Senin, 02 Januari 2012

Artikel Kimia

Artikel Kimia

A New Generation of Ca2+ Indicatorsw ith Greatly Improved Fluorescence Properties*

Posted: 02 Jan 2012 02:05 AM PST

A New Generation of Ca2+ Indicatorsw ith Greatly Improved Fluorescence Properties*

Grzegorz Grynkiewicz$, Martin Poenie, and Roger Y. TsienB
From the Department of Physiology-Anatomy, Uniuersitoyf California, Berkeley, California9 4720

Abstract: A new family of highly fluorescent indicators has been synthesized for biochemical studies of the physiological
role of cytosolic free Ca2+. The compounds combine an 8-coordinate tetracarboxylate chelating site with stilbene chromophores. Incorporation of the ethylenic linkage of the stilbene into a heterocyclic ring enhances the quantum efficiency and photochemical stability of the fluorophore. Compared to their widely used predecessor, "quina," the new dyes offer up to 30-fold brighter fluorescence, major changes in  wavelength not just intensity upon Ca2+ binding, slightly lower affinities for Ca2+, slightly longer wavelengths of excitation, and considerably improved selectivity for Ca2+ over other divalent cations. These properties, particularly the wavelength sensitivity to Ca2+, should make these dyes the preferred fluorescent indicators for many intracellular applications, especially in single cells, adherent cell layers, or bulk tissues.

Sources: THE JOURNAOLF BIOLOGICAL CHE MISTRY

Analysis and simulation of digital electron holograms from Polymer latexes

Posted: 01 Jan 2012 02:05 PM PST

Analysis and simulation of digital electron holograms from Polymer latexes

Y.C. Wang, T.M. Chou and M. Libera*
Department of Chemical, Biochemical and Materials Engineering, Stevens Institute of Technology, Hoboken, NJ

Abstract
This paper describes procedures being developed to quantitatively measure mean inner potential and use such data in holographic image simulations of polymer microstructure. Results from holographic imaging experiments are presented using polymer latexes whose spherical geometry leads to a simple relation between specimen thickness and lateral position in a phase image.

Success with this approach demands accurate determination of the center and radius in the image of a latex sphere. These parameters are found via a least-squares fit to experimental image data. Recursive methods to enhance the accuracy of the measured Φo values are being developed based on characteristic features in difference images generated using an experimental phase image and a simulated phase image calculated from experimentallyderived center, radius, and data.

Key Words: Latex sphere, multi-phase polymer microstructure, styrene-butadiene-styrene triblock copolymer, transmission electron holography, phase-contrast imaging, electron-optical refractive index, mean inner potential, simulation of phase image, least-squares fit, spherical geometry.

Sources: Scanning Microscopy Vol. 11, 1997 (Pages 427-436)
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Minggu, 01 Januari 2012

Artikel Kimia

Artikel Kimia

Bagaimakah Rumus Umum Alkuna

Posted: 01 Jan 2012 07:57 AM PST

Alkuna adalah senyawa hidrokarbon alifatik tak jenuh yang mengandung ikatan rangkap tiga.

Alkuna dengan jumlah atom C sebanyak 4 memiliki atom H sebanyak 6. Sedangkan untuk alkena dengan jumlah atom C sebanyak 4 memiliki atom H sebanyak 8.
Jadi, rumus umum alkuna adalah:

CnH2n – 2

Bagaimanakah Sifat-sifat Alkena

Posted: 01 Jan 2012 01:51 AM PST

 Sifat Fisis Alkena

Titik leleh dan titik didih alkena hampir sama dengan alkana yang sesuai. Pada suhu kamar, suku-suku rendah berwujud gas, sukusuku sedang berwujud cair, dan suku-suku tinggi berwujud padat.

 Reaksi-reaksi Alkena

Alkena jauh lebih reaktif daripada alkana karena adanya ikatan rangkap. Reaksi alkena terutama terjadi pada ikatan rangkap tersebut. Reaksi-reaksi alkena sebagai berikut.

a) Reaksi Adisi (penambahan atau penjenuhan)
Reaksi adisi, yaitu pengubahan ikatan rangkap menjadi ikatan tunggal dengan cara mengikat atom lain.

Zat-zat yang dapat mengadisi alkena adalah:
(1) Gas hidrogen (H2)
CH2 = CH2+ H2 ⎯⎯→ CH3– CH3
etena                                         etana

(2) Halogen (F2, Cl2, Br2, dan I2)

(3) Asam halida (HCl, HBr, HF, dan HI)
Jika alkena menangkap asam halida berlaku aturan Markovnikov, yaitu atom H dari asam halida akan terikat pada atom C berikatan rangkap yang telah memiliki atom H lebih banyak.
Contoh:

b) Reaksi Pembakaran (oksidasi dengan oksigen)
Pembakaran sempurna alkena menghasilkan CO2 dan H2O.
C2H4 + 3 O2 ⎯⎯→ 2 CO2 + 2 H2O
Pembakaran tidak sempurna alkena menghasilkan CO dan H2O.
C2H4 + 2 O2 ⎯⎯→ 2 CO + 2 H2O

c) Reaksi Polimerisasi
Reaksi polimerisasi adalah reaksi penggabungan molekulmolekul sederhana (monomer) menjadi molekul besar
(polimer).
Contoh:
Polimerisasi etena menjadi polietena
n CH2 = CH2 ⎯⎯→ – CH2 – CH2– ⎯⎯→ [– CH2 – CH2 –]n
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