3 research outputs found
Stacking and Branching in Self-Aggregation of Caffeine in Aqueous Solution: From the Supramolecular to Atomic Scale Clustering
The dynamical and structural properties
of caffeine solutions at
the solubility limit have been investigated as a function of temperature
by means of MD simulations, static and dynamic light scattering, and
small angle neutron scattering experiments. A clear picture unambiguously
supported by both experiment and simulation emerges: caffeine self-aggregation
promotes the formation of two distinct types of clusters: linear aggregates
of stacked molecules, formed by 2–14 caffeine molecules depending
on the thermodynamic conditions and disordered branched aggregates
with a size in the range 1000–3000 Å. While the first
type of association is well-known to occur under room temperature
conditions for both caffeine and other purine systems, such as nucleotides,
the presence of the supramolecular aggregates has not been reported
previously. MD simulations indicate that branched structures are formed
by caffeine molecules in a T-shaped arrangement. An increase of the
solubility limit (higher temperature but also higher concentration)
broadens the distribution of cluster sizes, promoting the formation
of stacked aggregates composed by a larger number of caffeine molecules.
Surprisingly, the effect on the branched aggregates is rather limited.
Their internal structure and size do not change considerably in the
range of solubility limits investigated
Molecular Dynamics and Neutron Scattering Studies of Mixed Solutions of Caffeine and Pyridine in Water
Insight into the
molecular interactions of homotactic and heterotactic
association of caffeine and pyridine in aqueous solution is given
on the basis of both experimental and simulation studies. Caffeine
is about 5 times more soluble in a 3 <i>m</i> aqueous pyridine
solution than it is in pure water (an increase from ∼0.1 <i>m</i> to 0.5 <i>m</i>). At this elevated concentration
the system becomes suitable for neutron scattering study. Caffeine–pyridine
interactions were studied by neutron scattering and molecular dynamics
simulations, allowing a detailed characterization of the spatial and
orientational structure of the solution. It was found that while pyridine–caffeine
interactions are not as strong as caffeine–caffeine interactions,
the pyridine–caffeine interactions still significantly disrupted
caffeine–caffeine stacking. The alteration of the caffeine–caffeine
stacking, occasioned by the presence of pyridine molecules in solution
and the consequent formation of heterotactic interactions, leads to
the experimentally detected increase in caffeine solubility
Comparison of Cellulose Iβ Simulations with Three Carbohydrate Force Fields
Molecular dynamics simulations of cellulose have recently
become
more prevalent due to increased interest in renewable energy applications,
and many atomistic and coarse-grained force fields exist that can
be applied to cellulose. However, to date no systematic comparison
between carbohydrate force fields has been conducted for this important
system. To that end, we present a molecular dynamics simulation study
of hydrated, 36-chain cellulose Iβ microfibrils at room temperature
with three carbohydrate force fields (CHARMM35, GLYCAM06, and Gromos
45a4) up to the near-microsecond time scale. Our results indicate
that each of these simulated microfibrils diverge from the cellulose
Iβ crystal structure to varying degrees under the conditions
tested. The CHARMM35 and GLYCAM06 force fields eventually result in
structures similar to those observed at 500 K with the same force
fields, which are consistent with the experimentally observed high-temperature
behavior of cellulose I. The third force field, Gromos 45a4, produces
behavior significantly different from experiment, from the other two
force fields, and from previously reported simulations with this force
field using shorter simulation times and constrained periodic boundary
conditions. For the GLYCAM06 force field, initial hydrogen-bond conformations
and choice of electrostatic scaling factors significantly affect the
rate of structural divergence. Our results suggest dramatically different
time scales for convergence of properties of interest, which is important
in the design of computational studies and comparisons to experimental
data. This study highlights that further experimental and theoretical
work is required to understand the structure of small diameter cellulose
microfibrils typical of plant cellulose