Juan-Manuel García-Ruiz

Crystal growth studies in microgravity with the APCF. I. Computer simulation of transport dynamics

Abstract A computer program has been designed to simulate the pre-nucleation history of protein crystal growth experiments in the Advanced Protein Crystallization Facility developed by ESA to conduct crystallization experiments under microgravity conditions. The program was fed with the initial conditions of a set of experiments performed by the ESA Diagnostic Group during the SpaceHab-01 mission. The time histories of the salt and protein concentrations and supersaturation at any point inside the reactor were obtained. The differences between Dialysis and Free Interface Diffusion techniques are discussed. Both DIA and FID expriments performed in the short protein chamber of the APCF are basically very slow mixing batch experiments although differences exist in the equilibration behavior. The kinetics of protein nucleation coupled with transport phenomena are analyzed, showing that the supersaturation rate modifies the supersaturation value at which nucleation occurs as well as waiting times for nucleation. Reactor geometry and initial conditions can be optimized. As a general rule, higher protein concentrations will produce crystals that nucleate at lower supersaturation and supersaturation rates, provided that the growth chamber is long enough to implement a true diffusive set-up. When these conditions are fulfilled, the growth system becomes self regulated and crystals nucleate under different supersaturation and supersaturation rate conditions, performing a kind of automatic, single-reactor screening experiment.

Crystal growth studies in microgravity with the APCF. II. Image analysis studies

Abstract Lysozyme crystallization experiments performed in the Advanced Protein Crystallization Facility during SpaceHab-01 mission are analyzed using pictures obtained during the flight. The temporal evolution of experiments performed using free interface diffusion (FID) and dialysis (DIA) techniques was studied to gain a deeper insight into the processes of nucleation and growth. Heterogeneous nucleation on the walls of the reactor predominates over nucleation in the bulk solution. Nucleation takes place simultaneously throughout the protein chamber, which means that the supersaturation inside the reactors has reached homogeneity before nucleation. In terms of precipitation behavior this means that the APCF reactors are working as very slow-mixing batch experiments. Measured growth rates range from 10 to 50A/s and decrease asymptotically to zero. L (a linear dimension of the crystals) versus time 1/2 plots are discussed. Large-scale growth rate fluctuations are observed and explained by the interaction between diffusive fields of neighboring crystals. Crystals not attached to the reactor walls are observed to move, with varying velocities and directions of movement, and at a given time, undergo a fast acceleration that projects them towards the walls of the reactor. The amount of information that can be derived from the final data set supplied after the space experiment is limited by the technical features implemented in the APCF. Some recommendations to enhance these features are proposed.

Visualization of the impurity depletion zone surrounding apoferritin crystals growing in gel with holoferritin dimer impurity

Colorless apoferritin (MD450 kDa) crystals were grown in the presence of redcoloredholoferritin d (MD900 kDa) within agarose gel by counterdiffusion with Cd 2+ as precipitant. In agreement with our previous quantitative measurements showing preferential trapping of dimers (segregation coefficient K ¼ 4), the apoferritin crystals became darkly colored while the gel solution around them was nearly colorless. This segregation coefficient and the absence of convective mixing resultedin a large colorless area surround ing the crystals which allowedus to demonstrate the existence of the impurity depletion zone by direct visual observation. r 2001 Elsevier Science B.V. All rights reserved.

Morphological behavior of inorganic precipitation systems

Inorganic precipitation processes are capable of producing a wide range of morphological outputs. This range includes shapes with both crystallographic and non-crystallographic symmetry elements. Among the latter, morphologies that mimic primitive living organisms are easily obtained under different physico-chemical conditions including those that are geochemically plausible. The application of this information to the problem of deciphering primitive life on the early Earth and Mars is discussed. It is concluded that morphology cannot be used unambiguously as a tool for primitive life detection.

Precipitation and Crystallization Kinetics in Silica Gardens

Abstract Silica gardens are extraordinary plant‐like structures resulting from the complex interplay of relatively simple inorganic components. Recent work has highlighted that macroscopic self‐assembly is accompanied by the spontaneous formation of considerable chemical gradients, which induce a cascade of coupled dissolution, diffusion, and precipitation processes occurring over timescales as long as several days. In the present study, this dynamic behavior was investigated for silica gardens based on iron and cobalt chloride by means of two synchrotron‐based techniques, which allow the determination of concentration profiles and time‐resolved monitoring of diffraction patterns, thus giving direct insight into the progress of dissolution and crystallization phenomena in the system. On the basis of the collected data, a kinetic model is proposed to describe the relevant reactions on a fundamental physicochemical level. The results show that the choice of the metal cations (as well as their counterions) is crucial for the development of silica gardens in both the short and long term (i.e. during tube formation and upon subsequent slow equilibration), and provide important clues for understanding the properties of related structures in geochemical and industrial environments.

A crystallographic study of crystalline casts and pseudomorphs from the 3.5 Ga dresser formation, Pilbara Craton (Australia)

Crystallographic methods are used to identify the primary mineral phase of pseudomorphs of crystals embedded in 3.48 Ga bedded carbonate-chert rocks from the Dresser Formation, Pilbara Craton, Australia. This identification provides valuable information on the chemical environments at the onset of life on Earth.