Expansion and differentiation of ex vivo cultured erythroblasts in scalable stirred bioreactors

Transfusion of donor-derived red blood cells (RBCs) is the most common form of cell therapy. Production of transfusion-ready cultured RBCs (cRBCs) is a promising replacement for the current, fully donor-dependent therapy. A single transfusion unit, however, contains 2 × 10¹² RBC, which requires large scale production. Here, we report on the scale-up of cRBC production from static cultures of erythroblasts to 3 L stirred tank bioreactors, and identify the effect of operating conditions on the efficiency of the process. Oxygen requirement of proliferating erythroblasts (0.55-2.01 pg/cell/h) required sparging of air to maintain the dissolved oxygen concentration at the tested setpoint (2.88 mg O₂ /L). Erythroblasts could be cultured at dissolved oxygen concentrations as low as 0.7 O₂ mg/ml without negative impact on proliferation, viability or differentiation dynamics. Stirring speeds of up to 600 rpm supported erythroblast proliferation, while 1800 rpm led to a transient halt in growth and accelerated differentiation followed by a recovery after 5 days of culture. Erythroblasts differentiated in bioreactors, with final enucleation levels and hemoglobin content similar to parallel cultures under static conditions.

Integrated Vacuum Stripping and Adsorption for the Efficient Recovery of (Biobased) 2-Butanol

Biobased 2-butanol offers high potential as biofuel, but its toxicity toward microbial hosts calls for efficient techniques to alleviate product inhibition in fermentation processes. Aiming at the selective recovery of 2-butanol, the feasibility of a process combining in situ vacuum stripping followed by vapor adsorption has been assessed using mimicked fermentation media. The experimental vacuum stripping of model solutions and corn stover hydrolysate closely aligned with mass transfer model predictions. However, the presence of lignocellulosic impurities affected 2-butanol recovery yields resulting from vapor condensation, which decreased from 96 wt % in model solutions to 40 wt % using hydrolysate. For the selective recovery of 2-butanol from a vapor mixture enriched in water and carbon dioxide, silicalite materials were the most efficient, particularly at low alcohol partial pressures. Integrating in situ vacuum stripping with vapor adsorption using HiSiv3000 proved useful to effectively concentrate 2-butanol above its azeotropic composition (>68 wt %), facilitating further product purification.

Perspectives for the microbial production of methyl propionate integrated with product recovery

A new approach was studied for bio-based production of methyl propionate, a precursor of methyl methacrylate. Recombinant E. coli cells were used to perform a cascade reaction in which 2-butanol is reduced to butanone using alcohol dehydrogenase, and butanone is oxidized to methyl propionate and ethyl acetate using a Baeyer-Villiger monooxygenase (BVMO). Product was removed by in situ stripping. The conversion was in line with a model comprising product formation and stripping kinetics. The maximum conversion rates were 1.14 g-butanone/(L h), 0.11 g-ethyl acetate/(L h), and 0.09 g-methyl propionate/(L h). The enzyme regioselectivity towards methyl propionate was 43% of total ester. Starting from biomass-based production of 2-butanol, full-scale ester production with conventional product purification was calculated to be competitive with petrochemical production if the monooxygenase activity and regioselectivity are enhanced, and the costs of bio-based 2-butanol are minimized.

Esters production via carboxylates from anaerobic paper mill wastewater treatment

This paper describes a new option for integrated recovery and esterification of carboxylates produced by anaerobic digestion at a pH above the pK_a. The carboxylates (acetate, propionate, butyrate, valerate and lactate) are recovered using a strong anion exchange resin in the bicarbonate form, and the resin is regenerated using a CO₂-expanded alcohol technique, which allows for low chemicals consumption and direct esterification. Paper mill wastewater was used to study the effect of pH and the presence of other inorganic anions and cations on the adsorption and desorption with CO₂-expanded methanol. Calcium, which is present in paper mill wastewater, can cause precipitation problems, especially at high pH. Esters yields ranged from 1.08±0.04mol methyl acetate/mol of acetate_in to 0.57±0.02mol methyl valerate/mol of valerate_in.

Prediction of protein retention times in hydrophobic interaction chromatography by robust statistical characterization of their atomic-level surface properties

The correlation between the dimensionless retention times (DRT) of proteins in hydrophobic interaction chromatography (HIC) and their surface properties were investigated. A ternary atomic-level hydrophobicity scale was used to calculate the distribution of local average hydrophobicity across the proteins surfaces. These distributions were characterized by robust descriptive statistics to reduce their sensitivity to small changes in the three-dimensional structure. The applicability of these statistics for the prediction of protein retention behaviour was looked into. A linear combination of robust statistics describing the central tendency, heterogeneity and frequency of highly hydrophobic clusters was found to have a good predictive capability (R2  = 0.78), when combined a factor to account for protein size differences. The achieved error of prediction was 35% lower than for a similar model based on a description of the protein surface on an amino acid level. This indicates that a robust and mathematically simple model based on an atomic description of the protein surface can be used for the prediction of the retention behaviour of conformationally stable globular proteins with a well determined 3D structure in HIC. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:372-381, 2016.

Ester production from bio-based dicarboxylates via direct downstream catalysis: succinate and 2,5-furandicarboxylate dimethyl esters

Two culture broths, one containing succinate produced de novo by C. glutamicum and the other containing 2,5-furandicarboxylate produced by whole cell biotransformation of 5-HMF by a recombinant P. putida, were used for dimethyl ester production.

Pervaporation of ethanol from lignocellulosic fermentation broth

Pervaporation can be applied in ethanol production from lignocellulosic biomass. Hydrophobic pervaporation, using a commercial PDMS membrane, was employed to concentrate the ethanol produced by fermentation of lignocellulosic hydrolysate. To our knowledge, this is the first report describing this. Pervaporation carried out with three different lignocellulosic fermentation broths reduced the membrane performance by 17-20% as compared to a base case containing only 3 wt.% ethanol in water. The membrane fouling caused by these fermentation broths was irreversible. Solutions containing model lignocellulosic components were tested during pervaporation at the same conditions. A total flux decrease of 12-15%, as compared to the base case, was observed for each component except for furfural. Catechol was found to be most fouling component whereas furfural permeated through the membrane and increased the total flux. The membrane selectivity increased in the presence of fermentation broth but remained unchanged for all selected components.

Simultaneous Clostridial fermentation, lipase-catalyzed esterification, and ester extraction to enrich diesel with butyl butyrate

The recovery of 1-butanol from fermentation broth is energy-intensive since typical concentrations in fermentation broth are below 20 g L(-1). To prevent butanol inhibition and high downstream processing costs, we aimed at producing butyl esters instead of 1-butanol. It is shown that it is possible to perform simultaneously clostridial fermentation, esterification of the formed butanol to butyl butyrate, and extraction of this ester by hexadecane. The very high partition coefficient of butyl butyrate pulls the esterification towards the product side even at fermentation pH and relatively low butanol concentrations. The hexadecane extractant is a model diesel compound and is nontoxic to the cells. If butyl butyrate enriched diesel can directly be used as car fuel, no product recovery is required. A proof-of-principle experiment for the one-pot bio-ester production from glucose led to 5 g L(-1) butyl butyrate in the hexadecane phase. The principle may be extended to a wide range of esters, especially to longer chain ones.

High-throughput protein precipitation and hydrophobic interaction chromatography: salt effects and thermodynamic interrelation

Salt-induced protein precipitation and hydrophobic interaction chromatography (HIC) are two widely used methods for protein purification. In this study, salt effects in protein precipitation and HIC were investigated for a broad combination of proteins, salts and HIC resins. Interrelation between the critical thermodynamic salting out parameters in both techniques was equally investigated. Protein precipitation data were obtained by a high-throughput technique employing 96-well microtitre plates and robotic liquid handling technology. For the same protein-salt combinations, isocratic HIC experiments were performed using two or three different commercially available stationary phases-Phenyl Sepharose low sub, Butyl Sepharose and Resource Phenyl. In general, similar salt effects and deviations from the lyotropic series were observed in both separation methods, for example, the reverse Hofmeister effect reported for lysozyme below its isoelectric point and at low salt concentrations. The salting out constant could be expressed in terms of the preferential interaction parameter in protein precipitation, showing that the former is, in effect, the net result of preferential interaction of a protein with water molecules and salt ions in its vicinity. However, no general quantitative interrelation was found between salting out parameters or the number of released water molecules in protein precipitation and HIC. In other words, protein solubility and HIC retention factor could not be quantitatively interrelated, although for some proteins, regular trends were observed across the different resins and salt types.

Techno-economic evaluation of an inclusion body solubilization and recombinant protein refolding process

Expression of recombinant proteins in Escherichia coli is normally accompanied by the formation of inclusion bodies (IBs). To obtain the protein product in an active (native) soluble form, the IBs must be first solubilized, and thereafter, the soluble, often denatured and reduced protein must be refolded. Several technically feasible alternatives to conduct IBs solubilization and on-column refolding have been proposed in recent years. However, rarely these on-column refolding alternatives have been evaluated from an economical point of view, questioning the feasibility of their implementation at a preparative scale. The presented study assesses the economic performance of four distinct process alternatives that include pH induced IBs solubilization and protein refolding (pH_IndSR); IBs solubilization using urea, dithiothreitol (DTT), and alkaline pH followed by batch size-exclusion protein refolding; inclusion bodies (IBs) solubilization using urea, DTT, and alkaline pH followed by simulated moving bed (SMB) size-exclusion protein refolding, and IBs solubilization using urea, DTT and alkaline pH followed by batch dilution protein refolding. The economic performance was judged on the basis of the direct fixed capital, and the production cost per unit of product (P(C)). This work shows that (1) pH_IndSR system is a relatively economical process, because of the low IBs solubilization cost; (2) substituting β-mercaptoethanol for dithiothreithol is an attractive alternative, as it significantly decreases the product cost contribution from the IBs solubilization; and (3) protein refolding by size-exclusion chromatography becomes economically attractive by changing the mode of operation of the chromatographic reactor from batch to continuous using SMB technology.

Development of a low pH fermentation strategy for fumaric acid production by Rhizopus oryzae

Dicarboxylic acids that are produced from renewable resources are becoming attractive building blocks for the polymers industry. In this respect, fumaric acid is very interesting. Its low aqueous solubility facilitates product recovery. To avoid excessive waste salt production during downstream processing, a low pH for fumaric acid fermentation will be beneficial. Studying the influence of pH, working volume and shaking frequency on cell cultivation helped us to identify the best conditions to obtain appropriate pellet morphologies of a wild type strain of Rhizopus oryzae. Using these pellets, the effects of pH and CO(2) addition were studied to determine the best conditions to produce fumaric acid in batch fermentations under nitrogen-limited conditions with glucose as carbon source. Decreasing either the fermentation pH below 5 or increasing the CO(2) content of the inlet air above 10% was unfavourable for the cell-specific productivity, fumaric acid yield, and fumaric acid titer. However, switching off the pH control late in the batch phase did not affect these performance parameters and allowed achieving pH of 3.6. A concentration of 20 gL(-1) of fumaric acid was obtained at pH 3.6 while the average cell mass specific productivity and fumaric acid yield were the same as at pH 5.0. Consequently, relatively modest amounts of inorganic base were required for pH control, while recovery of the acid should be relatively easy at pH 3.6.

Aerobic batch cultivation in micro bioreactor with integrated electrochemical sensor array

Aerobic batch cultivations of Candida utilis were carried out in two micro bioreactors with a working volume of 100 muL operated in parallel. The dimensions of the micro bioreactors were similar as the wells in a 96-well microtiter plate, to preserve compatibility with the current high-throughput cultivation systems. Each micro bioreactor was equipped with an electrochemical sensor array for the online measurement of temperature, pH, dissolved oxygen, and viable biomass concentration. Furthermore, the CO(2) production rate was obtained from the online measurement of cumulative CO(2) production during the cultivation. The online data obtained by the sensor array and the CO(2) production measurements appeared to be very reproducible for all batch cultivations performed and were highly comparable to measurement results obtained during a similar aerobic batch cultivation carried out in a conventional 4L bench-scale bioreactor. Although the sensor chip certainly needs further improvement on some points, this work clearly shows the applicability of electrochemical sensor arrays for the monitoring of parallel micro-scale fermentations, e.g. using the 96-well microtiterplate format.

Quantitative determination of glucose transfer between concurrent laminar water streams in a H-shaped microchannel

To explore the applicability of a laminar fluid diffusion interface (LFDI) for the controlled feeding of microbioreactors, glucose diffusion experiments were carried out in a rounded H-shaped microstructure etched in a glass substrate. The diffusion channel of the microstructure had a length of 4 mm and a depth of 50 microm with a trapezoidal cross section with a width of 100 microm at the bottom and 200 microm at the surface of the channel. The microchannel was operated at residence times of less than 1 s ensuring high-mass-transfer rates. It was confirmed, both by microscopic observations as well as computational fluid dynamics (CFD) studies that the flow characteristics in the microchannel were fully laminar. Special attention was paid to flow splitting at the end of the channel, because the CFD simulations indicated that the performance of the device was sensitive to unequal flow splitting. The difference in outflow volume of the two streams was measured to be small (1.25% +/- 0.6%). The measured glucose concentration in both exit ports at a fixed residence time was found to be stable in time and reproducible in multiple experiments. CFD simulation was shown to be a powerful tool for estimating the mass transfer in the LFDI, even at very short residence times. The results obtained in this work show the applicability of LFDI for the controlled diffusive supply of a solute to a water stream, with as possible application substrate and/or precursor feeding to microreactors.

Protein self-interaction chromatography on a microchip

This paper presents the development of a novel miniaturized experimental procedure for the measurement of protein-protein interactions through Self-Interaction Chromatography (SIC) on a microchip, without the use of chromatographic resins. SIC was recently demonstrated to be a relatively easy method to obtain quantitative thermodynamic information about protein-protein interactions, like the osmotic second virial coefficient B(22), which relates to protein phase behavior including protein crystallization. This successful miniaturization to microchip level of a measurement device for protein self-interaction data is a first key step to a complete microfluidic screening platform for the rational design of protein crystallizations, using substantially less expensive protein and experimentation time.

Lab-scale fermentation tests of microchip with integrated electrochemical sensors for pH, temperature, dissolved oxygen and viable biomass concentration

This article shows the development and testing of a microchip with integrated electrochemical sensors for measurement of pH, temperature, dissolved oxygen and viable biomass concentration under yeast cultivation conditions. Measurements were done both under dynamic batch conditions as well as under prolonged continuous cultivation conditions. The response of the sensors compared well with conventional measurement techniques. The biomass sensor was based on impedance spectroscopy. The results of the biomass sensor matched very well with dry weight measurements and showed a limit of detection of approximately 1 g/L. The dissolved oxygen concentration was monitored amperometrically using an ultra-microelectrode array, which showed an accuracy of approximately 0.2 mg/L and negligible drift. pH was monitored using an ISFET with an accuracy well below 0.1 pH unit. The platinum thin-film temperature resistor followed temperature changes with approximately 0.1 degrees C accuracy. The dimensions of the multi sensor chip are chosen as such that it is compatible with the 96-well plate format.

Fumaric acid production by fermentation

The potential of fumaric acid as a raw material in the polymer industry and the increment of cost of petroleum-based fumaric acid raises interest in fermentation processes for production of this compound from renewable resources. Although the chemical process yields 112% w/w fumaric acid from maleic anhydride and the fermentation process yields only 85% w/w from glucose, the latter raw material is three times cheaper. Besides, the fermentation fixes CO₂. Production of fumaric acid by Rhizopus species and the involved metabolic pathways are reviewed. Submerged fermentation systems coupled with product recovery techniques seem to have achieved economically attractive yields and productivities. Future prospects for improvement of fumaric acid production include metabolic engineering approaches to achieve low pH fermentations.

pH-gradient ion-exchange chromatography: an analytical tool for design and optimization of protein separations

This work demonstrates that a highly linear, controllable and wide-ranged pH-gradient can be generated through an ion-exchange chromatography (IEC) column. Such a pH-gradient anion-exchange chromatography was evaluated with 17 model proteins and found that acidic (pI<6) and basic (pI>8) proteins elute roughly at their pI, whereas neutral proteins (pI 6-8) elute at pH 8-9 regardless their pI values. Because of the flat nature of protein titration curves from pH approximately 6 to approximately 9, neutral proteins indeed exhibit nearly zero net charge at pH approximately 9. The elution-pH in pH-gradient IEC or the titration curve, but not the pI, was identified as the key parameter for pH optimization of preparative IEC in a fast and rational way. The pH-gradient IEC was also applied and found to be an excellent analytical tool for the fractionation of crude protein mixtures.

Phase behavior of an intact monoclonal antibody

Understanding protein phase behavior is important for purification, storage, and stable formulation of protein drugs in the biopharmaceutical industry. Glycoproteins, such as monoclonal antibodies (MAbs) are the most abundant biopharmaceuticals and probably the most difficult to crystallize among water-soluble proteins. This study explores the possibility of correlating osmotic second virial coefficient (B(22)) with the phase behavior of an intact MAb, which has so far proved impossible to crystallize. The phase diagram of the MAb is presented as a function of the concentration of different classes of precipitants, i.e., NaCl, (NH4)2SO4, and polyethylene glycol. All these precipitants show a similar behavior of decreasing solubility with increasing precipitant concentration. B(22) values were also measured as a function of the concentration of the different precipitants by self-interaction chromatography and correlated with the phase diagrams. Correlating phase diagrams with B(22) data provides useful information not only for a fundamental understanding of the phase behavior of MAbs, but also for understanding the reason why certain proteins are extremely difficult to crystallize. The scaling of the phase diagram in B(22) units also supports the existence of a universal phase diagram of a complex glycoprotein when it is recast in a protein interaction parameter.

In situ product recovery (ISPR) by crystallization: basic principles, design, and potential applications in whole-cell biocatalysis

The removal of inhibiting or degrading product from a bioreactor as soon as the product is formed is an important issue in industrial bioprocess development. In this review, the potential of crystallization as an in situ product removal (ISPR) technique for the biocatalytic production of crystalline compounds is discussed. The emphasis of this review is on the current status of crystalline product formation by metabolically active cells for application in fine-chemicals production. Examples of relevant biocatalytic conversions are summarized, and some basic process options are discussed. Furthermore, a case study is presented in which two conceptual process designs are compared. In one process, product formation and crystallization are integrated by applying ISPR, whereas a second, nonintegrated process is based on a known conventional process equivalent for the production of 6R-dihydro-oxoisophorone. The comparison indicates that employing ISPR leads to significant advantages over the nonintegrated case in terms of increased productivity and yield with a corresponding decrease in the number of downstream processing steps, as well as in the quantity of waste streams. This leads to an economically more interesting process alternative. Finally, a general outlook on the various research aspects of ISPR by crystallization is given.

Investigation and characterization of liquid two-phase systems for the separation of crystal mixtures by interfacial partitioning

The interfacial partitioning behavior of ampicillin and phenylglycine crystals in different two-phase systems has been investigated. The two-phase systems employed are water/dodecane, water/1-butanol, and water/pentane/methanol. By means of partition experiments and microscopic imaging, it has been shown that the mechanism of separation strongly depends on the choice of the two-phase system. While water/dodecane features a mechanism of sheer competitive adsorption at the interface, separation in water/1-butanol is mainly due to partitioning into both liquid phases, leading to a higher degree of separation. Experiments with water/pentane/methanol have illustrated the large potential of three-component systems, as slight variations in the composition can have large effects on the separation.

Design of self-interaction chromatography as an analytical tool for predicting protein phase behavior

Solution conditions under which proteins have a tendency to crystallize correspond to a slightly negative osmotic second virial coefficient (B22). A positive B22 value guarantees no crystallization to occur. On the other hand, a B22 value within the so called "crystallization slot" thermodynamically supports the crystallization processes but does not guarantee successful crystal growth. It is, however, a prerequisite for protein crystallization that the B22 value must be in the slightly negative regime. Self-interaction chromatography (SIC) is designed in this work as an analytical tool for determining B22 in a precise and reproducible way. The methodology was demonstrated in detail in terms of its theoretical basis, experimental methodology, troubleshooting and data analysis for different protein samples and solution conditions. The inherent error limit of SIC is found to be comparatively less than other B22 measurement techniques. The designed experimental approach was applied for mapping crystallization conditions of a model protein, i.e. lysozyme. Good agreement between the obtained lysozyme B22 values and literature values confirms the accuracy of the approach.

Novel method for the production of pure glycinin from soybeans

A novel method for the purification of glycinin from soy meal is presented. The method is based on the isoelectric precipitation of glycinin by using carbon dioxide as a volatile precipitant. Gaseous CO(2) was pressurized into the protein solution, thus lowering the pH and initiating glycinin precipitation. Pressurization and, consequently, acidification were done in a slow and controlled manner, with the end point of pH 6.4. The acidity of the protein solution was well controlled via the pressure of gaseous CO(2). In this way simultaneous precipitation of other soybean proteins was prevented and very pure glycinin was obtained. Approximately 40% of the glycinin present in the protein solution was recovered with purity as high as 98%. The purification process was successfully performed on both small and large scales, without affecting glycinin purity.

Quantification of Binary Diffusion in Protein Crystals

The use of confocal laser scanning microscopy for visualization and quantification of binary diffusion within anisotropic porous material is described here for the first time. The dynamics of adsorption profiles of dianionic fluorescein, zwitterionic rhodamine B, and their mixture in the cationic native orthorhombic lysozyme crystal were subsequently analyzed. All data could be described by a classical pore diffusion model. There was no change in the adsorption characteristics, but diffusion decreased with the introduction of a second solute in the solution. It was found that diffusion is determined by the combination of steric and electrostatic interactions,while adsorption is dependent on electrostatic and hydrophobic interactions. Thus, it was established that the outcome of binary transport depends on the solute, protein, and crystal characteristics.

Feasibility of acrylic acid production by fermentation

Acrylic acid might become an important target for fermentative production from sugars on bulk industrial scale, as an alternative to its current production from petrochemicals. Metabolic engineering approaches will be required to develop a host microorganism that may enable such a fermentation process. Hypothetical metabolic pathways for insertion into a host organism are discussed. The pathway should have plausible mass and redox balances, plausible biochemistry, and plausible energetics, while giving the theoretically maximum yield of acrylate on glucose without the use of aeration or added electron acceptors. Candidate metabolic pathways that might lead to the theoretically maximum yield proceed via beta-alanine, methylcitrate, or methylmalonate-CoA. The energetics and enzymology of these pathways, including product excretion, should be studied in more detail to confirm this. Expression of the selected pathway in a host organism will require extensive genetic engineering. A 100,000-tons/year fermentation process for acrylic acid production, including product recovery, was conceptually designed based on the supposition that an efficient host organism for acrylic acid production can indeed be developed. The designed process is economically competitive when compared to the current petrochemical process for acrylic acid. Although the designed process is highly speculative, it provides a clear incentive for development of the required microbial host, especially considering the environmental sustainability of the designed process.

Adsorption of xanthene dyes by lysozyme crystals

Adsorption characteristics of cross-linked lysozyme crystals of different morphologies (tetragonal, orthorhombic, triclinic and monoclinic) were examined using four anionic dyes (fluorescein, eosin, erythrosin, and rose bengal), one zwitterionic dye (rhodamine B), and one cationic dye (rhodamine 6G). The adsorption isotherms were of the Langmuir type for all examined systems with the exception of rhodamine B adsorption by monoclinic crystals. The weakest adsorption was observed for the cationic dye, rhodamine B, whereas dianionic dyes, eosin, rose bengal, and erythrosin were strongly adsorbed on the protein surface. The adsorption capacities of the crystals for the dyes were found to depend on both charge and hydrophobicity of the dye, reflecting the heterogeneous character of the lysozyme pore surface. The adsorption affinity of the crystals for the dyes was a function of the dyes' hydrophobicity. Furthermore, the crystal morphology was identified as an additional factor determining capacity and affinity for dye adsorption. Differences between crystals prepared in the presence of the same precipitant were lower than between morphologies prepared with different precipitants.

Relation between Pore Sizes of Protein Crystals and Anisotropic Solute Diffusivities

The diffusion of a solute, fluorescein, into lysozyme protein crystals with different pore structures was investigated. To determine the diffusion coefficients, three-dimensional solute concentration fields acquired by confocal laser scanning microscopy (CLSM) during diffusion into the crystals were compared with the output of a time-dependent 3-D diffusion model. The diffusion process was found to be anisotropic, and the degree of anisotropy increased in the order: triclinic, tetragonal and orthorhombic crystal morphology. A linear correlation between the pore diffusion coefficients and the pore sizes was established. The maximum size of the solute, deduced from the established correlation of diffusion coefficients and pore size, was 0.73 +/- 0.06 nm, which was in the range of the average diameter of fluorescein (0.69 +/- 0.02 nm). This proves that size exclusion is the key mechanism for solute diffusion in protein crystals. Hence, the origin of solute diffusion anisotropy can be found in the packing of the protein molecules in the crystals, which determines the crystal pore organization.