NAD+ is the most-handled coenzyme in biochemistry and one of the least stable reagents in the freezer. The same dinucleotide that shuttles electrons through every central metabolic pathway falls apart in water on a timescale of days, and it degrades under conditions that are the mirror image of what destroys its reduced partner NADH. This profile is a laboratory-reference treatment: the structure, the redox chemistry, why the molecule is so fragile in solution, and how it relates to the precursors — NMN and NR — that dominate the longevity-research conversation. It makes no clinical claims.
Structural identity
- Molecular formula (free acid): C₂₁H₂₇N₇O₁₄P₂
- Average mass: 663.43 g/mol
- CAS: 53-84-9 (NAD+); 58-68-4 (NADH)
- Class: Pyridine dinucleotide coenzyme. Also written β-NAD, β-DPN, or "cozymase" in the older literature.
The name is a literal description of the structure. Nicotinamide adenine dinucleotide is two nucleotides joined at their phosphates. One nucleotide carries adenine, the other carries nicotinamide (the amide of vitamin B3), and a bridging pyrophosphate links them. Each base sits on its own ribose. The nicotinamide ring is the business end, where the redox chemistry happens, and the bond between that ring and its ribose, the N-glycosidic bond, is the molecule's structural weak point.
A note on two molecular-weight conventions that trip people up. Suppliers list NAD+ as the neutral free acid, C₂₁H₂₇N₇O₁₄P₂, at 663.43 g/mol. The IUPAC and textbook convention writes the cationic species C₂₁H₂₈N₇O₁₄P₂⁺ at 664.4 g/mol, where the "+" denotes the quaternary nitrogen of the oxidized nicotinamide ring. The roughly 1-Da difference is a protonation convention, not a discrepancy. NADH, the reduced form, is C₂₁H₂₉N₇O₁₄P₂ at about 665.4 g/mol.
The redox couple: what NAD+ actually does
NAD+ is an electron carrier. In its oxidized form it accepts a hydride (a proton with two electrons) at the 4-position of the nicotinamide ring, becoming NADH. That single reaction, run forward and backward billions of times a second across a cell, is how reducing equivalents move from fuel molecules to the electron transport chain. The midpoint potential of the NAD+/NADH couple is −0.32 V, which places it near the electron-donating end of the biological redox range.
Otto Warburg pinned the redox activity to the nicotinamide ring in the 1930s, work that grew out of the original 1906 discovery of a heat-stable "cozymase" by Arthur Harden and William John Young. Harden shared the 1929 Nobel Prize in Chemistry with Hans von Euler-Chelpin for the fermentation studies that first isolated it. The modern interest in NAD+ comes from the enzymes that consume it outright rather than cycle it: sirtuins, PARPs, CD38, and SARM1 cleave NAD+ as a substrate in signaling, DNA repair, and immune pathways.
Why is NAD+ so unstable in solution?
Because it hydrolyzes. Reconstituted NAD+ holds up for only about a week at 4°C at neutral pH, and it decomposes far faster in acid or base. Lyophilized powder, by contrast, lasts years at −20°C. The difference is free water: in solution the labile bonds (the N-glycosidic bond, the pyrophosphate bridge, and the ring itself) are all open to attack.
Hydrolysis at the N-glycosidic bond releases free nicotinamide; scission at the pyrophosphate gives AMP and nicotinamide mononucleotide; the ribose can fall to ribose-5-phosphate. A stale NAD+ stock does not merely lose potency, it accumulates exactly the breakdown products you would least want in an assay. Cold storage matters because the kinetics are strongly temperature-dependent; degradation follows the usual Arrhenius pattern, so every drop in temperature buys disproportionate stability.
Inverse pH stability: NAD+ versus NADH
Here is the fact that catches people who store the two forms together. NAD+ and NADH have opposite pH requirements. NAD+ is degraded by base (alkaline conditions accelerate its breakdown), while NADH is degraded by acid. You cannot hold the redox pair under one happy condition. When both must coexist in a buffer, the practical compromise sits near pH 8.5, which minimizes the sum of the two degradation rates without being optimal for either.
| Property | NAD+ (oxidized) | NADH (reduced) |
|---|---|---|
| Degraded by | Alkaline / base | Acid |
| More stable at | Neutral to mildly acidic pH | Mildly alkaline pH |
| Approx. mass | 663.4 g/mol (free acid) | 665.4 g/mol |
| UV signature | 260 nm (adenine) | 260 nm + 340 nm |
| Role in redox | Electron acceptor | Electron donor |
The 340 nm absorbance of NADH, absent in NAD+, is the basis for the most common enzyme assay in all of biochemistry: dehydrogenase activity is tracked by watching 340 nm rise or fall as NAD+ and NADH interconvert. Buffer choice also affects longevity. In cofactor-stability work, Tris outperforms HEPES, and both outperform phosphate.
NAD+, NMN, and NR: the precursor landscape
NAD+ is rarely the molecule a cell takes up directly, which is why so much of the field works with precursors instead. The biosynthetic chain runs from the simplest building blocks upward: niacin and nicotinamide, then nicotinamide riboside (NR), then nicotinamide mononucleotide (NMN), then NAD+ itself. NMN is the immediate precursor, one enzymatic step from NAD+ via the NMNAT enzymes. NR sits one step further back and has to be phosphorylated to NMN first.
Two pathways feed the pool. The salvage pathway recycles nicotinamide back into NAD+ through NAMPT, the rate-limiting enzyme of the route. The de novo pathway builds NAD+ from the amino acid tryptophan by way of quinolinic acid. The salvage route carries most of the day-to-day flux, which is why NAMPT activity is a recurring variable in the metabolism literature.
Does intact NAD+ cross cell membranes?
Probably not on its own, and that open question is exactly why precursors dominate. Intact NAD+ does not freely diffuse across the cell membrane; it requires dedicated transport. Even for the smaller precursor NMN, whether it enters cells whole is unsettled. Grozio, Imai, and colleagues proposed in 2019 (Nature Metabolism) that the transporter Slc12a8 imports NMN directly. Schmidt and Brenner, in the same journal that year, rebutted the claim, arguing NMN is first dephosphorylated to NR before uptake.
The dispute is unresolved as of this writing, and it is more than academic. If cells cannot import NAD+ or NMN intact, then the form a reagent takes (NAD+, NMN, NR, or plain nicotinamide) determines what actually reaches the intracellular pool in a given model system. For a researcher choosing a compound, that is the single most consequential variable, and it is the reason the "NMN versus NR" question has its own literature.
Stability, salt forms, and handling notes
NAD+ ships as a hygroscopic powder and is sold in more than one salt form, which matters for anyone calculating molarity. The free acid (663.43 g/mol) is the common catalog form; disodium-salt and hydrate forms carry different molecular weights, so the exact figure on the lot's certificate is the one to use for reconstitution math, not a textbook value. Confirm the salt form and net content against the lot report; our CoA guide walks through where that appears, and every lot we release is in the open CoA library.
Reconstitution follows the same mechanical care as a peptide (stream diluent down the glass wall, swirl rather than shake), though the chemistry it protects against is hydrolysis and oxidation of the coenzyme rather than peptide aggregation. The broader handling framework is in our reconstitution guide and storage and stability article.
Common errors to avoid:
- Storing reconstituted NAD+ like a peptide. Many peptides last weeks refrigerated; NAD+ in solution is a days-to-weeks reagent at best, and far less at non-neutral pH.
- Holding NAD+ and NADH under the same conditions. Their pH-stability requirements are opposite. What protects one degrades the other.
- Using a textbook molecular weight for a salt-form vial. Free acid, disodium salt, and hydrate differ in mass. Use the CoA value.
- Assuming NAD+ enters cells intact. Membrane permeability of the intact dinucleotide is the open question the precursor field exists to work around.
- Leaving stocks at room temperature. Degradation is strongly temperature-dependent. Keep powder frozen and solutions on ice.
Where to find primary literature
The NAD+ record is vast; for the angles in this profile, useful entry points are the cofactor-stability literature (search "NAD stability pH degradation"), the salvage-pathway reviews indexed under "NAMPT NAD+ salvage," and the membrane-transport debate ("Slc12a8 NMN transporter," which returns both the Grozio/Imai 2019 paper and the Schmidt/Brenner rebuttal). The precursor comparison the search traffic actually wants, NMN versus NR, sits downstream of the membrane question covered above. HelixCore supplies NAD+ as a 500 mg lyophilized vial, paired with bacteriostatic water for reconstitution.
