Extension of the GROMOS 56a6CARBO/CARBO_R Force Field for Charged, Protonated, and Esterified Uronates

An extension of the GROMOS 5686(CARBO/CARBO_R) force field for hexopyranose-based carbohydrates is presented. The additional parameters describe the conformational properties of uronate residues. The three distinct chemical states of the carboxyl group are considered: deprotonated (negatively charged), protonated (neutral), and esterified (neutral). The parametrization procedure was based on quantum-chemical calculations, and the resulting parameters were tested in the context of (i) flexibility of the pyranose rings under different pH conditions, (ii) conformation of the glycosidic linkage of the (1 -> 4)-type for uronates with different chemical states of carboxyl moieties, (iii) conformation of the exocydic (i.e., carboxylate and lactol) moieties, and (iv) structure of the Ca2+-linked chain-chain complexes of uronates. The presently proposed parameters in combination with the 56a6(CARBO/CARBO_R) set can be used to describe the naturally occurring polyuronates, composed either of homogeneous (e.g., glucuronans) or heterogeneous (e.g., pectins, alginates) segments. The results of simulations relying on the new set of parameters indicate that the conformation of glycosidic linkage is nearly unaffected by the chemical state of the carboxyl group, in contrary to the ring conformational equilibria. The calculations for the poly(alpha-D-galacturonate)-Ca2+ and poly(alpha-L-guluronate)-Ca2+ complexes show that both parallel and anitiparallel arrangements of uronate chains are possible but differ in several structural aspects.