Hyaluronic acid: a double-helical structure in the presence of potassium at low pH and found also with the cations ammonium, rubidium and caesium.

An anti-parallel double-helical structure is proposed for hyaluronic acid in the presence of the cations K ⁺, NH⁺₄, Rb⁺ and Cs⁺. In particular the crystalline phase containing potassium ions, and in a defined pH range, has been determined and refined against the X-ray diffraction amplitudes. The reflections in the diffraction pattern index on a tetragonal unit cell (a = b = 1.714 nm, c (fibre axis) = 3.28 nm) with observed systematic absences of the form h + k + l = 2n for general hkl reflections and l = 4n (where n is an integer) for 001 reflections. Two double-helical molecules pass through the unit cell at positions ($\text{1}\text{4}\text{,}\text{1}\text{4}\text{, 0}$) and ($\text{3}\text{4}\text{,}\text{3}\text{4}\text{, 0}$) as defined by space group I4₁22. Each chain forms a contracted (axial advance per disaccharide repeat of 0.82 nm) 4-fold left-handed helix which intertwines with a neighbouring chain of opposite polarity about a common axis. Features of the structure are the positioning of the carboxyl group toward the core of the double helix and the presence of hydrophobic and hydrophilic pockets between adjacent double helices. The state of protonation of the hyaluronate molecule is discussed in the light of the infrared evidence. Common physicochemical features of these four cations which promote this particular doublehelical conformation for hyaluronate are considered.