Hydration of para-aminobenzoic acid (PABA) and its anion—The view from statistical mechanics
Introduction
For the functioning of many biomolecules a specific pattern of hydrophobic and hydrophilic patches at their surface is essential. Hydrophilic groups play an important role in controlling hydrophilicity and, thus, the interactions of the biomolecule with water and dissolved hydrophilic or/and charged compounds. Interactions between hydrophobic moieties such as aromatic rings and water are considered one of the important driving forces in different biochemical processes, for instance, in stabilization of biological structures, or in molecular recognition. It should be noted that recognition of aromatic functional groups plays a crucial role in drug design [1], [2]. Much experimental and theoretical effort is spent on exploring such systems. Nevertheless, molecular details of both hydrophilic hydration and especially hydrophobic hydration are not fully understood [3], [4], [5].
In this contribution we focus on the hydration of para-aminobenzoic acid (PABA, NH2C6H4COOH) and its anion (PABA−, NH2C6H4COO−) in aqueous solution. Both forms are existing in an aqueous environment at physiological pH [6]. PABA consists of specific functional groups typical for biological systems, namely a benzene ring substituted with an amino group and a carboxyl group. PABA is well known for its role in biological processes [7], [8], [9], [10], [11], [12]. It is a building block of PGA (pteroylglutamic acid), a form of folic acid, and considered as a B-complex factor. PABA enhances the metabolism of amino acids, improves the formation and health of red blood cells and assists the manufacture of folic acid in the intestines [13], [14], [15]. Rather recently [16], [17] it was established that PABA also stimulates interferon formation in living organisms. PABA also acts against viruses, modulates the immune system, is an antioxidant, an antithrombotic agent and a regulator of a water–salt exchange [18], [19], [20], [21]. In spite of the biological importance of PABA it is surprising that no information on the hydration of this rather simple molecule is available.
The hydration structure of PABA and PABA− was investigated by statistical mechanics, namely one-dimensional (1D-) and three-dimensional (3D-) RISM (Reference Interaction Site Model) theory. The RISM approach, initially introduced by Chandler and Andersen [22], provides detailed information about solute–solvent interactions and is a good method to investigate solvation phenomena. The 1D-RISM approach operates with site–site radial distribution functions (RDFs). With the 3D-RISM approach spatial molecule–atom distribution functions (SDFs) are obtained. The RISM theory is actively employed in the description of molecular and ion-molecular systems and was found to be an efficient tool in the investigation of hydration phenomena in aqueous biomolecular systems (see, for instance, Refs. [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]).
Section snippets
The theoretical methods and computational details
The basic concepts of the RISM theory are well known and can be found in the literature [22], [37], [38], [39], [40], [41], including a detailed description of the 1D-RISM method [22], [41]. For details of the 3D-RISM method we refer to Kovalenko [42]. Thus, it may suffice to give a brief outline of the main points of the 1D- and 3D-RISM methods.
PABA
The main peak of (w denotes the water molecule), characterizing the first hydration shell of the NH2 group, is located at a distance of 0.300 nm (Fig. 4), yielding a hydration number of is ~ 6.56 for the amino group (Table 2, ). The RDF is indicative for the nitrogen atom on the amino group acting as a H-bond acceptor with the hydrogen atoms on the water molecules. This function has the typical H-bond peak at a distance of 0.175 nm (Fig. 4, Table 2) with ~ 0.67 H-bonds formed by
Conclusions
The results of 1D- and 3D-RISM studies of the hydration structure of para-aminobenzoic acid and its anion form in water were presented. The first hydration shell of PABA and PABA− consists of ~ 28.4 and 27.2 water molecules respectively, indicating stable hydration shells for these compounds in water.
The average number of water molecules surrounding the amino group is ~ 6.6 for PABA and ~ 7 for PABA−. For both systems all atoms of the amino group form H-bonds with water molecules so that the total
Acknowledgments
This work was supported by funds from the European Union within its Seventh Framework Program (FP7-PEOPLE-2009-IRSES, Marie Curie Project) under grant agreement No. 247500.
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