In this work, a systematic 15N/13C solid-state NMR investigation is performed on three dopamine (DA) conjugates of hyaluronic acid, considered in both its native (HA) and NaIO4-oxidized (HAOx) forms. Two of them, here named HAEDC-DA and HAOx-DA, have been previously introduced as covalent conjugates involving DA amine nitrogen: the former by EDC-mediated amide bond formation, and the latter by reaction of the Schiff base with the aldehyde moieties presumed to exist in HAOx. The third conjugate, HA-DA, is reported here for the first time; it is obtained by simply mixing hyaluronan with DA∙HCl at pH 5. The 15N ss-NMR spectra were found to be consistent in all the systems, and the DA molecules were found to be in their charged -NH3+ form, which contradicts the HAEDC-DA/HAOx-DA covalent bonding schemes proposed in the literature. The 13C ss-NMR results add useful new insights into the structure and interaction patterns of the conjugates. All of our findings are relevant for future practical applications, for instance in developing novel HA-based hydrogels. In addition, the present study demonstrates the importance of using the most appropriate analytical tools when investigating composite systems due to the complexity of hyaluronic acid conjugates. Solid-state NMR proved essential to answering the question in the title: actually, there is nothing wrong with hyaluronic acid chemistry; the claimed covalent bonds between DA and the HA(HAOx) chain do not exist in these systems, because the conditions for their formation do not hold in practice.
Keywords: dopamine; hyaluronic acid; oxidized hyaluronic acid; solid-state NMR.