ELETTROFORESI CON ELETTROLITA LIBERO Problemi di convezione

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1 ELETTROFORESI CON ELETTROLITA LIBERO Problemi di convezione
BASI DI ELETTROFORESI Il campo di forze che causa il trasporto differenziale e' di tipo elettrico. L'elettroforesi ha avuto il suo ideale campo di applicazione in bioanalitica. La separazione si realizza in base al rapporto carica/raggio della molecola. ELETTROFORESI CON ELETTROLITA LIBERO Problemi di convezione ELETTROFORESI CAPILLARE ELETTROFORESI CON SUPPORTO

2 Electrophoretic mobility
Voltage difference, E = voltage applied/distance between electrodes; generally expressed as volts/cm Charge on molecule, q Frictional component, f, determined by size and shape of molecule, pore size of matrix, viscosity of buffer Velocity of particle, v= Eq/f Mobility of particle, µ = v/E = q/f Size/shape Charge Both size/shape and charge Separation can be effected by

3 Electrophoretic migration
V = IR Ohm law Voltage is a function of current and resistance Resistance decreases during electrophoretic run, therefore current increases if maintaining constant voltage Why minimize current increase during run? As current increases, power increases- much of power is dissipated as heat Heat affects electrophoretic separation- diffusion increases; samples can be sensitive to heat; buffer viscosity decreases therefore resistance decreases and uneven heating occurs due to best cooling at gel edges

4 EFFICIENZA IN ELETTROFORESI
Scan lucido DT= coeff. diff. totale q±1 in caso di altri meccanismi di dispersione

5 EFFICIENZA IN ELETTROFORESI
Scan lucido

6 F z E = - = D m z EX V F N zV = 20 N z V T = F R 2 J
EFFICIENZA IN ELETTROFORESI Il campo forza elettrica per mole di molecole sara' dato da F z E = - = D m ext z EX V F V= caduta potenziale Sostituendo si ottiene che: N z V T = F R 2 J NUMERO DI PIATTI In condizioni ideali si ottiene che: N zV = 20 Poiche' F C mol = 96000 / , V= 100÷50000V, z =1÷10 si ha che 6 N=2000÷10x10 ALTA EFFICIENZA

7 ELETTROFORESI SU SUPPORTO
Aumento del rapporto superficie/volume Tipi di supporto: acetato di cellulosa carta gel di poliacrilammide (PAGE) agarosio I supporti danno un effetto di setaccio per separare in base alle dimensioni, uno volta che le specie siano state caricate in ambiente tamponato Esempi di applicazioni: Analisi di proteine (Progetto “Proteoma”) Sequenziatura del DNA (Progetto “Genoma”)

8 Protein electrophoresis
Proteins are sequence of amino acids that can be ionized depend on their acid or basic character.  The N- and C- terminal and T-groups of the polypeptide can be ionized, contributing to the overall charge.  The protein’s net electric charge is the sum of the electric charges found on the surface of the molecule as a function of the environment. - At the pI of a specific protein, the protein molecule carries no net charge and does not migrate in an electric field. - At pH above the pI, the protein has a net negative charge and migrates   towards the anode. - At pH below the pI, the protein obtains a net positive charge on its   surface and migrates towards the cathode.

9 Polyacrylamide Gel Electrophoresis (PAGE)
Proteins, although can be used for separation of small DNAs Vertical electrophoresis setup with thin gels (0.2-2 mm) Can be analytical or preparative scale Can be denaturing (addition of SDS and reducing agent to sample and SDS to buffer; often also add denaturant to sample buffer; samples are heated before electrophoresis to ensure denaturation) or native conditions Separation: by size- denaturing; SDS treatment results in uniform charge density by charge and size/shape- native by charge/pI- isoelectric focusing

10 PAGE Total percentage of acrylamide- acrylamide and bis-acrylamide- determines pore size of gel Discontinuous gels are most common for highest resolution: Low percentage (3%) and low pH (6.8) are used for stacking gel- all proteins run readily through until hit higher percentage and pH (8.6) of running or separating gel (4-20%), then stack up due to change in pH.

11 Formazione di un gel PA Il setaccio tridimensionale si forma dalla co-polimerizzazione del monomero attivato (acrilammide) e del composto che forma i legami trasversali (metilen-bis-acrilammide)

12 Determinazione del MW via SDS PAGE
La mobilità elettroforetica delle proteine in un gel SDS PAGE è inversamente proporzionale al logaritmo del loro peso molecolare

13 SDS-PAGE: MW separation
1. Denaturing method relying on two components: SDS and reducing agents 2. Reducing agent ensures all disulfide bonds are reduced and SDS denatures and coats protein with basically uniform charge density 3. Native charge masked and native shape lost so separation primarily by size. Linear relationship of logMW and Mr allows MW estimation from comparison with standard curve 4. Separation may be quite different from gel to gel: protein standards should be included in each electrophoresis run. MW standards are also available to allow accurate MW determination of the proteins.

14 Charge of a protein vs. pH

15 Focalizzazione isoelettrica
Un gradiente di pH si forma nel gel prima di caricare il campione. Caricato il campione, viene applicato il voltaggio. Le proteine migreranno fino al punto in cui il pH è uguale al loro pI, dove la loro carica netta è nulla. Le proteine formano bande che possono essere tagliate e usate per ulteriori esperimenti.

16 2D PAGE in proteomica Un campione proteico è inizialmente frazionato nella prima dimensione mediante focalizzazione isoelettrica. Il gel di focalizzazione è quindi combinato con un PAGE in direzione ortogonale alla prima. Le proteine aventi stesso pI sono quindi separate in base al MW 2D PAGE del proteoma di E.coli: si tratta di più di 1000 proteine

17 ELETTROFORESI SU SUPPORTO

18 Capillary Zone Electrophoresis (CZE)
Major drawbacks of gel electrophoresis: speed of analysis.  Speed could be improved by increasing the electric current of the system. Large amount of heat would be generated: high convection CZE uses silica fused capillaries ranging from to millimeters in outer diameter to dissipate the heat produced. Increasing the electric fields produces very efficient separations and reduces separation times. Very small amount of sample (0.1 to 10 nL) is required.  The sample solution is injected at one end and a electric field of 100 to 700 volts/centimeter is applied across the capillary.

19 Electrophoresis in a buffer filled, narrow-bore capillaries
CZE– The Basics Electrophoresis in a buffer filled, narrow-bore capillaries Each capillary is about μm in internal diameter When a voltage is applied to the solution, the molecules move through the solution towards the electrode of opposite charge Depending on the charge, the molecules move through at different speeds Separation is achieved

20 CZE– The Basics / II A photocathode is then used to measure the absorbencies of the molecules as they pass through the solution The absorbencies are analyzed by a computer and they are represented graphically

21 CZE– The Basics/III The movement of ions solely due to the electric field, potential difference Cations should migrate toward cathode Anions should migrate toward anode Neutral molecules do not favor either

22 CZE– Basic theory

23 CZE– Basic theory/II

24 CZE– Basic theory/III

25 CZE– Basic theory/IV

26 CZE– Il flusso elettroosmotico

27 CZE– Flusso elettroosmotico anodico
Se la parete del capillare viene caricata positivamente allora:

28 CZE– Il flusso elettroosmotico/II
= 0

29 CZE– Il Potenziale z

30 CZE– Migrazione

31 CZE– Migrazione/II

32 CZE– Migrazione/III

33 CZE– Rivelatori

34 CZE– Rivelatori

35 CZE– UV/vis

36 CZE– UV/vis /II

37 Seminario Prof. De Lorenzi
CZE e tecniche ibride Seminario Prof. De Lorenzi Università di Pavia