The acid dissociation of a nitric acid HNO(3) molecule located at various depths in a water slab is investigated via Car-Parrinello molecular dynamics simulations. HNO(3) is found to remain molecular when it is adsorbed on top of the surface with two hydrogen-bonds, and to dissociate--although not always--by transferring a proton to a water molecule within a few picoseconds when embedded at various depths within the aqueous layer. The acid dissociation events are analyzed and discussed in terms of the proton donor-acceptor O-O hydrogen bonding distance and the configurations of the nearest-neighbor solvent waters of an HNO(3).H(2)O pair. Four key structural features for the HNO(3) acid dissociation are identified and employed to analyze the trajectory results. Key solvent motions for the dissociation include the decrease of the proton donor-acceptor O-O hydrogen bonding distance and a 4 to 3 coordination number change for the proton-accepting water. The Eigen cation (H(3)O(+)), rather than the Zundel cation (H(5)O(2)(+)), is found to be predominant next to the NO(3)(-) ion in contact ion pairs in all cases.

Gut microbiota play an important role in maintaining intestinal health and are involved in the metabolism of carbohydrates, lipids, and amino acids. Recent studies have shown that the central nervous system (CNS) and enteric nervous system (ENS) can interact with gut microbiota to regulate nutrient metabolism. The vagal nerve system communicates between the CNS and ENS to control gastrointestinal tract functions and feeding behavior. Vagal afferent neurons also express receptors for gut peptides that are secreted from enteroendocrine cells (EECs), such as cholecystokinin (CCK), ghrelin, leptin, peptide tyrosine tyrosine (PYY), glucagon-like peptide-1 (GLP-1), and 5-hydroxytryptamine (5-HT; serotonin). Gut microbiota can regulate levels of these gut peptides to influence the vagal afferent pathway and thus regulate intestinal metabolism via the microbiota-gut-brain axis. In addition, bile acids, short-chain fatty acids (SCFAs), trimethylamine-N-oxide (TMAO), and Immunoglobulin A (IgA) can also exert metabolic control through the microbiota-gut-liver axis. This review is mainly focused on the role of gut microbiota in neuroendocrine regulation of nutrient metabolism via the microbiota-gut-brain-liver axis.2 of 21 formate, acetate, propionate, and butyrate, which are related to maintaining intestinal epithelium and permeability [11]. Further, SCFAs regulate glucose and lipid metabolism as well as immune and inflammatory responses [12,13]. Hence, gut microbiota also plays an important role in immune systems, inflammation, and cancer prevention of the host [14,15]. The enteric nervous system (ENS) is reported to be involved in intestinal metabolic regulation, and enteric neurons and intestinal neurotransmitters play an important role in ENS regulation [16][17][18]. The gut contains full ENS reflex circuits, such as motor neurons, interneurons, and sensory neurons, and these neurons transfer information between the ENS and central nervous system (CNS). The vagal nerve pathway communicates between the CNS and ENS, which has remarkable impact on regulating gastrointestinal tract functions and feeding behavior [19,20]. Thus, the vagal nerve system is also involved in intestinal metabolic regulation through the gut-brain axis. Vagal afferent neurons express receptors for gut peptides, such as cholecystokinin (CCK), ghrelin, leptin, peptide tyrosine tyrosine (PYY), glucagon-like peptide-1 (GLP-1), 5-hydroxytryptamine (5-HT) and so on, which are secreted from enteroendocrine cells (EECs) [20][21][22]. When vagal afferent neurons sense these types of gut peptides, the corresponding gut information will transfer to the CNS and exert various reactions. At the same time, gut microbiota can regulate these gut peptides, such as CCK, ghrelin, leptin, PYY, GLP-1, 5-HT levels to influence vagal afferent pathway, and then regulated intestinal metabolic metabolism via the microbiota-gut-brain axis [23][24][25].The importance of the gut-brain axis in human health and disease has been known for a long period. However, it has only been re...