The mean hydration of carbohydrates as studied by normalized two- dimensional radial pair distributions Claus Andersson and Søren Balling Engelsen Food Technology, Department of Dairy and Food Science, The Royal Veterinary and Agricultural University, Frederiksberg, Denmark The hydration of carbohydrates plays a key role in many biological processes. Molecular dynamics simulations pro- vide an effective tool for investigating the hydration of complex solutes such as carbohydrates. In this article we devise an algorithm for the calculation of two-dimensional radial pair distributions describing the probability of find- ing a water molecule in a site defined by two reference atoms. The normalized 2D radial pair distribution is pro- posed as an effective tool for investigating and comparing localized or ordered water sites around flexible molecules such as carbohydrates when analyzing molecular dynamics simulations and the utility of 2D radial pair distributions is demonstrated using sucrose as an example. In this relatively simple structure, 2D radial pair distributions were able to characterize and quantify the importance of two unique interresidue hydration sites in which a water molecule is forming a bridge between the glycopyranosyl and fructo- furanosyl residues. The approach is proposed to be a valu- able tool for comparing and understanding the hydration of flexible biomolecules such as carbohydrates. © 2000 by Elsevier Science Inc. Keywords: pair distributions, hydration, molecular dynam- ics, sucrose INTRODUCTION The next frontier in the understanding of carbohydrate structure and functionality will most likely be focused around their hydration. In foods carbohydrates provide energy, sweet taste, and structure, and in all three aspects water is believed to play a decisive role. However, at the molecular level it is not known to what extent water contributes to energy metabolism and sweet taste perception as a structural element in addition to its carrier function. In carbohydrate-based gels, which are abun- dantly used in food products because of their excellent mouth- feel, chewing and flavor release properties, it is not known in detail how water functions as a structural element. In solid systems carbohydrates tend to cocrystallize with water and water is an important brick in building the structure of the starch granule. 1 To obtain a better understanding of the dy- namic and static interactions between carbohydrates and water, one feasible route is to perform molecular dynamics simula- tions with explicitly present water molecules. Molecular dynamics (and Monte Carlo) simulations of sol- utes in water provide a unique possibility for detailed exami- nation of water–solute interactions. However, when such sim- ulations are performed, the question arises: “How does one characterize water solvating a complex solute?” 2 The large number of water molecules, the complexity of the solute, and the high degree of mobility in such simulations require a statistical approach to describe the hydration. Radial pair dis- tributions, orientational pair distribution, and static water den- sities with the solute as reference are the most commonly statistical approaches employed to examine water–water and water–solute interactions. Radial pair distributions have the longest history and an experimental background. In a pure water system the radial oxygen– oxygen pair distribution functions, g(r Ow ,r Ow ), can be measured from X-ray scattering experiments, assuming that the X-ray intensities are dominated by spherical scattering centered at the oxygens. 3 Radial pair distributions of specific solute nuclei and the oxygen nuclei in water have been used in the literature to describe the large differences in the first and second hydration shells of different types of atoms, e.g., around carbon in methyl and methylene groups, which have a typical hydrophobic behavior, and around oxygen in hydroxyl groups, which have a typical hydrophilic behavior. 4 Radial pair distri- butions have also been used to scrutinize the detailed hydration pattern around solute atoms. 5–7 However, because of compli- Color Plates for this article are on pages 131–133. Corresponding author: Dr. Søren Balling Engelsen, Food Technology, Department of Dairy and Food Science, The Royal Veterinary and Agri- cultural University, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark. Tel.: +45 35 28 32 05; fax: +45 35 28 32 45. E-mail address: se@kvl.dk (Søren Balling Engelsen) Journal of Molecular Graphics and Modelling 17, 101–105, 1999 © 2000 by Elsevier Science Inc. 1093-3263/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S1093-3263(99)00022-4