= Nucleoside diphosphate
A \b[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 $\mathrm{C_{10}H_{15}N_5O_{10}P_2}$.<ADP in ChEBI>[CHEBI:16761, Definition, Formula and structure]{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>.<Enzyme Nomenclature by the IUBMB>[EC 2.7.4.3, accepted reaction]{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.
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