ADSORPTION OF SOME AMINO ACIDS BY CHRYSOTILE.
Hideo Hashizume
National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
Fax: +81-29-860-4667
E-mail: HASHIZUME.Hideo@nims.go.jp(Received 28 April, 2006 Accepted 30 January, 2007)
Abstract
Adsorption of 10 types of amino acids (Gly,Ala, Val, Leu, Ile, Pro, Ser, Asp, Glu, and Phe) has
been studied using chrysotile, which is a mineralbelonging to the serpentine group.
The adsorptionof Asp and Glu by chrysotile was strong while Gly,
Ala, Val, Leu, Ile, Pro, Ser and Phe were hardly
adsorbed by chrysotile. It was found that adsorption
behavior of these amino acids was mainly affected by
the dissociation behavior of the adsorbent and theadsorbates, and also by the solubility of the adsorbate
into water.
Around at pH 8, chrysotile would havepositive charges, and Asp and Glu possessed negative
charges. Therefore, Asp and Glu could be adsorbed
well on chrysotile by electrostatic interaction. It
was shown that the adsorption of amino acids wasinversely correlated with their solubility into water.
The adsorption behavior of amino acids by chrysotile
could be explained on the basis of the dissociation ofamino acids and their solubility into water. This
trend of the adsorption behavior of amino acids bychrysotile was similar to that by kaolinite [9]. This
fact suggested that chrysotile would possess thesimilarity of crystal structure to kaolinite.
Additionally, chiral separation of D- and L-aminoacid would be difficult by chrysotile.
(Keywords) adsorption, amino acid, chrysotile,
discrimination between optical isomers.
1. Introduction
Clay minerals have several functions such as
catalytic and adsorptive activities for organic
compounds [1, 2]. The relationships between theadsorptive ability and catalytic activity of clay
minerals for bio-molecules have been investigated [3– 8]. Hedges and Hare (1987) investigated adsorptionof 15 amino acids by montmorillonite and kaolinite indiluted aqueous solutions [9].
The adsorption
studies showed that basic amino acids were adsorbed
strongly by both montmorillonite and kaolinite andacidic amino acids were only adsorbed well bykaolinite. Neutral amino acids were hardly adsorbed
by either montmorillonite or kaolinite. Siffert andNaidja (1992) investigated selective adsorption of
racemic Asp and Glu using a natural montmorillonite,where Mg(II) cation was enriched in the silicate layer[10]. An isotherm showed that the adsorption of DAsp was stronger than that of L-Asp. On the other
hand, the adsorption of L-Glu was stronger than thatof D-Glu.
Montmorillonite might have an
asymmetric structure by exchanging an Al with Mg in

Viva Origino 35 (2007) 60 - 65
© 2007 by SSOEL Japan- 60 -

octahedral site of the layer [11]. Therefore, Mg
enriched montmorillonite could result in the selective
adsorption of optical isomers.
For instance,
Hashizume et al. (2000) showed the discrimination
between D-alanyl-D-alanine and L-alanyl-L-alanineby allophane [12]. Allophane has a hallow spherule
structure and its outer diameter is from 3.5 to 5.5 nm
and several perforations on the surface. Some AlOH or Si-OH groups that would be related to theadsorption site for amino acids are exposed on thewall of the perforation. Allophane generally causes
an aggregate.
The discrimination between dialanine enatiomers was expected on the basis of the
shape of the perforation, the OH groups, and theformation of the aggregate.
It is known thatkaolinite has levorotatory and dextrorotatory crystals
as well as α-quartz [13].
It was shown by
theoretical calculation that one of these types ofkaolinite existed slightly more frequently than theother types [14]. If amino acids were synthesizedfrom the precursors in the presence of such a type ofkaolinite, the chiral discrimination of D- or L-aminoacid would have been occurred. Moreover, it was
shown that kaolinite could adsorb D- or L-amino acidselectively [14, 15]. Further, an edge of kaolinite
might be capable to discriminate D- and L-aminoacids, where Al-OH and Si-OH groups existed at theedge of kaolinite. Organic molecules could be
adsorbed on the OH group of the edge by hydrogenbonding. It had been shown that the edge might
have a chiral center by a specific arrangement of the
OH group [16].
Concerning the origin of life, the surface of theearly earth had been mainly covered with basaltic
rocks, of which mainly involved Mg, Si and Al [17].
According to the chemical composition analyses ofthe early crust, not only clay minerals such as
kaolinite,
pyrophyllite,
andmontmorillonite
consisting of the system of Al2O3-SiO2-H2O, but also
serpentine and talc consisting of the system of MgOSiO2-H2O could have been formed from the basaltic
glasses or rocks on the surface of the early earth. It
would be necessary and important to investigate therelationship between the characteristics of clay
minerals involving of MgO, SiO2 and OH groups andthe adsorption of bioorganic compounds by theminerals. Clay minerals consisting of MgO, SiO2
and H2O mainly belong to serpentine group(chrysotile, lizardite and anitgorite) and talc. They
are generally formed by weathering of olivine,
pyroxine, and basaltic glass.
Serpentine group is a layered silicate, andinvolves three different types, that is to say chrysotile,Viva Origino 35 (2007) 60 - 65
lizardite, and antigorite. Serpentine group minerals
are composed of a tetrahedral (Si site) and an
octahedral (Mg site) sheet.
The chemicalcomposition of serpentine group is ideally
Chrysotile consists of tubular
Mg3Si2O5(OH)4.
fibers with such chemical composition, where thelayer sheet rolls up; the octahedral sheet of Mg site islarger than the tetrahedral sheet of Si site [18].
Lizardite forms a plane sheet. In the case of natural
lizardite, cations such as Fe3+ or Al3+ are included in
the octahedral sheet. Thus, the sheet of lizardite is
not curved. Antigorite is also a plane sheet silicate[18].
The octahedral sheet of antigorite iscontinuous as shown in Fig. 1. In the period (A) in
Fig. 1, the octahedral sheet is not bonded to the justabove octahedral sheet but the just below tetrahedral
sheet, while in the period (B) the octahedral sheet ofthe bottom is not bonded to the just below tetrahedral
sheet but the the just above tetrahedral sheet. In
other words, an octahedral sheet is bonded to one
adjacent tetrahedral sheet of one side in one period,and in another period the octahedral sheet is bonded
to the another adjacent tetrahedral sheet (Fig. 1).
Therefore, antigorite forms a plane sheet. The
crystal structure of serpentine group minerals issimilar to that of kaolinite. Kaolinite is a layered
silicate and is composed of the stratification of theoctahedral (Al site) and tetrahedral (Si site) sheet.
In order to balance the charges of the tetrahedral
sheet with those of the octahedral sheet, theoctahedral sheet is not filled by Al3+, that is to saythat there are vacant sites in the octahedral sheet.
For that reason, kaolinite does not show a tubular
fiber structure, Kaolinite has dextrorotatory andlevorotatory forms in its structure, because of the
distribution of the vacant sites in the octahedral sheet[14]. Also, it was expected that the edge of kaolinite
might have chiral centers by presence of hydroxyl
groups [16]. In the serpentine group, chrysotile andlizardite are almost of the same crystal structure as
kaolinite. Chrysotile has almost the ideal chemicalcomposition Mg3Si2O5(OH)4, and forms the tubular
fiber.
Therefore, chrysotile was chosen in thepresent research.
Adsorption of amino acids by chrysotile has
been investigated in this study. Chrysotile wassynthesized. Fine particles of synthesized chrysotile,which were fractionated smaller than 2 µm, were
Period (A)Period (B)Tetrahedral sheet (T)

(O)
(T)

(T)

(O)
(O)
(T)
(T)
(O)
:Si :Mg:O :O :OH

Octahedral sheet (O)Fig. 1 Schematic illustration of antigorite.

- 61 -

used for the adsorption experiments. Ten aminoacids, glycine (Gly), alanine (Ala), valine (Val),
leucine (Leu), isoleucine (Ile), proline (Pro), serine(Ser), phenylalanine (Phe), aspartic acid (Asp) andglutamic acid (Glu), were chosen. These aminoacids could have been formed under the prebiotic
conditions [19]. In addition, these have alreadybeen found in meteorites [20].
Wong (1988)
proposed the hypothesis for the co-evolution of thegenetic code [21], in which 6 to 10 amino acids might
have been used for 61 codones in the first stage oforigin of the genetic code. Based on these facts, 10amino acids were examined in this work.
Furthermore, it was investigated whether chrysotile
could discriminate D- and L-amino acids byadsorption or not.
2. Experimental
Chyrsotile used was prepared by hydrothermal
synthesis, in which MgO and SiO2 (reagent grade,
Wako Pure Chemical Industries Ltd) were mixed
according to the chemical composition of chrysotile(i.e. MgO:SiO2 = 3:2 in molar ratio). Five g of the
mixture of MgO and SiO2, and 5 g of deionized water
were put into a reaction vessel. The reaction vessel
was placed in an oven and was heated at 200 °C for 6
weeks. The product was analyzed by an XRD to
confirm that chrysotile was synthesized. Chrysotile
was ground softly, and then it was dispersed indeionized water. Chrysotile particles smaller than 2µm were fractionated by the difference of asedimentation ratio.
After the fractionation,
chrysotile was treated using 10 % of H2O2 at 100 ℃,
and dialyzed with deionized water.
Chrysotile
fractionated as adsorbent was dried at 60 °C in an
oven. The dried chrysotile was confirmed by the
XRD which did not include any other materials.
Nine racemic amino acids (Ala, Val, Leu, Ile,Pro, Ser, Phe, Asp, and Glu) and Gly were used of thereagent grade (Wako Pure Chemical Industries Ltd).
Amino acid solutions containing 5 mmol/l amino acid
were prepared. It was added 7 ml of amino acidsolution and 150 mg chrysotile in a glass bottle with astopper. After the shaking of the bottle for 65 h, pHof the suspension was measured. The pH was 8.0± 1.0. The suspension was centrifuged to separate
supernatant and chrysotile. The extents of aminoacid in the initial solution and the supernatant after
the reaction were measured 5 – 7 times by a totalorganic carbon analyzer (Shimadzu TOC-5000A).
Concentrations of amino acids in the initial solution
and the supernatant were obtained by their averagecarbon contents. Adsorption capacity of aminoacids by chrysotile was estimated by theconcentration of amino acids in the initial solution
and the supernatant. For the confirmation of theadsorption capacity by chrysotile, the adsorption
treatment was performed 3 - 4 times to use the freshchrysotile and amino acid solutions.
For discrimination between D- and L-aminoacids by chrysotile, the extent of amino acids insupernatant after adsorption by chrysotile wasViva Origino 35 (2007) 60 - 65
measured by ahighperformanceliquidchromatogram (Hitachi 4200 series) on a Shimpack
FC-ODS using an eluent containing 0.1%
trifruoracetic acid at 0.9 ml/min with a circulardichroism detector Jusco CD-2015 (HPLC-CD).
The CD detector is capable to monitor ultraviolet(UV) absorption as well as CD at 220 nm. The initialconcentrations of amino acid before adsorption werealso measured by HPLC-CD to confirm that the same
concentrations of D- and L-amino acid existed. It
was confirmed that there were no peaks found by CDdetector for the initial solutions.
3. Results and discussion.
3.1 Adsorption of 10 amino acids
The average adsorption capacities of chrysotile
for 10 amino acids were depicted in Fig. 2, where theerror bars were indicated. Adsorption of Asp was
highest and adsorption of Glu was second highest
among 10 amino acids. Except for Asp and Glu,
there is a trend that amino acids possessing large
molecular weights are adsorbed on chrysotile
strongly . For instance, Gly and Ala were lessadsorbed on chrysotile, but Phe was adsorbed
strongly. Ile and Ser may be exceptional (see Table
1).
Adsorption is generally related to theabundance of charges of an adsorbent and anadsorbate as well as the solubility of the adsorbate
into water. Dissociation constants of amino acids
used were shown in Table 1. The values of pH of
the suspension after adsorption treatment were 7 to 9.
Thus, the amino acids exist as neutral or slightly
negatively charged forms. Asp and Glu would
possess two negative charges. On the other hand,the dissociation behavior of the surface of chrysotile
is not known. The dissociation constant for silanolgroup (pKa) at the silicate surface is 9.86 [22]. At
pH 8, silanol group located on the edge of chrysotile
may slightly have a positive charge such as Si-OH2+.

- 62 -

Therefore, since Asp and Glu have the negative
charges these will be capable to be more strongly
adsorbed on chrysotile than other amino acids.
There is a trend that molecules which arereadily adsorbed possess low solubility into water
within a water-adsorbent system. Generally, amolecule possessing high solubility into water does
not be adsorbed strongly by an adsorbent, while themolecules with low solubility into water have a highadsorption capacity by the adsorbent. The solubilityof 10 amino acids into water was shown in Table 1.
The relationship between solubility into water andamino aids and the relationship between adsorption
and amino acids was also added in Fig. 3. It was
found that the strength of adsorption of amino acids
are inversely correlated with the solubility of aminoacid into water (Fig. 3), while Gly and Glu were
exceptions. In the case of Glu, the effect on thenegative charges of Glu may be stronger than that on
solubility into water. The determination of the

0.020
0.015
0.010
0.005
0

GlyAlaValLeu Ile ProSerAspGluPhe

Amino acid
Fig. 2 Adsorption of individual amino acids bychrysotile.

Viva Origino 35 (2007) 60 - 65

0.8
0.6

-2.6

0.4
0.2
0
-0.2

-2.8
-3.0

GlyAlaValLeu IleProSerAspGluPhe

Amino acid
Fig. 3 Relationship between solubility to water [23]and adsorption of individual amino acids. Solidcircles show adsorption (A) and triangles show
solubility (S).

3.2 Possibility of discrimination between D- and Lamino acids
Chromatograms using the CD and UV
detectors for supernatants including amino acids after
the adsorption experiments were shown in Fig. 4.
HPLC peaks in all the chromatograms appeared using
the UV detector, while any peaks were not detected
clearly using the CD detector. The fact that theadsorption of amino acids by chrysotile was notstrong might be the reason that it was difficult to
detect any peaks on the chromatogram using CD.
Nevertheless, the chromatograms for CD of Pro and

CD
2

4

6

8

10

CD

0

2

CD
4

6

0

2
4
6
8 10
Retention time, min

0

4

6

8

10

4

6

6

8

10

UV
CD
2
4
6
8 10
Retention time, min

CD

0

8 10

Glu

UV

0

2

4
Phe
UV

CD
2
4
6
8 10
Retention time, minFig. 4 Chromatograms for UV and CD of individual amino acids except gly.

- 63 -

CD, 220 nm

CD2CD, 220 nm

UV

CD, 220 nm

UV, 220 nm

Asp

0

2

Ser

CD

8 10

CD

0

UV

UV, 220 nm

2

8 10

UV

CD, 220 nm

CD, 220 nm

UV

0

6

Pro
UV, 220 nm

Ile

4CD, 220 nm

UV, 220 nm

0

UV

Leu
CD, 220 nm

CD, 220 nm

UV

UV, 220 nm

Ala

Val
CD, 220 nm

UV, 220 nm

capacity of adsorption of Gly involved a fairly large
error as shown in Fig. 2. The relationship between
the adsorption and the solubility into water for Gly
may be correlated with the fact that the relatively
large error may be involved for the determination ofadsorption. Generally, the solubility of large aminoacids into water is low and that of small amino acids
is high. Therefore, it may be true that large aminoacids in size could be more readily adsorbed thansmall amino acids. The adsorption of an amino acidby chrysotile would be affected well on the
dissociation behavior of the amino acid and theUV, 220 nm-2.4

UV, 220 nm-2.2

UV, 220 nm-2.0

solubility to water.
Chrysotile possesses the same type crystal
structure as kaolinite. Chrysotile has Mg cation in
the octahedral site, while kaolinite has Al cation in
the octahedral site. Adsorption of 15 amino acids bykaolinite was reported [9].
According to thecomparison between the adsorption of 9 amino acids(i.e. Gly, Ala, Val, Leu, Ile, Ser, Asp, Glu and Phe) bychrysotile with that by kaolinite, both of chrysotile
and kaolinite adsorbed Asp and Glu strongly, while
their clay minerals hardly adsorbed Ala, Val, Ile andLeu etc. The trend for adsorption by chrysotile wassimilar to that by kaolinite, where the initialconcentration of amino acids used in the present
study was much higher than that used in the previous
study [9]. That trend of adsorption by chrysotile
and kaolinite will depend on the similarity of thecrystal structure of chrysotile to that of kaolinite.

1.2
1.0

Solubility (S)Adsorption (A)Viva Origino 35 (2007) 60 - 65
Asp might indicate a small positive peak and thechromatogram for CD of Phe might show a negative
peak as shown by the arrow in Fig. 4, though thesedata are ambiguous hidden in signal noises. Thismay be due to that L-Pro, L-Asp, and D-Phe might beconcentrated in supernatant so that D-Pro, D-Asp,and L-Phe might be adsorbed by chrysotile morestrongly than L-Pro, L-Asp and D-Phe.
If the discrimination between D- and L-aminoacids by chrysotile occurs, chrysotile should havespecific adsorption sites for the discrimination. It isconsidered that such the specific adsorption sites canbe formed in chrysotile structure because of thefollowing reasons.
Chrysotile hardly has any
surface charges, since Mg cation is located in theoctahedral site without any vacant sites [2].
Chrysotile, however, has positive or negative charges
on the edge according to the dissociation of OH
group [2]. It is expected that the amount of anamino acid is adsorbed on an edge of chrysotile.
The size of the octahedral sheet for Mg cation isslightly larger than that of the tetrahedral sheet for Si
cation in the chrysotile structure. Because of thisreason, the chrysotile sheet forms spiral cylindricalstructure as the outer surface of chrysotile is of theoctahedral sheet. The inner diameter of the cylinderof chryositle is 5 to 10 nm [18]. A scheme for theadsorption site for discrimination between D- and Lamino acid is shown in Fig. 5. When some Si-OH
groups of the edge are dissolved into water in Fig. 5(a), the edge of the tetrahedral sheet will havepositive charges. If the edge in the cylinder is
Chrysotile
SiO2 nH2O

Dissolution Tetrahedral sheet (Si site)(b)
COOOH2+
H
C
NH3+
OR

(a)

SiO2 surface
: Si

: Mg

Octahedral sheet (Mg site): OH: Hydrogen: OH 2+
bonding:O

Fig. 5 A possible schematic illustration of a selective
adsorption site on the edge of chrysotile is shown in(a). SiO4 tetrahedron is dissolved into water from theedge more easily than MgO. After the dissolution of
SiO4 tetrahedron, the structure of the edge would
become a hydrated form as shown in (b), where SiOH2+ and Mg-O- exist in the edge. Thus, amino acids
are easily adsorbed on the edge through hydrogenbonding. The side chains of the amino acids in thiswork are relatively hydrophobic ones except for Asp
and Glu.
In addition, the surface of chrysotile
generally has no charges. Based on these reasons, theside chain of amino acids should be placed on thesurface of chrysotile so that this may cause a selective
adsorption of L-amino acid on the edge of chrysotile.

formed as shown in Fig. 5 (b), an L-amino acidwould be likely adsorbed on the edge of the cylinder.
The side chain of the amino acid will be placed on ornear SiO2 surface of the adjacent layer, because there
are any charges on the chrysotile surface except theedge. The carboxyl and amino group of the aminoacid will be bonded to Si-OH2+ of the tetrahedral
sheet and Mg-O- in the octahedral sheet by thehydrogen bonding, respectively.
Therefore, theadsorptive formation of the amino acid to chrysotile
as shown in Fig 5 (b) would be expected. However,chrysotile involves both the clockwise andanticlockwise spiral structures in the almost the sameratio. Furthermore, the existence of the alternative
structure of which the edge prefers to adsorb Damino acid can be naturally supposed. Therefore, it
is not straightforwardly determined whether
chrysotile prefers to adsorb D-Pro, D-Asp, and L-Pheor their enantiomers. On the CD chromatogram, asmall peak, which was showed by the arrow,appeared in the same time as the peak on UV
chromatogram (Fig. 4). However, the peaks using
the CD detector was so small that it may not be
distinguished from noise. Thus, it is difficult toconclude whether chrysotile possesses capability forthe discrimination of one of the enantiomers of anamino acid from the other one.
Further
investigations will be necessary.
Conclusively, the adsorption behavior of 10
amino acids by chrysotile was investigated, wherechrysotile in the system of MgO-SiO2-H2O had neverbeen used for adsorption. Asp and Glu were morestrongly adsorbed by chrysotile than the other aminoacids. Amino acids examined in the present study
except for Asp and Glu were not adsorbed strongly
by chrysotile. There was a trend that amino acids
possessing a large side chain were more strongly
adsorbed than small amino acids except for Asp, Glu,
Ile, and Ser. It was found that the strength ofadsorption of amino acids by chrysotile wasdominated by the dissociation of both amino acids
and chrysotile, and also the solubility of amino acids
into water. This trend of the adsorption of aminoacids by chrysotile is similar to that by kaolinite [9].
This is probably due to the fact that the crystal
structure of chrysotile is almost the same as that ofkaolinite. Selectivity of D- or L-amino acids bychrysotile was not detected clearly in the present
work. It was difficult to identify small peaks of thechromatogram for CD. The peaks were too small to
distinguish whether the peaks indicate the selective
adsorption of D- or L-amino acid by chrysotile ornoises.
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