetate from purification is often present and may affect mass balance. Certificates of analysis should state the analytical method, the column and gradient used, and the lot-specific retention time so that results can be compared across suppliers.
G6PD is generally found as a dimer of two identical monomers (see main thumbnail). Depending on conditions, such as pH, these dimers can themselves dimerize to form tetramers. Each monomer in the complex has a substrate binding site that binds to G6P, and a catalytic coenzyme binding site that binds to NADP+/NADPH using the Rossman fold. For some higher organisms, such as humans, G6PD contains an additional NADP+ binding site, called the NADP+ structural site, that does not seem to participate directly in the reaction catalyzed by G6PD. The evolutionary purpose of the NADP+ structural site is unknown. As for size, each monomer is approximately 500 amino acids long (514 amino acids for humans). Functional and structural conservation between human G6PD and Leuconostoc mesenteroides G6PD points to three widely conserved regions on the enzyme: a nine-residue peptide in the substrate binding site, RIDHYLGKE (residues 198–206 on human G6PD), a nucleotide-binding fingerprint, GxxGDLA (residues 38–44 on human G6PD), and a partially conserved sequence EKPxG near the substrate binding site (residues 170–174 on human G6PD), where we have use "x" to denote a variable amino acid. The crystal structure of G6PD reveals an extensive network of electrostatic interactions and hydrogen bonding involving G6P, three water molecules, three lysine residues, one arginine, two histidines, two glutamic acids, and other polar amino acids. The proline at position 172 is thought to play a crucial role in positioning Lys171 correctly with respect to the substrate, G6P.
The Aviation and Transportation Security Act made the federal government, rather than airports, responsible for airport security. The law created the Transportation Security Administration to inspect passengers and luggage, causing long delays and concern over passenger privacy. After suspected abuses of the USA Patriot Act were brought to light in June 2013 with articles about the collection of American call records by the NSA and the PRISM program, Representative Jim Sensenbrenner (of Wisconsin), who introduced the Patriot Act in 2001, said that the NSA overstepped its bounds. Criticism of the war on terror has focused on its morality, efficiency, and cost. According to a 2021 report by the Costs of War Project, the several post-9/11 wars participated in by the United States in its war on terror have caused the displacement, conservatively calculated, of 38 million people in Afghanistan, Pakistan, Iraq, Libya, Syria, Yemen, Somalia, and the Philippines. They estimated these wars caused the deaths of 897,000 to 929,000 people directly and cost US$8 trillion. In a 2023 report, the Costs of War Project estimated that there have been between 3.6 and 3.7 million indirect deaths in the post-9/11 war zones, with the total death toll being 4.5 to 4.6 million. The report defined post-9/11 war zones as conflicts that included significant United States counter-terrorism operations since 9/11, which in addition to the wars in Iraq, Afghanistan and Pakistan, also includes the civil wars in Syria, Yemen, Libya and Somalia.
Therefore, they require multipurpose plants instead of the dedicated plants prevailing in the industry so far. At the same token, outsourcing is gaining ground. Globalization results in a shift of fine chemical production from the industrialized to developing countries. The latter benefit not only from a "low cost/high skill" advantage, but also from a rapidly rising domestic demand for Western medicine. Despite the mantras of Western industry leaders, the cost advantage of the Asian producers is going to persist. As the pharmerging countries mainly use generics, their market share continues to grow to the detriment of originator pharmaceuticals and agrochemicals. This is also the case for biosimilars, the generic versions of biopharmaceuticals. As a consequence of the harsh business climate, many Western fine chemical companies or divisions created during the "irrational exuberance" at the end of the 20th century have already exited from the sector. Others will follow suit or will be acquired by private equity firms. Survival strategies include implementing lean production principles originally developed by the automotive industry and extending the business model to also include contract research at the beginning and active drug formulation towards the end of the added value chain. This latter strategy, however, is not finding unanimous approval by industry experts.
Sources: en.wikipedia.org
The first fast reactor built and operated was the Los Alamos Plutonium Fast Reactor ("Clementine") in Los Alamos, NM. Clementine was fueled by Ga-stabilized delta-phase Pu and cooled with mercury. It contained a 'window' of Th-232 in anticipation of breeding experiments, but no reports were made available regarding this feature. Another proposed fast reactor is a fast molten salt reactor, in which the molten salt's moderating properties are insignificant. This is typically achieved by replacing the light metal fluorides (e.g. LiF, BeF2) in the salt carrier with heavier metal chlorides (e.g., KCl, RbCl, ZrCl4). Several prototype FBRs have been built, ranging in electrical output from a few light bulbs' equivalent (EBR-I, 1951) to over 1,000 MWe. As of 2006, the technology is not economically competitive to thermal reactor technology, but India, Japan, China, South Korea, and Russia are all committing substantial research funds to further development of fast breeder reactors, anticipating that rising uranium prices will change this in the long term. Germany, in contrast, abandoned the technology due to safety concerns. The SNR-300 fast breeder reactor was completed in 1985 but did not obtain authorization to start up. The project was cancelled in March 1991.
=== Research on storage === The need for refrigeration is a barrier to vaccination in less-developed areas, as a power outage could make an entire community's supply of vaccines a write-off. In 2025, a team from University of Colorado Boulder reported that their experimental sapphire-coated CCEEV maintained its potency for three months at 104 °F (40 °C).
Sichuan was then ruled by general Wang Lingji, who Chiang had appointed as governor in April the past year. Wang's rule was unpopular with the Sichuan warlords, and Liu organized a movement to oppose Wang, joined by Xiong Kewu and Xiang Chuanyi. This association blocked Wang's military budget and unsuccessfully attempted appealing his removal. The communist authors Peng Dixian and Shu Guofan consider this to be a form of opposition to the Nationalist government. By late 1949, Liu had come into contact with fellow Sichuanese generals Deng Xihou and Pan Wenhua to prepare an uprising. At the same time, Chiang Kai-shek had fled to Chongqing after the falls of Nanjing and Guangzhou, and attempted to make a stand in the Sichuan-Xikang area. However, the strategy collapsed when Chongqing fell to the Communists on October 30, and Chiang was once again forced to flee to Chengdu. At the time, Liu and his associates were also in Chengdu, and Yan Xishan had informed them that planes had been prepared for their final retreat to Taiwan. On 2 December, Chiang dispatched Zhang Qun to demand that Liu cooperate with General Hu Zongnan in battle, as his 24th Army was not fighting against the communists. Liu continued to stall for more time until Chiang invited him for a meeting on 7 December. Believing this to be a trap, Liu and Deng Xihou fled to Peng County in hiding, with Pan Wenhua later arriving on 10 December.
Although trials are commonly conducted at major medical centers, some participants are excluded due to the distance and expenses required for travel, leading to hardship, disadvantage, and inequity for participants, especially those in rural and underserved communities. "Decentralized clinical trials" minimize or eliminates the need for patients to travel to sites, and are now more widespread; a capability improved by telehealth and wearable technologies.
Sources: en.wikipedia.org
In 2014, the European Association for Palliative Care recommended oxycodone by mouth as a second-line alternative to morphine by mouth for cancer pain. In children between 11 and 16, the extended-release formulation is FDA-approved for the relief of cancer pain, trauma pain, or pain due to major surgery (for those already treated with opioids, who can tolerate at least 20 mg per day of oxycodone) – this provides an alternative to Duragesic (fentanyl), the only other extended-release opioid analgesic approved for children. Oxycodone, in its extended-release form or in combination with naloxone, is sometimes used off-label in the treatment of severe and refractory restless legs syndrome.
4-Nitro-L-phenylalanine (1) was converted to its phthalimide by heating with phthalic anhydride, and this was converted to its ethyl ester (2). Catalytic hydrogenation produced the corresponding aniline. Heating in acid with oxirane, followed by treatment with phosphorus oxychloride provided the bischloride, and removal of the protecting groups by heating in hydrochloric acid gave melphalan (3).
=== Vertebrates === Serotonin, also referred to as 5-hydroxytryptamine (5-HT), is a neurotransmitter most known for its involvement in mood disorders in humans. It is also a widely present neuromodulator among vertebrates and invertebrates. Serotonin has been found having associations with many physiological systems such as cardiovascular, thermoregulation, and behavioral functions, including: circadian rhythm, appetite, aggressive and sexual behavior, sensorimotor reactivity and learning, and pain sensitivity. Serotonin's function in neurological systems along with specific behaviors among vertebrates found to be strongly associated with serotonin will be further discussed. Two relevant case studies are also mentioned regarding serotonin development involving teleost fish and mice. In mammals, 5-HT is highly concentrated in the substantia nigra, ventral tegmental area and raphe nuclei. Lesser concentrated areas include other brain regions and the spinal cord. 5-HT neurons are also shown to be highly branched, indicating that they are structurally prominent for influencing multiple areas of the CNS at the same time, although this trend is exclusive solely to mammals.
Sources: en.wikipedia.org
Dry powder is best kept sealed, protected from light, and held at minus 20 degrees Celsius or below. Desiccant packaging helps limit moisture uptake because the material is hygroscopic. A sealed vial should be allowed to equilibrate to room temperature before opening to reduce condensation.
Hydrolysis of peptide bonds and deamidation of arginine and proline residues are the main degradation routes. Alkaline pH and higher temperatures accelerate both processes. Buffered, near-neutral solutions stored cold generally degrade more slowly than unbuffered ones.
Reverse-phase chromatography gives the clearest single view of related impurities and truncated sequences. Mass spectrometry then confirms that the main peak carries the expected mass. Neither test detects enantiomeric impurities, which require a separate chiral assay.
Purity is usually reported as an HPLC area percentage, most often measured at 214 nm. Identity is confirmed separately by mass spectrometry. A certificate of analysis should state both the method and the observed value.