Adenosine diphosphate (ADP) plays a crucial role in cell biochemistry, especially in metabolic pathways and energy storage. ADP itself, as well as many of its analogues, such as adenosine hypodiphosphate (AhDP), has been studied extensively, in particular in terms of enzymatic activity. However, structural studies in the solid state, especially for AhDP, are still missing. An analogue of ADP, i.e. adenosine hypodiphosphate ester, has been synthesized and characterized in the crystalline form as two hydrated sodium salts of 2':3'-isopropylideneadenosine 5'-hypodiphosphate (HAhDP, CHNOP for the neutral form), namely pentasodium tetrakis(2':3'-isopropylideneadenosine 5'-hypodiphosphate) tetracosahydrate, 5Na·3CHNOP·CHNOP·24HO or Na(HAhDP)(HAhDP)·24HO, (I), and sodium tetrakis(2':3'-isopropylideneadenosine 5'-hypodiphosphate) pentadecahydrate, Na·CHNOP·2CHNOP·CHNOP·15HO or Na(HAhDP)(HAhDP)(HAhDP)·15HO, (II). Crystal structure analyses of (I) and (II) reveal two nucleoside hypodiphosphate ions in the asymmetric units with different ionization states of the hypodiphosphate unit and adenine base. For all AhDP nucleotides, the same anti conformation about the N-glycosidic bond and similar puckering of the ribose ring have been found. AhDP geometry and interactions have been compared to ADP nucleotides deposited in the Cambridge Structural Database. The adenine-hypodiphosphate interactions, identified as defining nucleotide self-assembly, have been analysed in model systems, i.e. the adenine (Ade) salts of hypodiphosphoric acid, namely bis(adeninium) hypodiphosphate dihydrate, 2CHN·HPO·2HO or (AdeH)(HPO)·2HO, (III), and bis(adeninium) hypodiphosphate, 2CHN·HPO or (AdeH)(HPO), (IV).

Organic-inorganic salts and ionic co-crystals of hypodiphosphoric acid (H4P2O6) with 2-aminopyridne (2ap) have been synthesized and characterized by X-ray crystallography, thermogravimetry, element analysis and Fourier-transform infared spectroscopy. (2apH)4(H3P2O6)2(H2P2O6)∙2H2O (1), (2apH)2(H2P2O6)∙2H2O...

Two imidazolium hypodiphosphates, (C3H5N2)(H3P2O6) (I) and (C3H5N2)2(H2P2O6) (II), have been synthesized and structurally characterized. In both metal-free organic–inorganic hybrids (I) and (II), the hypodiphosphate mono- and dianions, (H3P2O6)− and (H2P2O6)2−, form hydrogen-bonded frameworks of different types, to which the organic cations are linked via N—H...O and C—H...O hydrogen bonds. The purity of the compounds was confirmed by powder X-ray diffraction. Differential scanning calorimetry of compound (I) revealed two structural phase transitions: continuous at 311.8 K [cooling/heating; from high-temperature phase (HTP) to room-temperature phase (RTP)] and a discontinuous one at 287.9/289.2 K [RTP → low-temperature phase (LTP)]. Compound (I) is characterized in a wide temperature range by single-crystal and powder X-ray diffraction methods. Crystal structures of high- and low-temperature phases are determined, which show orthorhombic (HTP, Pnna, No. 52) → monoclinic (LTP, P21/n11, No. 14, a-axis doubled) structural change on cooling with an intermediate incommensurately modulated phase (RTP). Dynamic properties of polycrystalline (I) were studied by means of dielectric spectroscopy. The dielectric behaviour is explained by the motion of imidazolium cations.

Hypodiphosphoric acid (H4P2O6) -the structural analogue of diphosphoric acid (H4P2O7) -contains two phosphorus atoms at the +4 oxidation state, which are connected by a direct covalent bond. After its synthesis was described in the 19th century, the mainstream of scientific research focused on the synthesis and physicochemical properties of its inorganic salts [1,2].