Dendrimer-Guest Interactions: Challenging Conventional Wisdom

Solutions to many future challenges - including water purification, drug delivery, and energy storage - will require innovative new materials. Dendrimers are a class of materials with wide-ranging applications whose behavior is not fully understood. In many potential applications, dendrimers interact with small molecules. Our work focuses on describing the fundamental mechanisms governing the interactions between dendrimers and hydrocarbons using molecular modeling and computer simulations. A common view of dendrimer host-guest interactions is that the guest molecules are encapsulated in protected interior voids within the dendrimer structure. Our results present an alternative picture and show that the association of a model aromatic hydrocarbon, naphthalene (NPH), involves temporary pockets formed by the dendrimer branches and interactions between the NPH molecules themselves

Fluctuations of water near extended hydrophobic and hydrophilic surfaces

We use molecular dynamics simulations of the SPC-E model of liquid water to derive probability distributions for water density fluctuations in probe volumes of different shapes and sizes, both in the bulk as well as near hydrophobic and hydrophilic surfaces. To obtain our results, we introduce a biased sampling of coarse-grained densities, which in turn biases the actual solvent density. The technique is easily combined with molecular dynamics integration algorithms. Our principal result is that the probability for density fluctuations of water near a hydrophobic surface, with or without surface-water attractions, is akin to density fluctuations at the water-vapor interface. Specifically, the probability of density depletion near the surface is significantly larger than that in bulk. In contrast, we find that the statistics of water density fluctuations near a model hydrophilic surface are similar to that in the bulk

Sitting at the edge: How biomolecules use hydrophobicity to tune their interactions and function

Water near hydrophobic surfaces is like that at a liquid-vapor interface, where fluctuations in water density are substantially enhanced compared to that in bulk water. Here we use molecular simulations with specialized sampling techniques to show that water density fluctuations are similarly enhanced, even near hydrophobic surfaces of complex biomolecules, situating them at the edge of a dewetting transition. Consequently, water near these surfaces is sensitive to subtle changes in surface conformation, topology, and chemistry, any of which can tip the balance towards or away from the wet state, and thus significantly alter biomolecular interactions and function. Our work also resolves the long-standing puzzle of why some biological surfaces dewet and other seemingly similar surfaces do not.Comment: 12 pages, 4 figure

Seasonal distribution of chlorophyll-<i>α</i> in the Exclusive Economic Zone (EEZ) of India

292-297Seasonal distribution of chlorophyll-α (chl-α) in the different sectors of the EEZ of India was studied based on data from 430 stations over the period from 1962 to 1988. The annual average chl-α for the entire euphotic zone of EEZ was 12.0 mg m-2 whereas the seasonal average was calculated to be 14.2, 12.8 and 7.4 mg m-2 during premonsoon (February-May), SW monsoon (June-September) and postmonsoon (October-January) respectively. The maximum average (18.0 mg m-2) was noticed in the Arabian Sea during premonsoon followed by the Bay of Bengal sector during SW monsoon (14.8 mg m-2) and the Lakshadweep Sea during premonsoon (14.5 mg m-2). The range of chl-α (0.2 to 100.6 mg m-2) was found maximum during premonsoon followed by SW monsoon in the euphotic zone of EEZ of India

Temperature error in digital bathythermograph data

234-236Simultaneous Digital Bathythermograph (DBT) and Nansen Cast data collected during two cruises of R.V. Gaveshani (GV-117 and GV-118) and archived in Indian Oceanographic Data Centre (IODC) are used to determine existing temperature errors in DBT. The resulting mean error for DBT data from the GV-117 cruise varies from -0.5 to - 1 oC, while it varied between -0.3 and -0.6 oC for data from cruise GV-118. For both the data sets, the error shows consistently negative bias from surface to 800 m depth, however there is no apparent or measurable systematic dependence of the error on depth. Considering the given temperature accuracy of 0.05 oC, the observed DBT error, varying from -0.3 to -1 oC, is significant and such offsets should be removed from DBT archives. It is found that a corrective measure of +0.5 oC, equivalent to the mean surface offset obtained from two cruises, can considerably reduce the temperature error at all DBT depths

PAMAM Dendrimers and Graphene: Materials for Removing Aromatic Contaminants from Water

We present results from experiments and atomistic molecular dynamics simulations on the remediation of naphthalene by polyamidoamine (PAMAM) dendrimers and graphene oxide (GrO). Specifically, we investigate 3^rd–6^th generation (G3-G6) PAMAM dendrimers and GrO with different levels of oxidation. The work is motivated by the potential applications of these emerging nanomaterials in removing polycyclic aromatic hydrocarbon contaminants from water. Our experimental results indicate that GrO outperforms dendrimers in removing naphthalene from water. Molecular dynamics simulations suggest that the prominent factors driving naphthalene association to these seemingly disparate materials are similar. Interestingly, we find that cooperative interactions between the naphthalene molecules play a significant role in enhancing their association to the dendrimers and GrO. Our findings highlight that while selection of appropriate materials is important, the interactions between the contaminants themselves can also be important in governing the effectiveness of a given material. The combined use of experiments and molecular dynamics simulations allows us to comment on the possible factors resulting in better performance of GrO in removing polyaromatic contaminants from water