3-Hydroxyisonicotinaldehyde (HINA), also known as 3-hydroxypyridine-4-carboxaldehyde, is a derivative of

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...

, with

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

and

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

substituents. It has been studied as a simple analogue of vitamin B₆. In 2020, it was reported as having the lowest molecular weight of all

dyes A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

which exhibit green

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

Preparation

3-Hydroxyisonicotinaldehyde was first prepared in 1958 by oxidation of 3-hydroxy-4-pyridinemethanol with

manganese dioxide Manganese dioxide is the inorganic compound with the formula . This blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for is for dry-cell ...

. Alternative syntheses have also been reported.

Spectroscopic properties

The

absorption spectrum Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...

of HINA has been the subject of studies dating back to the 1950s, owing to its relationship to vitamin B₆ and

pyridoxal Pyridoxal is one form of vitamin B6. Some medically relevant bacteria, such as those in the genera ''Granulicatella'' and ''Abiotrophia'', require pyridoxal for growth. This nutritional requirement can lead to the culture phenomenon of satellit ...

, of which it is a simple analogue. However, its fluorescent properties were not described until 2020. It is noteworthy for having a green-emitting

fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound that can re-emit light upon light excitation. Fluorophores typically contain several combined aromatic groups, or planar or cyclic molecules with se ...

with a wavelength of maximum emission (λ_em,max) at 525 nm in aqueous solution at alkaline pH, making it the compound of lowest

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

to display that property. In acidic solutions, the fluorescence is less intense and becomes blue; the compound has

isosbestic point In spectroscopy, an isosbestic point is a specific wavelength, wavenumber or frequency at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. The word derives from two Greek words: ...

s at 270 and 341 nm. : HINA-pH

The molecular basis of the observed properties is the presence of a push-pull fluorophore, a feature of many fluorescent and

luminescent Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a cryst ...

compounds. At pH above 7.1 in aqueous solutions, HINA is in its

anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

form, with its absorbance peak at 385 nm and emission peak at 525 nm. The anion contains just 13 atoms, with a

molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...

of 122 Da. The

quantum yield The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system. Applications Fluorescence spectroscopy The fluorescence quantum yield is defined as the ratio of the numb ...

for the emission is 15%, with an emission lifetime of 1.0 ns. The observed

Stokes shift __NOTOC__ Stokes shift is the difference (in energy, wavenumber or frequency units) between positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) of the same electronic transition. It ...

of 6900 cm⁻¹ is typical of push-pull dyes.

Uses

In mechanistic studies of vitamin B₆

HINA has been used as an analogue of pyridoxal 5′-phosphate, the active form of the

coenzyme A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...

vitamin B₆. It is an especially good mimic for the enzyme-bound form of that compound, better than the vitamin or pyridoxal. The enzyme mechanism involves

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bo ...

formation, giving a Schiff's base, and such derivatives of HINA with

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

have been studied for their

reaction kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in wh ...

, leading to insights about the enzymes which use pyridoxal 5-phosphate.

As a dyestuff

Stable dyes of low molecular weight which are water soluble are useful in biological systems. HINA has been used to detect and quantify the presence of

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

in aqueous solutions.

References