A Labile Metallo-Porphyrin as Tool to Control J-Aggregates Chiroptical Properties

The zinc(II) metal derivative of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) is quite labile and readily demetallates under acidic conditions, affording the parent diacid porphyrin in a monomeric form. The rate of this process is first order on [ZnTPPS4] and second order on [H+], allowing a precise control of the monomer release in solution. Under high ionic strength, this latter species is able to self-assemble into J-aggregates, whose kinetics of growth are largely modulated by pH. The aggregation kinetics have been treated according to a well-established model, in which the formation of an initial nucleus is the rate determining step preceding the autocatalytic growth of the whole assembly. The extinction spectra of the aggregates suggest the occurrence of a dipolar coupling mechanism very similar to that operating in metal nanoparticles. Spontaneous symmetry breaking takes place in these aggregates as evidenced by unusual circular dichroism spectra. The intensity and sign of the effect is controlled by the aggregation rate and therefore can be tuned through a proper choice of initial conditions.

Size regulation of multiple organelles competing for a shared subunit pool

How cells regulate the size of intracellular structures and organelles, despite continuous turnover in their component parts, is a longstanding question. Recent experiments suggest that size control of many intracellular assemblies is achieved through the depletion of a limiting subunit pool in the cytoplasm. While the limiting pool model ensures organelle size scaling with cell size, it does not provide a mechanism for robust size control of multiple co-existing structures. Here we propose a kinetic theory for size regulation of multiple structures that are assembled from a shared pool of subunits. We demonstrate that a negative feedback between the growth rate and the size of individual structures underlies size regulation of a wide variety of intracellular assemblies, from cytoskeletal filaments to three-dimensional organelles such as centrosomes and the nucleus. We identify the feedback motifs for size control in these structures, based on known molecular interactions, and quantitatively compare our theory with available experimental data. Furthermore, we show that a positive feedback between structure size and growth rate can lead to bistable size distributions arising from autocatalytic growth. In the limit of high subunit concentration, autocatalytic growth of multiple structures leads to stochastic selection of a single structure, elucidating a mechanism for polarity establishment.

Ligand Mediated Nanocluster Formation with Classical and Autocatalytic Growth

We describe a kinetic model of ligand-mediated nanocluster (diameter < 2 nm) formation. We investigate two growth pathways: classical single-monomer addition, and autocatalytic growth. We compare our results to some relevant experiments and we discuss and generally describe the optimal conditions for "bottom up" nanocluster synthesis.<br>

Ligand Mediated Nanocluster Formation with Classical and Autocatalytic Growth

We describe a kinetic model of ligand-mediated nanocluster (diameter < 2 nm) formation. We investigate two growth pathways: classical single-monomer addition, and autocatalytic growth. We compare our results to some relevant experiments and we discuss and generally describe the optimal conditions for "bottom up" nanocluster synthesis.<br>

Ligand Mediated Nanocluster Formation with Classical and Autocatalytic Growth

We describe a kinetic model of ligand-mediated nanocluster (diameter < 2 nm) formation. We investigate two growth pathways: classical single-monomer addition, and autocatalytic growth. We compare our results to some relevant experiments and we discuss and generally describe the optimal conditions for "bottom up" nanocluster synthesis.<br>