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A nucleoside diphosphate is a nucleotide whose nucleoside carries a diphosphate chain: two phosphate groups joined through a phosphoanhydride bond. For the usual 5′ species, the phosphate nearest the sugar is attached at its 5′ position. Adenosine diphosphate is an example, with adenine as its nucleobase and ribose as its sugar. ChEBI records its acid structure as ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{N}}_5{\mathrm{O}}_{10}{\mathrm{P}}_2$.^[ref]

Counting two phosphorus atoms is necessary for this example but does not establish that connectivity. A molecule bearing two separate phosphate esters on different sugar positions would have different connectivity from a single diphosphate chain. The chemical structure, including the bridging oxygen, distinguishes the alternatives.

The adenylate kinase reaction makes the relationship with nucleoside monophosphates and nucleoside triphosphates explicit: one adenosine triphosphate and one adenosine monophosphate interconvert with two adenosine diphosphates.^[ref] The number of nucleotide molecules stays two while their phosphate counts change from three plus one to two plus two.

An original comparison also shows why identifying a nucleoside diphosphate is more informative than measuring total phosphorus. A sample containing $1\mathrm{mmol}$ of pure adenosine diphosphate contains $2\mathrm{mmol}$ of phosphorus atoms. A different sample containing $1\mathrm{mmol}$ each of adenosine monophosphate and free phosphate has that same phosphorus total, but different molecules and reaction possibilities. Total phosphorus alone cannot discriminate the samples.

A phosphoanhydride bond is the oxygen-bridged connection between phosphorus-containing groups in a phosphoric-acid anhydride, conventionally recognized as a $\mathrm{P}-\mathrm{O}-\mathrm{P}$ linkage. In adenosine triphosphate, the phosphate chain contains two such connections, whereas the attachment to the ribose is a phosphate ester. Adenosine diphosphate retains one phosphoanhydride connection. These distinctions concern connectivity, not simply the presence of phosphorus.^[ref][ref]

For an original connection count, draw the phosphate chain schematically asThe subscripts label the three phosphorus-containing groups, not bare phosphorus atoms bonded directly to adenosine. The two intergroup bridges are α–β and β–γ. A cleavage giving ADP and free phosphate leaves the α–β connection intact; a cleavage giving AMP and diphosphate leaves a phosphate–phosphate connection in the diphosphate product. Breaking one connection does not necessarily destroy every anhydride connection in the products.

The phrase “high-energy bond” must be read in the context of a complete chemical reaction. Isolated bond breaking requires an energy input; whether hydrolysis lowers Gibbs free energy depends on the full reactant/product states and conditions. A structural count alone supplies neither a reaction rate nor a free-energy change.^[ref] Calling every reaction that breaks a phosphoanhydride connection identical would erase the different chemical products and coupling possibilities.