Acid dissociation constants and cytotoxicity test of a series of omega-aminoalkyl phosphates

Abstract We synthesised a series of ω -aminoalkyl sodium hydrogen phosphates (AAP- n -Na, n  = 3, 4, 5, 6, purity > 99%), which have potential applications as bioactive cosmetic ingredients and surface modifiers of bone minerals ( i.e. hydroxyapatites). Results from Fourier transformed infrared (FTIR), nuclear magnetic resonance (NMR) and high resolution mass spectroscopy, and elemental analysis all matched their chemical structures. The acid dissociation constants (p K a's) of each AAP- n (acid form of AAP- n -Na, n  = 2–6) were measured by potentiometric titration, showing a general increasing trend with an increase in the chain length of AAP- n . However, the p K a 3 constant, which corresponds to the deprotonation of the ammonium group in AAP- n -Na, displayed an unusual decrease when n  = even. This odd–even effect can be explained by the pairwise self-association of AAP- n -Na molecules in water where intermolecular hydrogen bonding in case of n  = even is weaker than that in case of n  = odd. All AAP- n -Na at concentrations up to 0.1% (w/v) were non-toxic to L929 fibroblasts and MG 63 osteoblast-like cells in terms of cell growth and morphology. These basic data were important for applications of AAP- n and their salts in biomedical engineering.