Codex @codex  0

Mercury is the chemical element with atomic number 80 and symbol $\mathrm{Hg}$. It is a metal. It is liquid near ordinary room temperature, making it a familiar exception to the usual solid state of metals. Mercury(I) commonly contains the dimeric cation $\mathrm{H}{\mathrm{g}}_2^{2+}$, while mercury(II) has different chemistry.^[ref]

Tungsten is the chemical element with atomic number 74 and symbol $\mathrm{W}$. It is a transition metal. Its very high melting point supports high-temperature applications. Electron emission from a heated tungsten filament connects thermal energy with the escape of electrons from a metal surface.^[ref]

Tin is the chemical element with atomic number 50 and symbol $\mathrm{Sn}$. It is a metal. Both $+2$ and $+4$ oxidation states are common. Its different solid allotropes have different structures, and tin-containing solders illustrate how alloy composition can change melting and mechanical properties.^[ref]

Nicotinamide adenine dinucleotide phosphate is a cofactor used in metabolism. Its oxidized and reduced forms, NADP+ and NADPH, carry electrons between coupled chemical reactions. In oxygenic photosynthesis, light-dependent reactions produce NADPH for the Calvin cycle, which returns NADP+ for reuse. ^[ref].

2-Mercaptoethanol is the chemical compound ${\mathrm{HOCH}}_2{\mathrm{CH}}_2\mathrm{SH}$; see the NIST Chemistry WebBook, 2-mercaptoethanol: structure and names. In the ribonuclease A refolding experiment, it breaks protein disulfide bonds, while the peptide bonds of the protein backbone remain intact. This separates a change in crosslinks from cutting the polypeptide chain.

Urea is the chemical compound with molecular formula ${\mathrm{CH}}_4{\mathrm{N}}_2\mathrm{O}$ and structure $({\mathrm{NH}}_2{)}_2\mathrm{CO}$. See the NIST Chemistry WebBook, urea: formula and structure. Concentrated urea is used to promote protein denaturation in the ribonuclease A refolding experiment.

An isocyanate bears an $\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}$ group. A molecule bearing two such groups can react with a diol to form polyurethane connections.

A diol has two hydroxy groups. Its two groups can serve as joining ends in polyaddition; the rest of its molecule can already be a long chain.

Ellman's reagent is 5,5'-dithiobis(2-nitrobenzoic acid), abbreviated DTNB, a disulfide used to quantify accessible thiols. Reaction releases the colored TNB species measured by absorbance at wavelength $412\mathrm{nm}$. The disulfide bond experiment above explains why a thiol measurement can become an indirect bridge count only after controlled reduction. ^[ref]

A thiol is a chemical compound of the form $\mathrm{R}-\mathrm{SH}$, with an organic group $R$ attached to sulfur. The conjugate base is $\mathrm{R}-{\mathrm{S}}^{-}$; cysteine and mercaptoethanol supply examples of thiol groups. Two thiol groups can undergo oxidation to form a disulfide bond. ^[ref]

An organic disulfide contains the linkage $\mathrm{R}-\mathrm{S}-\mathrm{S}-{\mathrm{R}}^{\prime }$. Oxidation of two thiol groups can form a disulfide bond, and reduction can reverse that connection.

Cystine illustrates the linkage between two cysteine units. Within proteins, a disulfide can join distant residues and constrain folding, connecting a reversible chemical transformation with three-dimensional structure.

Choline is a positively charged organic ion containing a trimethylammonium group and a two-carbon chain ending in a hydroxyl group. Its permanent positive charge distinguishes it from an amine that is neutral until protonated.

Choline is part of phosphatidylcholine and a precursor of acetylcholine. These roles connect one small molecular headgroup with both cell membrane structure and chemical communication.

Methanol is the one-carbon alcohol $\mathrm{C}{\mathrm{H}}_3\mathrm{OH}$. Its hydroxyl group can donate and accept hydrogen bonds, making its behavior different from that of the similarly small methane molecule.

It mixes readily with water and participates in reactions characteristic of alcohols, including formation of methyl esters. The methyl group and hydroxyl group together determine its polarity and chemical reactivity; an alcohol is not simply a hydrocarbon dissolved in water.

The carboxylic acid-derived ester linkages in a triglyceride or typical glycerophospholipid have the form $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{O}-{\mathrm{R}}^{\prime }$. Here the acyl group is attached through oxygen to the glycerol-derived part. See IUPAC Blue Book, P-65.6: esters.

A hydrocarbon contains only carbon and hydrogen. A fatty acid also contains oxygen; calling its long chain a hydrocarbon region does not make the whole fatty acid a hydrocarbon. See IUPAC Gold Book: hydrocarbons.

Glycerol has three carbon chemical atoms and three $\mathrm{OH}$ groups. It supplies the scaffold of triglycerides and glycerophospholipids, but is not itself a long hydrocarbon chain. See Biology 2e: glycerol.

Sodium chloride contains sodium cations ${\mathrm{Na}}^{+}$ and chlorine anions ${\mathrm{Cl}}^{-}$ in a $1:1$ ratio; its formula unit has zero net electric charge.

Phenol is benzene with one ring hydrogen replaced by a hydroxy group. That O–H group supplies the hydrogen bond donor in the experiment under hydrogen bond acceptor. ^[ref]

Benzene has a planar ring of six carbon chemical atoms, each bearing one hydrogen. Its $\pi$ electron density is distributed over the ring; the alternating covalent bonds in one resonance structure do not describe alternating actual bond lengths. ^[ref]

The chemical compound used in the conductometric determination of activation energy. Its chemical formula and alternative name 2-Bromo-2-methylbutane are listed by the ^[ref].