General principles of biomineralization: Biologically induced mineralization

Biologically induced mineralization involves the adventitious precipitation of inorganic minerals by reaction of extraneous ions with metabolic products extruded across or into the cell wall.

The mineral products are closely associated with the cell wall and crystallochemically heterogeneous.

 

Ca2+ + 2HCO3- CaCO3 + CO2 + H2O

Biologically induced magnetite nanoparticles

20 nm

Biologically controlled mineralization involves the specialized regulation of mineral deposition and results in functional materials with species-specific crystallochemical properties.

General principles: Biologically controlled mineralization

uniform particle sizes

well-defined structures and compositions

high levels of spatial organization

complex morphologies

controlled aggregation and texture

preferential crystallographic orientation

higher-order assembly

hierarchical structures

General principles: Site-directed biomineralization

H2O

Intercellular –in the spaces between closely packed cells

0.5 mm

Intracellular – bilayer lipid vesiclesenclosed compartments within the cell

Extracellular – on or within an insoluble macromolecular framework secreted outside the cell

calcified coral

eggshell formation

PhysiochemicalSpatialStructuralMorphologicalConstructional

General principles: Site requirements

Spatial delineation – physical boundary for size and shape control

Chemical regulation– for increasing ionic concentrations (ionic pumping)

Diffusion limited ion flow– for controlling solution composition

Organic surface – nucleation

General principles: Control mechanisms

Gene pool Bioenergetics Biochemical

Boundary-organized biomineralization

The delineation of biological environments is of key importance in boundary-organized biomineralization because it provides sites of controlled chemistry that are spatially defined.

Control functions:

spatial delineation – size, shape and organization of the mineral phase

diffusion limited ion flow – ionic activities, solution composition, supersaturation

mineral passivation – surface stabilization against dissolution and transformation ion accumulation and transport – supplying chemicals to remote intra- and

extracellular mineralization sites

mineral nucleation – regulating interfacial energies

mineral transportation – moving mineralized structures to new construction sites.

Boundary-organized biomineralization; coccolith calcification

Light-dependent uptake: some CO2 fixed by photosynthesis influence of light on calcification

Some calcification occurs in dark using stored metabolic energy.

2HCO3

-

Ca2+

CaCO3

CO2

SO42-

SO42-

Photosynthesis

2HCO3

-

Ca2+ Ca2+

HCO3

-

CO32-CO2 OH

-

H2O

Sugars

Polysaccharides

Golgi vesicle

Medium Cell

H+

Coccoliths

+ Ca2+

uniport

CA

symport

Chemical control of biomineralization

Solubility

The solubility (S) of an inorganic salt depends on the balance between lattice energy (L) and ion hydration (H) ion pairing (IP) and complexation (C) in aqueous solution.

GS = GL - (GH + GIP + GC)

Lattice substitutions are important in controlling the solubility of biological apatite

Undersaturated

[Ca ]2+

/M

HAP

FAP

Plaque fluid

Saliva

Supersaturated

pH

100

10

1

0.15 6 7

Fluoride and tooth decay

Chemical control of biomineralization: Solubility product

The solubility product is a critical factor in determining the thermodynamic limit for the onset of inorganic precipitation. When the solubility product is less than the activity product (AP) of a solution then precipitation will occur until Ksp = AP.

MnXm(solid) nM+(aq) + mX-

(aq)

 Ksp = {M+}n . {X-}m

Problems

Complexation with biological ligands (citrate/oxo/hydroxy)

Covalent polymers (silica)

Crystal size heterogeneity - Ostwald ripening

Kinetic effects organic sheaths etc

Chemical control of biomineralization: Supersaturation

Supersaturation is a measure to what extent a solution is out of equilibrium and represents the thermodynamic driving force for inorganic precipitation.

Relative supersaturation, Absolute supersaturation  

SR = AP/Ksp SA = (AP – Ksp)/Ksp

 The difference in chemical potential between the supersaturated solution and a solution at equilibrium with the solid is related to SR by

  = kT lnSR

Supersaturation is highly regulated in biology through the process of boundary-organized biomineralization.

Supersaturation control in spatial boundaries

Mn+

Mn+

M (n+1)+

MX

Matrix

X-

X-

Mn+

E1 E2

H2O H2O

MC

A+

A+

B-

B-

H+H

+

Direct mechanisms to increase S

ion pumping ( + redox)

ion complexation/decomplexation

enzymic regulation of anions carbonic anhydrase (HCO3

-) alkaline phosphatase (HPO4

2-)

Indirect mechanisms

ionic strength - Na+ and Cl- transport

water extrusion – silica deposition?.proton pumping – pH changes

Chemical control of biomineralization: Nucleation

Homogeneous nucleation

The free energy of formation of a spherical nucleus, GN, is given by the

difference between the surface (interfacial, I) and bulk (B) energies,  

GN = GI - GB

 GI = + 4r2

 where = interfacial free energy per unit surface area, and  GB = - 4r3 Gv

3 Vm

 where Gv = per mole solid-liquid phase

change, and Vm () is the molar

(molecular) volume.

G

G N*

G I

G N

G B

rr*

r* = critical sizeGN

* = activation energy

GN* = 1632 r* = 2Vm

3(kTlnSR)2 Gv

 Rate; JN = Aexp(-GN

*/kT)

Nucleation control in biomineralization

JN

A

B

CSR* SR

Heterogeneous

Homogeneous

JN

SR*

 SR increase GN* decrease JN increase

GN* (lnSR)-2

Need to control over catastrophic nucleation rate in pure solution at SR

*

GN* 3

small reductions in can have a marked effect on JN and r*

External substrate (dust etc) reduces and hence increases JN at given value of SR.

Heterogeneous nucleation occurs at lower SR and under control if SR

* is not breached.

GN* and JN determined by and SR and

biologically controlled by organic matrices and the membrane regulation of concentration gradients.

Extraneous particles with equal (A), variable (B) nucleation efficiencies. (C) without extraneous particles).

Oriented nucleation (epitaxy)

A B C

(A) non-oriented, (B) mosaic with crystals aligned only perpendicular to the substrate, (C) iso-oriented array with 3-D crystallographic alignment.

Structural control in biomineralization involves the preferential nucleation of a specific crystal face or axis on the surface of an organic matrix.

Key concepts: molecular recognition, lattice matching, electrostatic, stereochemical and structural complementarity.

  % mismatchCaCO3//KBr 3

CaCO3//NaI 1

CaCO3 //KCl -2

CaF2//NaBr 8

CaF2//NaCl 3

CaF2//KBr 21

Chemical control of biomineralization: Crystal growth

mass transport (diffusion-limited) very high values of SA (x = 0)

polynucleation (growth islands) high SA (x > 2)

layer-by-layer growth moderate SA (x = 1)

screw dislocation growth low SA (x = 2)

Bulk diffusion

Surface adsorption + dehydration2D

diffusion

1D KINK

Crystal growth and termination are dependent on the level of supersaturation and occur through surface-controlled processes (active sites). STEP

Rate of growth, JG = k (SA)x  

k = rate constant, SA = absolute supersaturation

Active sites - kinks, steps

screw dislocation

A

B

C

Chemical control of biomineralization: Crystal growth inhibitors

Soluble additives bind at steps/kinks, inhibit growth, change composition, structure and form.

Time / h

pH

0 1 2

7.0

7.2

7.4

Mg2+

control

F-

apatite crystallization

Time / min

pH

5 15

7.0

7.5

8.0

control

10

calcite crystallization + coccolith polysaccharides

t= 0

t = 4 min

intercalation texture50 m

sea urchin

Chemical control of biomineralization: Crystal morphology

CaCO3

Crystal morphology (habit) is determined by the relative rates of growth of different crystal faces, with the slow growing surfaces

dominating the final form.

controlLi+

HPO42-

010

110

100

Σ s{hkl}. A{hkl} = minimum

[001]

[110]

[110]

Chemical control of biomineralization: Habit modification

Molecular-specific interactions: electrostatic, stereochemical and structural matching modify the surface energy or mechanism of growth, or both.

CaCO3 +[malonate]2-

CaCO3 + polyaspartate

Chemical control of biomineralization: Polymorphism

JN

S (amorph)

S (crystal)

Amorphous Crystal

Ksp

(crystal) Ksp

(amorph) X Activity product

periodic order

Ostwald-Lussac law of stages

Crystallization proceeds along a series of structures with decreasing solubility and increasing thermodynamic stability. The structure of the critical nucleus is an important factor in controlling the crystallization pathway.

disordered/hydrated

A

B

G

Solution(M+

(aq) + X-(aq))

Amorphous

GN(B)Gg(B)

+

GT1

GN(A) Gg(A)+

GT2

GT3

Final mineral (crystalline)

The chemical control of crystallization pathways that involve a sequence of kinetic inhibition and phase transformation can result in a high degree of selectivity in crystal structure and composition.

Polymorphism and phase transformations

Accelerators and inhibitors

CALCIUM CARBONATE CALCIUM PHOSPHATE

Amorphous CaCO3

Vaterite

Aragonite

Calcite

Amorphous CaP

Brushite

Octacalcium phosphate

Hydroxyapatite

amorphous silica - biologically stable

Phase transformations - examples

Calcium phosphate

slow transformations of amorphous phase

octacalcium phosphate to hydroxyapatite

in situ solid state hydrolytic transformationx1 OCP (d100 = 1.868 nm) x2 HAP (2d100 = 1.632 nm

Calcium carbonate

fast transformations of amorphous phase unless stabilized by organic sheaths

high Mg2+ level in ACC high Mg calcites (30 mol%)

eg. sea urchin spicules

CDL

OEOCP

HAP

Traces of OCP precursor are left as a central dark linein enamel HAP crystals.

Larval sea urchin spicule - early growth stage

1m

Phase transformations - iron oxides

Solid state transformationpH 7, 80 C

Dissolution Reductive dissolution

RapidSlow-low Fe-low O2

-[H] agents-complexation

-Fe2O3

FERRITINFe2O3·nH2O

FeIII(aq) + FeII

(aq)Magnetite

Amorphous Ferrihydrite Goethite

FeIII

FeIII

O

OH

OH

+ FeII OH+(aq)

FeIII

FeIII

O

O

O

FeII + H+ + H2O

Fe3O4 + H+ + H2O

[FeIIIFeII(O)x(OH)y]n+

ferrihydrite magnetite (bacteria, chitons)

-10.0 -5.0 0.0 5.0 10.0

0.85

0.90

0.95

1.00

A1

A2C

B

Velocity (mm/sec)

Inte

nsi

ty

57Fe Mössbauer