Mechanical Molecular Dynamics Research Articles

Quantitative Analysis of QM/MM Boundary Artifacts and Correction in Adaptive QM/MM Simulations.

A quantum chemical treatment of solvation effects using the standard quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations is challenging due to QM and MM solvent exchange near the QM solute. To this end, various adaptive QM/MM methods have been proposed; free solvent exchanges are allowed via flexible switching of their identities between QM and MM systems depending on their distances from the QM solute. However, temporal and spatial discontinuities remain in the standard implementations of adaptive QM/MM approaches and continue to hamper stable and accurate dynamics simulations. We previously demonstrated that the size-consistent multipartitioning (SCMP) method achieves temporal continuity while, to some extent, avoiding spatial discontinuities. In the present study, we demonstrate that residual spatial discontinuities may lead to severe artifacts under certain conditions. Through quantitative analyses, we show that all multiscale including QM/MM methods might experience these problems, which so far have not been investigated in depth. To alleviate these artifacts, we propose a correction scheme in the framework of the SCMP approach and demonstrate its effectiveness using bulk water simulations.

QM/MM Benchmarking of Cyanobacteriochrome Slr1393g3 Absorption Spectra.

Cyanobacteriochromes are compact and spectrally diverse photoreceptor proteins that are promising candidates for biotechnological applications. Computational studies can contribute to an understanding at a molecular level of their wide spectral tuning and diversity. In this contribution, we benchmark methods to model a 110 nm shift in the UV/Vis absorption spectrum from a red- to a green-absorbing form of the cyanobacteriochrome Slr1393g3. Based on an assessment of semiempirical methods to describe the chromophore geometries of both forms in vacuo, we find that DFTB2+D leads to structures that are the closest to the reference method. The benchmark of the excited state calculations is based on snapshots from quantum mechanics/molecular mechanics molecular dynamics simulations. In our case, the methods RI-ADC(2) and sTD-DFT based on CAM-B3LYP ground state calculations perform the best, whereas no functional can be recommended to simulate the absorption spectra of both forms with time-dependent density functional theory. Furthermore, the difference in absorption for the lowest energy absorption maxima of both forms can already be modelled with optimized structures, but sampling is required to improve the shape of the absorption bands of both forms, in particular for the second band. This benchmark study can guide further computational studies, as it assesses essential components of a protocol to model the spectral tuning of both cyanobacteriochromes and the related phytochromes.

(Invited) Fundamental Understanding of Faradaic and Non-Faradaic Processes in Carbon-Based Supercapacitors

Nanostructured carbon materials, such as graphene and nanoporous carbon, have become increasingly prominent for use as electrodes in supercapacitors, owing to their high specific and volumetric surface area and good electrical conductivity. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer, and the low energy to limited electrode surface area (SA). As such, low-dimensional carbon nanostructures have been extensively explored. However, anomalous and nonlinear relationships between observed capacitances and SAs have prompted a need to revise our mechanistic understanding of charge storage. In this talk, we present our recent findings regarding charge storage mechanisms, along with possible faradaic reactions, in carbon-based supercapacitors from combined quantum mechanical and classical molecular dynamics simulations. Our first-principles studies suggest that the total interfacial capacitance (CT ) of graphene-based supercapacitors is determined from both electrode (CQ ) and electric double layer (CD ) capacitances which are in series. It has also been demonstrated that the CQ , and thereby CT , can be significantly enhanced as a result of changes to the electronic structure through chemical functionalization, doping, and/or structural deformation. Furthermore, our voltammetric molecular dynamics simulations have revealed the complex non-equilibrium processes associated with extreme confinement of ionic liquid electrolytes within subnanometer pores. Based on these results, we will also discuss a mechanistic perspective centered around ion reorganization kinetics, particularly the dependence of ion migration efficiency, and relatedly, the capacitance, on pore morphology. This talk will also briefly touch on possible faradaic reactions that may cause self-discharge.

Hydrated Excess Protons in Acetonitrile/Water Mixtures: Solvation Species and Ultrafast Proton Motions

The solvation structure of protons in aqueous media is highly relevant to electric properties and to proton transport in liquids and membranes. At ambient temperature, polar liquids display structural fluctuations on femto- to picosecond time scales with a direct impact on proton solvation. We use two-dimensional infrared (2D-IR) spectroscopy to follow proton dynamics in acetonitrile/water mixtures with the Zundel cation H5O2+ prepared in neat acetonitrile as a benchmark. The 2D-IR spectra of the proton transfer mode of H5O2+ demonstrate stochastic large-amplitude motions in the double-minimum proton potential, driven by fluctuating electric fields. In all cases, the excess proton is embedded in a water dimer, forming an H5O2+ complex as the major solvation species. This observation is rationalized by quantum mechanics/molecular mechanics molecular dynamics simulations including up to four water molecules embedded in acetonitrile. The Zundel motif interacts with its closest water neighbor in an H7O3+ unit without persistent proton localization.

Adsorption and dissociation of water molecules at the α-Al2O3(0001) surface: A 2-dimensional hybrid self-consistent charge density functional based tight-binding/molecular mechanics molecular dynamics (2D SCC-DFTB/MM MD) simulation study

Interfacial adsorption and dissociation of water molecules at the α-Al2O3(0001) surface were studied using a novel hybrid 2D-periodic Self-Consistent Charge Density Functional Tight-Binding/Molecular Mechanics Molecular Dynamics (SCC-DFTB/MM MD) simulation strategy. The (0001) surface of α-Al2O3 (C-plane) was considered to assess the structural and dynamical behavior of adsorbed water at the interface region. In-plane distribution profiles of oxygen and hydrogen with respect to the non-periodic z-coordinate provide detailed information on the ordering of monolayer adsorbed water molecules at the interface. Dissociation of water molecules occured on the picosecond time scale, indicated by the formation of an additional plane-H peak located at 0.056 nm. Along simulation 4.2 OH groups per nm2 were formed at the α-Al2O3(0001) surface.

Hybrid QM/MM vs Pure MM Molecular Dynamics for Evaluating Water Distribution within p21N-ras and the Resulting GTP Electronic Density.

p21ras protein activity, regulated by GTP hydrolysis, constitutes an active field of research for the development of cancer targeted therapies that would concern ∼30% of human tumors to which specific mutations have been associated. Indeed, the catalyzing mechanisms provided by the protein environment during GTP hydrolysis and how they are impaired by specific mutations remain to be fully elucidated. In this article, we present results from molecular mechanics (MM) molecular dynamics (MD) simulations and density functional theory (DFT) calculations carried out for wild-type p21 N-ras and six Gln 61 mutants. In the first part, we present the water distribution within the active site of the wild-type protein according to MM MD. Significant differences are observed when comparing the results to the previous distribution assessed through quantum mechanics/molecular mechanics (QM/MM) MD. Such method-dependent results highlight the importance of accounting for the electrostatic coupling between the protein complex and the solvent molecules in identifying hydration sites. In the second part, we present the results from DFT calculations performed to determine the electronic distribution of the GTP ligand, considering the wild-type active site arrangement according to both classical and hybrid approaches. Only in the QM/MM-based configuration is the ligand electronic density similar to that of a GDP-like state observed experimentally. For this reason, in the last set of calculations carried out for p21 N-ras Gln 61 mutants, only the active site structural conformations obtained through hybrid MD are considered. Through the analysis of the GTP electronic density, we conclude that the wild-type active site arrangement according to QM/MM MD is closer to a catalytically efficient conformation of the protein than the arrangement according to MM MD. Hence, water distribution according to the hybrid approach must correspond to the optimal placement of solvent in the active site. Within all of the studied Gln 61 substituted proteins, p21ras major catalyzing effect, which consists of stabilizing a more GDP-like state, is lost.

Can multiscale simulations unravel the function of metallo-enzymes to improve knowledge-based drug discovery?

Metallo-enzymes are a large class of biomolecules promoting specialized chemical reactions. Quantum-classical quantum mechanics/molecular mechanics molecular dynamics, describing the metal site at quantum mechanics level, while accounting for the rest of system at molecular mechanics level, has an accessible time-scale limited by its computational cost. Hence, it must be integrated with classical molecular dynamics and enhanced sampling simulations to disentangle the functions of metallo-enzymes. In this review, we provide an overview of these computational methods and their capabilities. In particular, we will focus on some systems such as CYP19A1 a Fe-dependent enzyme involved in estrogen biosynthesis, and on Mg2+-dependent DNA/RNA processing enzymes/ribozymes and the spliceosome, a protein-directed ribozyme. This information may guide the discovery of drug-like molecules and genetic manipulation tools.

Reaction intermediates during operando electrocatalysis identified from full solvent quantum mechanics molecular dynamics

Electrocatalysis provides a powerful means to selectively transform molecules, but a serious impediment in making rapid progress is the lack of a molecular-based understanding of the reactive mechanisms or intermediates at the electrode-electrolyte interface (EEI). Recent experimental techniques have been developed for operando identification of reaction intermediates using surface infrared (IR) and Raman spectroscopy. However, large noises in the experimental spectrum pose great challenges in resolving the atomistic structures of reactive intermediates. To provide an interpretation of these experimental studies and target for additional studies, we report the results from quantum mechanics molecular dynamics (QM-MD) with explicit consideration of solvent, electrode-electrolyte interface, and applied potential at 298 K, which conceptually resemble the operando experimental condition, leading to a prototype of operando QM-MD (o-QM-MD). With o-QM-MD, we characterize 22 possible reactive intermediates in carbon dioxide reduction reactions ([Formula: see text]RRs). Furthermore, we report the vibrational density of states (v-DoSs) of these intermediates from two-phase thermodynamic (2PT) analysis. Accordingly, we identify important intermediates such as chemisorbed [Formula: see text] ([Formula: see text]), *HOC-COH, *C-CH, and *C-COH in our o-QM-MD likely to explain the experimental spectrum. Indeed, we assign the experimental peak at 1,191 cm-1 to the mode of C-O stretch in *HOC-COH predicted at 1,189 cm-1 and the experimental peak at 1,584 cm-1 to the mode of C-C stretch in *C-COD predicted at 1,581 cm-1 Interestingly, we find that surface ketene (*C=C=O), arising from *HOC-COH dehydration, also shows signals at around 1,584 cm-1, which indicates a nonelectrochemical pathway of hydrocarbon formation at low overpotential and high pH conditions.

Dcdftbmd: Divide-and-Conquer Density Functional Tight-Binding Program for Huge-System Quantum Mechanical Molecular Dynamics Simulations.

Dcdftbmd is a Fortran 90/95 program that enables efficient quantum mechanical molecular dynamics (MD) simulations using divide-and-conquer density functional tight-binding (DC-DFTB) method. Based on the remarkable performance of previous massively parallel DC-DFTB energy and gradient calculations for huge systems, the code has been specialized to MD simulations. Recent implementations and modifications including DFTB extensions, improved computational speed in the DC-DFTB computational steps, algorithms for efficient initial guess charge prediction, and free energy calculations via metadynamics technique have enhanced the capability to obtain atomistic insights in novel applications to nanomaterials and biomolecules. The energy, structure, and other molecular properties are also accessible through the single-point calculation, geometry optimization, and vibrational frequency analysis. The available functionalities are outlined together with efficiency tests and simulation examples. © 2019 Wiley Periodicals, Inc.

Effects of Moisture on the Mechanical Properties of Microcrystalline Cellulose and the Mobility of the Water Molecules as Studied by the Hybrid Molecular Mechanics–Molecular Dynamics Simulation Method

ABSTRACTA hybrid molecular mechanics–molecular dynamics simulation method has been performed to study the effects of moisture content on the mechanical properties of microcrystalline cellulose (MCC) and the mobility of the water molecules. The specific volume and diffusion coefficient of the water increase with increasing moisture content in the range studied of 1.8–25.5 w/w%, while the Young's modulus decreases. The simulation results are in close agreement with the published experimental data. Both the bound scission and free‐volume mechanisms contribute to the plasticization of MCC by water. The Voronoi volume increases with increasing moisture content. It is related to the free volume and the increase enhances the mobility of the water molecules and thus increases the coefficient of diffusion of the water. Moreover, with increasing moisture content, the hydrogen bonding per water molecule between MCC–water molecules decreases, thus increasing the water mobility and number of free water molecules. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 454–464

Mechanism of Passenger Cleavage in Autotransporter EspP Explored with QM/MM Molecular Dynamics Simulation

EspP is an extracellular serine protease and one of the major virulence factors expressed by E. coli. As in all autotransporters, it contains a C-terminal β-barrel domain embedded in the outer membrane and an N-terminal extracellular passenger domain. EspP belongs to the family of serine protease autotransporters of Enterobacteriaceae (SPATE), whose passengers can cleave various mammalian host proteins. The sequence of SPATE proteins is highly conserved, particularly around the passenger cleavage site. It has been shown that cleavage occurs through asparagine cyclization and that several amino acids in the vicinity of the reacting asparagine are essential for the reaction. Surprisingly, presence of the protein-cleaving serine protease domain or other serines and histidines, commonly employed in enzymatic protein cleavage reactions was not required for cleavage to occur. Therefore, mechanisms involving several aspartate residues and a water molecule have been proposed, although the plausibility of these mechanisms have not been previously explored by computational methods. Here, we employ state of the art hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations and free energy methods to investigate several paths for asparagine cyclization and the consequent cleavage of the passenger domain in EspP. It is shown that the reaction involves several consecutive proton transfers and that presence of a water molecule and multiple amino acids at the catalytic site greatly reduces the reaction barrier. Our results can be generalized for other SPATE proteins and facilitate development of SPATE inhibitors in order to combat bacterial infections.

One‐Pot Bioconversion of l‐Arabinose to l‐Ribulose in an Enzymatic Cascade

AbstractThis work reports the one‐pot enzymatic cascade that completely converts l‐arabinose to l‐ribulose using four reactions catalyzed by pyranose 2‐oxidase (P2O), xylose reductase, formate dehydrogenase, and catalase. As wild‐type P2O is specific for the oxidation of six‐carbon sugars, a pool of P2O variants was generated based on rational design to change the specificity of the enzyme towards the oxidation of l‐arabinose at the C2‐position. The variant T169G was identified as the best candidate, and this had an approximately 40‐fold higher rate constant for the flavin reduction (sugar oxidation) step, as compared to the wild‐type enzyme. Computational calculations using quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) showed that this improvement is due to a decrease in the steric effects at the axial C4‐OH of l‐arabinose, which allows a reduction in the distance between the C2‐H and flavin N5, facilitating hydride transfer and enabling flavin reduction.

One-Pot Bioconversion of l-Arabinose to l-Ribulose in an Enzymatic Cascade.

This work reports the one-pot enzymatic cascade that completely converts l-arabinose to l-ribulose using four reactions catalyzed by pyranose 2-oxidase (P2O), xylose reductase, formate dehydrogenase, and catalase. As wild-type P2O is specific for the oxidation of six-carbon sugars, a pool of P2O variants was generated based on rational design to change the specificity of the enzyme towards the oxidation of l-arabinose at the C2-position. The variant T169G was identified as the best candidate, and this had an approximately 40-fold higher rate constant for the flavin reduction (sugar oxidation) step, as compared to the wild-type enzyme. Computational calculations using quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) showed that this improvement is due to a decrease in the steric effects at the axial C4-OH of l-arabinose, which allows a reduction in the distance between the C2-H and flavin N5, facilitating hydride transfer and enabling flavin reduction.

The HCM-causing Y235S cMyBPC mutation accelerates contractile function by altering C1 domain structure

Mutations in cardiac myosin binding protein C (cMyBPC) are a major cause of hypertrophic cardiomyopathy (HCM). In particular, a single amino acid substitution of tyrosine to serine at residue 237 in humans (residue 235 in mice) has been linked to HCM with strong disease association. Although cMyBPC truncations, deletions and insertions, and frame shift mutations have been studied, relatively little is known about the functional consequences of missense mutations in cMyBPC. In this study, we characterized the functional and structural effects of the HCM-causing Y235S mutation by performing mechanical experiments and molecular dynamics simulations (MDS). cMyBPC null mouse myocardium was virally transfected with wild-type (WT) or Y235S cMyBPC (KOY235S). We found that Y235S cMyBPC was properly expressed and incorporated into the cardiac sarcomere, suggesting that the mechanism of disease of the Y235S mutation is not haploinsufficiency or poison peptides. Mechanical experiments in detergent-skinned myocardium isolated from KOY235S hearts revealed hypercontractile behavior compared to KOWT hearts, evidenced by accelerated cross-bridge kinetics and increased Ca2+ sensitivity of force generation. In addition, MDS revealed that the Y235S mutation causes alterations in important intramolecular interactions, surface conformations, and electrostatic potential of the C1 domain of cMyBPC. Our combined in vitro and in silico data suggest that the Y235S mutation directly disrupts internal and surface properties of the C1 domain of cMyBPC, which potentially alters its ligand-binding interactions. These molecular changes may underlie the mechanism for hypercontractile cross-bridge behavior, which ultimately results in the development of cardiac hypertrophy and in vivo cardiac dysfunction.

Counterion Effects on Aggregate Structure of 12-Hydroxystearate Salts in Hexane: A Quantum Mechanical and Molecular Dynamics Simulation Study.

Salts of 12-hydroxystearate are important organogelators and grease thickeners, but a structural rationale for their rheological properties remains elusive. We use quantum mechanical calculations and molecular dynamics (MD) simulations to analyze aggregate structures for (1) ( R)-12-hydroxystearic acid (( R)-12HSA), (2) lithium ( R)-12-hydroxystearate (( R)-Li12HS), and (3) sodium ( R)-12-hydroxystearate (( R)-Na12HS). First, quantum mechanical calculations were used to establish the structure and complexation energies of dimers of acetic acid, lithium acetate, and sodium acetate. The expected acetic acid dimer is predicted, and both the lithium acetate and sodium acetate dimer formed a C2 h-symmetric structure. All dimers were sufficiently stable to allow modeling them as pseudocovalent complexes in all-atom, explicit solvent MD. After microsecond-long MD, all systems produced strong ringlike ordered nuclei. The C2 h lithium salt molecules produced aggregates that had the most efficient packing at the head group and a higher frequency of hydroxyl hydrogen bonding compared to the sodium salt. This ordering propensity explains the high melting temperature of ( R)-Li12HS. Also, the higher frequency of hydrogen bonding leads to fewer solvent-exposed hydrogen bond partners. This explains why lithium is a common counterion in high-temperature and water-resistant greases.

Ion-Ion Repulsions and Charge-Shielding Effects Dominate the Permeation Mechanism through the OmpF Porin Channel.

OmpF is a wide channel bacterial porin frequently employed to study selective ionic translocation. The cationic preference of this porin is mainly determined by electrostatic forces between the translocated ion and the protein and the formation of ion pairs (e.g., K+···Cl-) being previously pointed as the main cause to favor the cationic transport through the constriction zone. Hybrid quantum mechanics/molecular mechanics-molecular dynamics simulations, which have provided polarization-containing potentials of mean force profiles for different permeation scenarios, reveal significant new insights related with the ion translocation mechanism. Results show that the permeation is dominated by electrostatic interactions, which in turn affect ion-protein interactions at the constriction zone. However, it is observed that ion flow is favored by ion-ion repulsions and, in a lesser extent, by charge-shielding effects, instead of the previously pointed ionic pair formation.

Accelerating Pmn21-BAlNP properties prediction by machine learning based on first-principles calculation

In this paper, the CASTEP program and machine learning algorithm are used to study Pmn21-BxAl1-xNyP1-y. The mechanical stability, thermodynamic stability and molecular dynamics stability of several typical structures of Pmn21-BxAl1-xNyP1-y are estimated by CASTEP program. The results show that all the structures are stable. The data such as lattice constants and elastic properties of Pmn21-BxAl1-xNyP1-y are used as training sets of machine learning, then, the average absolute percent error (MAPE) of each model is calculated by using 7-fold cross-validation, and the lattice constant prediction model and shear modulus prediction model of Pmn21-BxAl1-xNyP1-y are selected respectively. Linear regression algorithm is used for modeling lattice constants, and Xgboost algorithm is used for modeling shear modulus. On the test sets, it is found that the error is within the acceptable range by comparing the data predicted by the models with the real data. Therefore, the machine learning prediction models of lattice constants, and shear modulus G have strong generalization ability. The electrical properties of Pmn21-BxAl1-xNyP1-y are calculated by using hybrid PBE0 functional. The direct band gaps are found in AlN, BP, AlP, B0.25Al0.75N, B0.75Al0.25P, B0.5Al0.5P, B0.25Al0.75P, AlN0.75P0.25, AlN0.5P0.5 and AlN0.25P0.75.

Ambimodal Dipolar/Diels-Alder Cycloaddition Transition States Involving Proton Transfers.

Quantum mechanical computations and molecular dynamics simulations have been used to elucidate the factors that control reaction outcomes in ambimodal transition states leading to both dipolar and Diels-Alder cycloaddition products, which can interconvert via α-ketol rearrangements. The dipolar cycloaddition pathways were found to be disadvantaged due to the persistence of charge separation after the second C-C formation en route to the dipolar cycloaddition adducts. Structural modifications that result in the stabilization of the charge-separated species lead to an increase in the amount of dipolar cycloadducts formed.

Investigation of the preferential solvation and dynamical properties of Cu+ in 18.6% aqueous ammonia solution using ab initio quantum mechanical charge field (QMCF) molecular dynamics and NBO analysis

Structural and dynamical properties of Cu+ in 18.6% aqueous ammonia solution at 298.15 K have been investigated via quantum mechanical charge field molecular dynamics (QMCF MD). The QM region was set to a radius of 6.7 Å to include the first and second solvation shell. The Hartree-Fock (HF) level was applied to calculate the ion-ligand and ligand-ligand interactions in the QM region using the LANL2DZ-ECP basis set for the ion and DZP-Dunning for the ligands. The Cu+N and Cu+O radial distribution functions showed maximum first shell probabilities at distances of 2.23 and 2.30 Å, respectively. Predominantly, four NH3 molecules were found to form a tetrahedral [Cu(NH3)4]+ complex, although the formation of a short-lived intermediate [Cu(NH3)3H2O]+ complex was also observed. The mean residence times of NH3 and H2O ligands in the first solvation shell were estimated as 14.6 ps and 1.3 ps, respectively, reflecting the strong interaction between Cu+ and ammonia as well as the occurrence of rapid water exchange. The vibrational power spectrum of the Cu+N vibration in the first solvation shell revealed a wave number of 252 cm−1 with a corresponding force constant of 43.0 Nm−1. In addition, an NBO analysis was carried out, confirming the strong electrostatic character of the Cu+NH3 and Cu+H2O interaction, and highlighting that the presence of H2O ligands may destabilize the first solvation shell.

Structure, Dynamics, and Hydration Free Energy of Carbon Dioxide in Aqueous Solution: A Quantum Mechanical/Molecular Mechanics Molecular Dynamics Thermodynamic Integration (QM/MM MD TI) Simulation Study.

The solvation of carbon dioxide in solution represents a key step for the capture and fixation CO2 in nature, which may be further influenced by the formation of (bi)carbonate species and/or the formation of CO2 clusters in solution. The latter processes are strongly dependent on the exact environment of the liquid state (e.g., pH value, solvated ions, etc.) and may interfere with the experimental determination of structural, dynamical, and thermodynamic properties. In this work a hybrid quantum mechanical/molecular mechanical (QM/MM) simulation approach at correlated ab initio level of theory resolution-of-identity second-order Møller-Plesset Perturbation Theory (RI-MP2) has been applied in the framework of thermodynamic integration (TI) to study structure, dynamics, and the hydration free energy of a single carbon dioxide molecule in aqueous solution. A detailed analysis of the individual QM/MM potential energy contributions demonstrate that the overall potential remains highly consistent over the entire sampling phase and that no artificial contributions are influencing the determination of the hydration free energy. The latter value of 0.01 ± 0.92 kcal/mol was found in very good agreement with the values of 0.06 and 0.24 kcal/mol obtained via quasi-chemical theory and experimental measurements, respectively. In order to obtain detailed information about the C- and O C-water interaction, conically restricted regions with respect to the main axis of the CO2 molecule have been employed in structural analysis. The presented data not only provide detailed information about the hydration properties of CO2 but act as a critical validation of the simulation technique, which will be beneficial in the study of nonaqueous solvents such as pure and aqueous NH3 solutions, which have been suggested as potential candidates to capture CO2 from anthropogenic sources.