The structure of monosodium phosphoenolpyruvate

C<SUB>3</SUB>H<SUB>4</SUB>O<SUB>6</SUB>P<SUP>-</SUP>.Na<SUP>+</SUP>.H<SUB>2</SUB>O, M<SUB>r</SUB> =208·0, is monoclinic,Cc, a=11.423(2),b=23.253(5),c=6.604(1)Å, β=123.63(1)<SUP>0</SUP>,U=1460.6Å<SUP>3</SUP>, D<SUB>x</SUB>=1.89 Mgm<SUP>-3</SUP>, Z=8,λ (Mo Kα) = 0·1701Å,μ(Mo Ka)= 0.44mm<SUP>-1</SUP>,F(OOO) = 840. Final R= 0·063 for 1697 reflections.The two crystallographically independent molecules of phosphoenolpyruvate (PEP) (If and B) are almost mir­ror images of each other, the mirror being the plan­ar enolpyruvate group. The torsion angle C(3)-C(2)­-O(l)-P(I) is 122·6 in A and -112·0<SUP>0</SUP> in B, in contrast to -209·1<SUP>0</SUP> in PEP.K. The enolic C(2)-O(1) has a partial double-bond character [1.401 A, 1·386 Å (B)]. The high-energy P-O bond (1·595 and 1·610 Å) is comparable to that in PEP. K (1.612Å). Na(1) has six nearest neighbours while Na(2) has only five. The Ne+ ions are involved in binding only the phosphates of different molecules, in contrast to the K<SUP>+</SUP> ion in PEP. K. which binds to both the phosphate and carboxyl ends of the same molecule. The planar carboxyl groups stack on each other at rut average distance of 3·2 Å instead of forming hydrogen-bonded dimers usually found in carboxylate structures