Thirteen unrelated compounds sorted by one structural question — the same question you already asked of a terpene, asked now of a vitamin.
~12 min read
Look at what those six oxygens on ascorbic acid are doing. Four are hydroxyls — the same group you met on linalool, doing what it does there: donating a hydrogen bond, accepting one, holding on to water. The other two close a lactone ring. On six carbons, that leaves almost no non-polar surface.
The B group reaches the same place with different hardware: ring nitrogens, carboxyls, and phosphate groups carrying a full negative charge rather than a partial one. Thiamine is the extreme case. Its thiazolium ring holds a nitrogen with four bonds instead of three, and a nitrogen in that arrangement is permanently positive, whatever the pH. A permanent charge is about as water-loving as a molecule gets.
The behaviour follows without further explanation. A polar molecule dissolves straight into the water compartment of blood and needs no carrier to be soluble there. What dissolves freely in plasma is what the kidney filters freely, so a surplus is leaving within hours of arriving: resupply runs on a short clock, and accumulating one is difficult. There is a ceiling on the way in too — plasma ascorbate levels off near 70 micromoles per litre, because the transporters reclaiming it are finite and saturate. The same saturation happens at the other end, in the intestinal wall.
There is no carboxyl anywhere on ascorbic acid, and it is still an acid. What it has instead is two hydroxyls on adjacent carbons either side of a double bond — an arrangement called an enediol. One of the two gives up its hydrogen at a pKa of about 4.2, properly acidic, comparable to the acid in vinegar.
The reason is what is left behind. When that hydrogen leaves, the negative charge does not sit on one oxygen; it spreads across the double bond and over both oxygens at once. A charge spread out is a charge stabilised, and a molecule that can stabilise what it becomes is willing to let the hydrogen go. At the pH of blood, 7.4, more than ninety-nine per cent of it exists as the ascorbate anion — not slightly polar but fully charged. That is the real answer to why it is the most water-soluble of the thirteen.
The same feature explains the behaviour it is best known for. A group that lets go of a hydrogen easily lets go of an electron easily, and in laboratory assays — in a tube, on cells in a dish, which is not the same as what happens in a person — ascorbate donates electrons readily. Free radicals and antioxidants takes that apart with the qualifiers it needs.
The other four share an architecture rather than a formula: a large hydrocarbon body carrying one small oxygen-bearing head. Each behaves the way that shape says it will.
Vitamin A — retinol
Twenty carbons, one hydroxyl at the end. Four isoprene units, which makes retinol a diterpene alcohol — the construction method of terpenes, one class above what survives steam distillation. A run of alternating double bonds stretches down the chain, and that run is why it interacts with light and why oxygen attacks it.
Vitamin D — cholecalciferol
Twenty-seven carbons, one hydroxyl. It starts as 7-dehydrocholesterol in skin, where ultraviolet B light breaks one bond and opens one of its four rings. That opened ring is the difference between a sterol and a vitamin.
Vitamin E — alpha-tocopherol
A two-ring head carrying one hydroxyl, with a saturated sixteen-carbon tail. The tail is the point: it anchors the molecule in the fatty middle of a membrane and leaves the hydroxyl at the surface, where the water is.
Vitamin K — phylloquinone
A two-ring head with two oxygens double-bonded onto it, making it a quinone, and again a twenty-carbon tail. The head does the chemistry; the tail decides which compartment it reaches.
None of the four dissolves in plasma, so none travels as itself. They move the way a fat moves: emulsified by bile into a micelle, packaged into a lipoprotein, routed into the lymph before they reach blood. A meal with no fat releases no bile, which means no micelle — absorption and bioavailability follows that route step by step.
Storage is the other consequence, and it is solubility again. A body has a great deal of fat to dissolve them in and no sieve to remove them, so the clock is long: circulating 25-hydroxyvitamin D has a half-life measured in weeks, liver stores of retinol in months. Accumulation is possible in a way it is not for vitamin C — which is why published upper intake levels exist for preformed vitamin A and vitamin D and not for most of the B group.
| Water-soluble — B group, C | Fat-soluble — A, D, E, K | |
|---|---|---|
| What makes it so | Many oxygens and nitrogens on few carbons; often a full charge | A large hydrocarbon body, one small oxygen head |
| How it travels | Dissolved in plasma, unaided | Micelle, then lipoprotein, then the lymph |
| Where a surplus goes | Filtered by the kidney, out in urine within hours | Nowhere — it dissolves into fat |
| Resupply clock | Days | Weeks to months |
| Can it accumulate | Hard to do | Yes, which is why ceilings are published |
Where the split leaks
Vitamin B12 is water-soluble by structure and held in the liver for years, because it never travels loose: it is handed from one binding protein to the next and reclaimed from bile rather than lost. Folate has a smaller version of the same. A rule this useful still has exceptions, and the exception is always a protein carrying the molecule.
Filtration in the kidney is a mechanical sieve before it is anything else. Molecules below roughly five thousand daltons pass freely; albumin, at about sixty-six thousand, does not. Ascorbate is 176 daltons and riboflavin 376, so keeping either is an active job — transporters reclaiming it from the filtrate, at a rate they can sustain and no faster.
The fat-soluble four are never presented to that sieve, and the reason is not size. Retinol rides a binding protein of about twenty-one thousand daltons, which would filter easily alone — so it circulates locked to a second protein, transthyretin, and the pair is far too large to pass. “Water-soluble vitamins leave in urine” is a statement about a sieve and a set of carrier proteins, not about vitamins.
Vitamin E is not a compound. It is eight of them. Each of the thirteen names a group of related molecules the body can put to the same use, and those relatives are vitamers.
Vitamin E means four tocopherols and four tocotrienols — alpha, beta, gamma and delta of each. What separates the tocopherols is where methyl groups sit on the ring; what separates a tocopherol from a tocotrienol is three double bonds in the tail. Eight molecules, one name, not interchangeable: the liver carries a transfer protein that binds one in preference to the rest.
Vitamin A arrives by two routes: as retinol, already a vitamin, or as beta-carotene — C40H56, forty carbons, eight isoprene units, not one oxygen atom. Beta-carotene is a pure hydrocarbon, a terpene of the largest class, and it is not vitamin A. An enzyme cuts it in half at its central double bond to give two retinals, which are then reduced to retinol. That conversion is neither efficient nor equal in everyone — roughly twelve to one by weight from a plant source — which is why a label reports vitamin A in retinol activity equivalents.
Diagram to come
Three stages left to right at one consistent scale. Stage one: beta-carotene drawn full length as a skeletal structure, its forty carbons visibly long, with the eight isoprene units marked by faint dividing ticks along the chain so a reader can count them, and both end rings drawn. Mark the central double bond distinctly, in a second colour or heavier weight, labelled as the cut point. Stage two: the molecule separated into two mirror-image halves at that bond, each now carrying an oxygen double-bonded at its new end, labelled as retinal. Stage three: one of those halves with the oxygen now shown as a hydroxyl, labelled as retinol. Below the figure, one line giving the formula of beta-carotene and noting that it contains no oxygen at all. The reader should finish able to say that a carotene becomes a vitamin by being cut and oxygenated, not by being absorbed.
Folate and folic acid are used as synonyms and are not the same molecule. Folic acid is fully oxidised and carries a single glutamate; folate in a leaf is reduced and carries a chain of them. What the body uses is 5-methyltetrahydrofolate, reduced twice over from folic acid by an enzyme whose capacity in humans is modest. A methylfolate on a label has had that done already.
B12 is the clearest case, because the difference between its vitamers is one attached group. Cobalamin is a corrin ring with a cobalt ion at its centre — the only vitamin built around a metal, and the place where minerals and vitamins stop being separate categories. What sits on that cobalt is what the prefix names: cyanide in cyanocobalamin, a methyl group in methylcobalamin, a hydroxyl in hydroxocobalamin. The body exchanges it for whichever the reaction in front of it needs, and an exchange is a step.
Why a label says “as”
A panel line reading vitamin E (as d-alpha-tocopherol), or folate (as L-5-methyltetrahydrofolate), is not padding. The amount alone does not say which molecule is in the bottle, and two bottles carrying the same figure can hold different compounds. Reading a supplement label takes a real panel apart. Which forms are sold also differs by market.
Alpha-tocopherol has three carbons where four different groups meet. Each is a chiral centre — a point buildable in either of two mirror-image arrangements that cannot be superimposed however you turn them. Three of them give two by two by two: eight distinct molecules sharing one formula.
A plant builds exactly one. It is written RRR-alpha-tocopherol, and on an older label it appears as d-. Synthesis has no such preference and produces all eight in roughly equal parts, written all-rac, or dl- — so seven-eighths of a synthetic alpha-tocopherol is a stereoisomer no plant makes. The liver’s transfer protein binds RRR in preference, which is why vitamin E is one of the few nutrients whose unit conversions depend on which molecule you started from.
None of this is a nutritional oddity. It is the chemistry of an oil bottle. Carvone comes in two mirror images: one smells of spearmint, the other of caraway, from the same atoms in the same order. A receptor is itself handed, and so is a transfer protein. To anything with a shape, mirror images are two different molecules.
A vitamin is a molecule, so it can be broken — and what breaks one is never general fragility. It is a named bond in a named molecule, and knowing which bond tells you which conditions matter.
Oxygen, on ascorbic acid
The enediol that makes vitamin C an acid makes it the easiest target of the thirteen. Two electrons away and it is dehydroascorbic acid, which the body can still reduce back. Leave it longer, the ring hydrolyses open, and that is not recoverable. Copper and iron ions catalyse the first step, which is why a cut apple browns faster than air alone would manage.
Light, on riboflavin
Riboflavin absorbs visible light and breaks down to lumiflavin. Not a laboratory curiosity — it is why milk stopped being sold in clear glass bottles.
Alkali and heat, on thiamine and folate
Thiamine holds its two rings together with a single carbon bridge, and the bridge is the weak point: raise the pH and it comes apart. Folate is the most heat-sensitive of the thirteen — prolonged boiling costs a vegetable more than half of it.
Oxygen and light, on retinol and carotene
Both carry long runs of alternating double bonds, and a double bond is where oxygen attacks — the same chemistry, on the same kind of bond, that makes an old bottle of oil smell unlike a fresh one. Purity covers that side.
One of the thirteen resists nearly all of it: B12 comes through cooking temperatures largely intact, protected by the ring it sits in.
Leaching is not degradation. A water-soluble vitamin gone into the cooking water is intact — it moved, it did not break, and steaming the vegetable or using the water keeps it. Oxidation is a chemical change and does not come back.
Which makes the storage instructions on a supplement bottle the same as on an oil bottle, for the same reasons in the same order. Cool, dark, closed tightly, not kept for years. A cupboard rather than a windowsill; not the shelf above the cooker, not a bathroom that fills with steam twice a day. The chemistry that quietly turns linalyl acetate back into linalool in a warm bathroom thins out a vitamin the same way, and neither announces itself.
One honest limit. Structure tells you how a molecule is handled — where it goes, what breaks it, whether it can be stored. It does not tell you what it does once it arrives, and for most of the B group that is a second molecule entirely: enzymes and cofactors.
Keep supplements out of reach of children, and keep oils away from eyes and inner ears. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. A supplement does not replace a varied diet, and nothing here replaces professional medical care. Nothing here is intended to diagnose, treat, cure or prevent any disease.
One question — polar or not — sorted thirteen unrelated molecules into two families and predicted how each is carried, stored, lost and spoiled. The next class of nutrient does not answer it at all: a mineral is not a molecule. It is an element with a charge, and the charge does the work.
Lesson 14 of 18 · Science