4. Discussion-Carrageenan gels bec

4. Discussion
-Carrageenan gels because helices from different chains associate laterally. Therefore a necessary condition to form a gel is
to induce the coil–helix transition. This transition occurs below
a critical temperature that is strongly dependent on the type
and concentration of ions that are present including the counterions. However, the critical temperature is independent of the
polysaccharide concentration as long as the contribution of the
counterions can be neglected. The association of helices accelerates
with increasing concentration and for a given concentration with
decreasing temperature, as was reported in more detail elsewhere
(Meunier et al., 1999).
It is remarkable that the gel stiffness increased with increasing
the cation concentration in the case of pure K
+
, at least up to
100 mM, but the elastic modulus decreased in the case of pure
Ca
2+
. As was mentioned in the introduction, it has been reported
that Gel increases at low Ca
2+
and decreases only at higher Ca
2+
concentrations, while here we only observe a decrease. However,
the initial increase may have been observed for samples for which
G

had not yet reached its steady state value, because the gelation
kinetics can be very slow at lower salt concentrations. Another
remarkable difference between gels induced by pure K
+
and by
pure Ca
2+
is that the latter become increasingly turbid when
more ions are present, while the former remain transparent. The
most likely cause for the difference between gelation induced
by pure K
+
and pure Ca
2+
is that in the presence of higher Ca
2+
concentrations the helices stack into thicker strands that form
more heterogeneous gels.
In the presence of pure K
+
the association of the helices was
very slow at low polymer concentrations even 15 degrees below
Tc. Most likely the association was inhibited by electrostatic repulsion between the helices, which explains why adding NaCl strongly
sped up this process. NaCl did not modify the coil–helix transition
induced by 10 mM KCl, at least below 0.2 M, but it screened the electrostatic repulsion. By itself NaCl induced the formation of weak
turbid gels only above 0.1 M.
The synergistic effect on gelation of adding K
+
and Ca
2+
together
was very strong and cannot be explained only in terms of the ionic
strength. Especially at low ionic strengths the effect of adding Ca
2+
on K
+
induced gelation was much stronger than that of Na+
even
though neither of the ions influenced the coil–helix transition at
these lower ionic strengths. This observation indicates that specific
binding of Ca
2+
to -car occurred as was already suggested by Doyle
et al. (2002), which reduced the charge density of the chains much
more effectively than merely screening.
Inversely, addition of as little as 2 mM KCl completely modified
Ca
2+
induced gelation. Instead of decreasing with increasing Ca
2+
concentration in pure CaCl2, the elastic modulus increased with
increasing Ca
2+
concentration when KCl was present. We note that
by itself 2 mM KCl did not give rise to a significant increase of the
shear modulus. Clearly, such small amounts of K
+
do not influence
the electrostatic interactions significantly, so the effect must be
attributed to the influence on the helix formation. The much lower
turbidity in the presence of K
+
even at high Ca
2+
concentrations also
indicates that different types of gels are formed that are much more
homogeneous. Remarkably, gels were formed rapidly in 2 mM KCl
even for [CaCl2] < 5 mM, i.e. under conditions were pure salts did
not cause a significant increase of the shear modulus.
The case of 5.3 mM CaCl2 is peculiar in that addition of KCl
led to a decrease of Gelup to 1 mM. At higher KCl concentrations
Gel increased again, but remained slightly lower than without
KCl However, the gels with added KCl were formed much more
rapidly, compare Figs. 3b and 6. The minimum of Gelas a function
of the KCl concentration at 5.3 mM CaCl2can be understood from
the fact that when KCl is added the gel is no longer induced by
Ca
2+
, but by K
+
. At higher CaCl2 concentrations the K
+
induced
gel is stronger than the Ca
2+
induced gel, but in the presence of
5.3 mM CaCl2it is slightly weaker.
The increase of the stiffness of K
+
induced gels by the reduction
of electrostatic repulsion should also apply to screening by KCl at
temperatures significantly below Tc. The origin for the increase of
the elastic modulus with increasing KCl concentration could therefore be, at least in part, the effect of screening rather than the effect
of increasing Tc. In fact, if we consider the further addition of KCl
in excess of 10 mM in the same way as that of NaCl and CaCl2, we
find that the increase of the Gelwith increasing ionic strength by
adding more KCl shown in Fig. 2a is intermediate between that by
adding NaCl and by adding CaCl2.
5. Conclusion
-Car gels are formed below the coil–helix transition temperature in the presence of KCl or CaCl2, but with different structures.
B.T. Nguyen et al. / Carbohydrate Polymers 112 (2014) 10–15 15
In KCl the gel stiffness increases with increasing salt concentration
and the gels remain transparent, while in CaCl2the gel stiffness
decreases with increasing salt concentration and they become
increasingly turbid.
Addition of NaCl reduces the electrostatic repulsion between the
helices which facilitates their association for K
+
induced gelation.
As a consequence the gel stiffness and the gelation rate increase
with increasing NaCl concentration. A similar effect on K
+
induced
-car gelation is observed when CaCl2is added. However, the effect
is much stronger than for NaCl at the same ionic strength indicating
specific binding of Ca
2+
to -car.
The presence of a small amount of KCl strongly modifies the Ca
2+
induced gelation of -car. Especially at higher CaCl2concentrations
it leads to a strong increase of the elastic modulus and a reduction of
the turbidity. The synergistic between Ca
2+
and K
+
is most striking
when the gels are weak in the pure salts

Hasil (Bahasa Indonesia) 1:

4. diskusi-Carrageenan gel karena helices dari rantai berbeda mengasosiasikan lateral. Oleh karena itu adalah kondisi yang diperlukan untuk membentuk geluntuk menginduksi transisi coil-helix. Transisi ini terjadi di bawahsuhu kritis yang sangat tergantung pada jenisdan konsentrasi ion yang ada termasuk counterions. Namun, suhu kritis independen darikonsentrasi polisakarida selama kontribusicounterions dapat diabaikan. Mempercepat Asosiasi helicesdengan meningkatnya konsentrasi dan konsentrasi yang diberikan denganpenurunan suhu, seperti yang dilaporkan di selengkapnya di tempat lain(Meunier et al., 1999).Hal ini luar biasa bahwa gel kekakuan meningkat dengan meningkatnyakonsentrasi kation dalam kasus murni K+, setidaknya sampai100 mM, tetapi modulus elastis menurun dalam kasus murniCA2 +. Seperti yang telah disebutkan dalam pendahuluan, telah dilaporkanbahwa Gel Peningkatan di Ca rendah2 +dan menurun hanya di Ca lebih tinggi2 +konsentrasi, sementara di sini kita hanya mengamati penurunan. Namun,peningkatan awal mungkin telah diamati untuk sample yangGmemiliki belum mencapai nilai mapan, karena gelationkinetika bisa sangat lambat pada konsentrasi garam yang lebih rendah. Lainperbedaan yang luar biasa antara gel yang disebabkan oleh murni K+danCa murni2 +adalah bahwa yang terakhir menjadi semakin keruh KapanIon lain hadir, sementara mantan tetap transparan. Thekemungkinan menyebabkan perbedaan antara gelation diinduksioleh murni K+dan Ca murni2 +adalah bahwa hadapan Ca lebih tinggi2 +konsentrasi helices tumpukan ke helai tebal yang berupasemakin heterogen gel.Hadapan murni K+adalah asosiasi helicessangat lambat pada rendah polimer konsentrasi bahkan 15 derajat di bawah iniTC. Kemungkinan Asosiasi dihambat oleh elektrostatik tolakan antara helices, yang menjelaskan mengapa menambahkan NaCl kuatmempercepat proses ini. NaCl Apakah tidak memodifikasi transisi coil-helixdisebabkan oleh 10 mM KCl, setidaknya di bawah 0, 2 M, tapi itu diputar tolakan elektrostatik. Dengan sendirinya NaCl disebabkan pembentukan lemahgel keruh hanya di atas 0.1 M.Efek sinergis pada gelation menambahkan K+dan Ca2 +bersama-samasangat kuat dan tidak dapat dijelaskan hanya dalam hal ionikkekuatan. Terutama pada rendah ionik kekuatan efek menambahkan Ca2 +pada K+diinduksi gelation adalah jauh lebih kuat daripada Na +bahkanMeskipun tak satu pun dari ion yang dipengaruhi coil-helix transisi diini menurunkan kekuatan ionik. Pengamatan ini menunjukkan yang khasmengikat CA2 +-Mobil terjadi seperti itu sudah disarankan oleh Doyleet al. (2002), yang mengurangi biaya kepadatan rantai jauhlebih efektif daripada hanya pemutaran.Terbalik, penambahan sesedikit 2 mM KCl benar-benar diubahCA2 +diinduksi gelation. Bukannya menurun dengan peningkatan Ca2 +konsentrasi di CaCl2 murni, modulus elastis meningkat denganincreasing Ca2+concentration when KCl was present. We note thatby itself 2 mM KCl did not give rise to a significant increase of theshear modulus. Clearly, such small amounts of K+do not influencethe electrostatic interactions significantly, so the effect must beattributed to the influence on the helix formation. The much lowerturbidity in the presence of K+even at high Ca2+concentrations alsoindicates that different types of gels are formed that are much morehomogeneous. Remarkably, gels were formed rapidly in 2 mM KCleven for [CaCl2] < 5 mM, i.e. under conditions were pure salts didnot cause a significant increase of the shear modulus.The case of 5.3 mM CaCl2 is peculiar in that addition of KClled to a decrease of Gelup to 1 mM. At higher KCl concentrationsGel increased again, but remained slightly lower than withoutKCl However, the gels with added KCl were formed much morerapidly, compare Figs. 3b and 6. The minimum of Gelas a functionof the KCl concentration at 5.3 mM CaCl2can be understood fromthe fact that when KCl is added the gel is no longer induced byCa2+, but by K+. At higher CaCl2 concentrations the K+inducedgel is stronger than the Ca2+induced gel, but in the presence of5.3 mM CaCl2it is slightly weaker.The increase of the stiffness of K+induced gels by the reductionof electrostatic repulsion should also apply to screening by KCl attemperatures significantly below Tc. The origin for the increase ofmodulus elastis dengan meningkatnya konsentrasi KCl karena itu bisa, setidaknya sebagian, efek skrining bukan efekdari meningkatnya Tc. Pada kenyataannya, jika kita mempertimbangkan penambahan lebih lanjut KCllebih dari 10 mM dengan cara yang sama seperti yang dari NaCl dan CaCl2, kamimenemukan bahwa peningkatan Gelwith meningkatkan kekuatan ionik olehmenambahkan lebih KCl, ditampilkan dalam Fig. 2a antara yang olehmenambahkan NaCl dan dengan menambahkan CaCl2.5. kesimpulan-Mobil gel dibentuk di bawah suhu transisi coil-helix hadapan KCl atau CaCl2, tapi dengan struktur yang berbeda.B.T. Nguyen et al. / polimer karbohidrat 112 (2014) 15 10-15Dalam KCl kekakuan gel meningkatkan dengan bertambahnya konsentrasi garamdan gel tetap transparan, sementara CaCl2the gel kekakuanmenurun dengan peningkatan konsentrasi garam dan mereka menjadisemakin keruh.Penambahan NaCl mengurangi elektrostatik tolakan antarahelices yang memfasilitasi mereka Asosiasi k+diinduksi gelation.Akibatnya gel kekakuan dan gelation tingkat kenaikandengan meningkatnya NaCl konsentrasi. Efek serupa pada K+diinduksi-Mobil gelation diamati ketika CaCl2is ditambahkan. Namun, efeklebih kuat daripada untuk NaCl di sama ionik kekuatan menunjukkanspesifik mengikat CA2 +ke - mobil.Kehadiran sejumlah kecil KCl sangat memodifikasi Ca2 +diinduksi gelation dari - mobil. Terutama pada tinggi CaCl2concentrationsitu mengarah pada peningkatan yang kuat dari modulus elastis dan pengurangankekeruhan. Sinergis antara Ca2 +dan K+yang paling mencolokKetika gel lemah dalam garam murni