Presentation on theme: "Chemistry and Physics of Electrophoresis Bio-Rad Biotechnology Explorer™ Dye/STEM Kit."— Presentation transcript:
Chemistry and Physics of Electrophoresis Bio-Rad Biotechnology Explorer™ Dye/STEM Kit
Biotechnology Explorer™ | explorer.bio-rad.com 2 Instructors - Bio-Rad Curriculum and Training Specialists Sherri Andrews, Ph.D. Damon Tighe, Leigh Brown, M.A.
Biotechnology Explorer™ | explorer.bio-rad.com 3 Electrophoresis separates molecules by CHARGE and SIZE Electrophoresis means “to carry with electricity” Molecular sieve Buffer with charged ions Electricity Electrode
Biotechnology Explorer™ | explorer.bio-rad.com 4 Components of electrophoresis – The Buffer Buffer with charged ions –Must buffer the DNA and not change pH significantly with increase in temperature –Must be capable of carrying charge –Must be capable of solubilizing a gel matrix molecule –Must maintain an orderly distribution of the electric field –Must not interfere with future reactions (ie isolation of DNA fragments, ligation of DNA fragments, cloning of DNA) or worded differently, must not chemically react with the samples –Must not heat up too much during a run
Biotechnology Explorer™ | explorer.bio-rad.com 5 Components of electrophoresis – The Buffer Most developments for slab gel electrophoresis occurred near 1971 and since then…not much has changed with respect to buffers, electrodes and molecular sieves… –Most design was based on Protein gel work and it was assumed that properties would carry over to DNA electrophoresis –Buffers for DNA agarose gel electrophoresis have almost exclusively been TAE – Tris(2-amino-2-(hydroxymethyl)-1,3-propanediol) acetate EDTA TBE – Tris borate EDTA
Biotechnology Explorer™ | explorer.bio-rad.com 6 Components of electrophoresis – The Buffer – Tris based “For reasons not fully evident today, Tris became established as the favored cation for DNA Electrophoresis.” J.R. Brody, S.E. Kern/Analytical Biochemistry 333 (2004) 1-13
Biotechnology Explorer™ | explorer.bio-rad.com 7 TAE Buffer –Pros : Does not interfere with subsequent enzymatic reactions such as ligations –Cons: Lower buffering capacity and higher conductivity than TBE buffer, temperature dependent pH TBE Buffer –Pros: Higher buffering capacity and lower conductivity than TAE buffer –Cons: Borate can interfere with downstream DNA enzymatic reactions due to interactions with sugar groups in DNA, temperature dependent pH Components of Electrophoresis – The Buffers
Biotechnology Explorer™ | explorer.bio-rad.com 8 Components of Electrophoresis – The Electrodes Electrodes must be capable of carrying charge with minimal chemical transformations occurring while immersed in a salty solution –Must be relatively chemically inert in a salty solution –Must be capable of carrying a charge with a voltage difference of between V DC –Must be maleable enough to mold to desired dimensions –Reusable for fairly permanent fixation into an instrument (ie do not want to have to replace regularly)
Biotechnology Explorer™ | explorer.bio-rad.com 9 Commercial electrodes are exclusively made of platinum –Pros: High conductivity, Extremely low reactivity –Cons: Expensive Components of Electrophoresis – The Electrodes
Biotechnology Explorer™ | explorer.bio-rad.com 10 Components of Electrophoresis – Molecular Sieve The Molecular Sieve must be capable of separating molecules via size –Should be easily moldable –Should not chemically interact with the molecules being separated –Should have a high enough melting point that electrophoretic runs will not melt it –If polymeric, should be of molecular purity such that there is no batch to batch differences –Must be able to form a variety of pore sizes
Biotechnology Explorer™ | explorer.bio-rad.com 11 Components of Electrophoresis – Molecular Sieve Most commonly used for horizontal electrophoresis is agarose –Complex polysaccharide agar-bearing marine algae –Neutrally charged, less chemical complexity than agar (which also contains agaropectin which has heavily modified acidic side-groups) –Low likelihood to react with DNA –Forms pore sizes amenable to separating DNA of 100 basepairs and up in size –Gelling temperature : 35-40ºC –Melting temperature : 86-90ºC
Biotechnology Explorer™ | explorer.bio-rad.com 12 So how do we design an electrophoresis chamber? Dye Electrophoresis Commercial versus built box comparisons
Biotechnology Explorer™ | explorer.bio-rad.com 13 The Chemistry of Electrophoresis Electrolysis always occurs during electrophoresis.
Biotechnology Explorer™ | explorer.bio-rad.com 14 Electrochemistry in Action What other chemical reactions will occur? –If the buffer is Tris acetate EDTA Electrodes made of copper Electrodes made of galvanized steel Electrodes made of aluminum –If the buffer is Tris borate EDTA Electrodes made of copper Electrodes made of galvanized steel Electrodes made of aluminum
Biotechnology Explorer™ | explorer.bio-rad.com 15 Polymer Chemistry in Action What is the impact of different percentages of agarose gels on separation? What is the impact of a different polymer on separation? –Gelatin versus agarose
Biotechnology Explorer™ | explorer.bio-rad.com 16 Simple DC Circuit model Measure Voltage and Current, calculate Resistance Determine impact of different buffers on resistance Physics of Electrophoresis R = Electrophoresis system V = Battery tower V = IR
Biotechnology Explorer™ | explorer.bio-rad.com 17 Physics of Electrophoresis More complex model of resistance – contribution of electrophoresis components on resistance gel Side view of gel box Direction of current buffer R1 R2 R3 R4 V = Battery tower R TOT R TOT = 1 (1/(R1+R2+R3) + 1/R4)
Biotechnology Explorer™ | explorer.bio-rad.com 18 Physics of Electrophoresis – Measurements versus Calculations Use a conductivity meter to measure the conductivity of an electrophoresis buffer and then convert to conductance –Conductance = Conductivity (V/m) / kc (conductivity constant) – For the Vernier probe kc = 1 m -1 Create a simple DC circuit using the gel box, buffer, leads and electrodes, and measure the voltage and current of the system using a multimeter. Calculate the resistance of the electrophoresis buffer and then convert to conductance. –Resistance (ohm) = Voltage (V)/current (Amp) –Conductance = 1/Resistance Are the measured and calculated values the same?
Biotechnology Explorer™ | explorer.bio-rad.com 19 Physics of Electrophoresis – Ohmic heating Energy dissipated per unit time = Energy dissipated per charge passing through resistor x Charge passing through resistor per unit time Q I 2 R which comes from P=VI=I 2 R=V 2 /R Also know that Q = vC T where C is the volume specific heat capacity of the substance, v is the volume in ml, and T is the change in temperature –Joule – a unit of energy –Coulomb – a unit of electrical charge –Volt – a unit of electrical potential (joule per coulomb) –Watt – a unit of power or energy per unit time (joule per second) –Ampere – a unit of current flow (coulomb per second) –Specific heat – the amount of energy required to raise the temperature of a given volume of a substance 1ºC
Biotechnology Explorer™ | explorer.bio-rad.com 20 Experimental setup Do you have a bomb calorimeter lab? Use it to measure the specific heat capacity of your buffer and agarose setup! –Measure the volume of buffer and estimate total volume of buffer+gel –Prepare an electrophoresis chamber with your buffer of choice –Measure the voltage of your power supply (batteries) –Measure the initial temperature of your system –Measure the initial current of your system –Run electrophoresis for 20 minutes –Measure the final voltage, final temperature, and final current Physics of Electrophoresis – Ohmic heating – Experimental
Biotechnology Explorer™ | explorer.bio-rad.com 21 Physics of Electrophoresis – Ohmic heating – Calculations P ave = V ave x I ave –Calculate the average voltage and average current and use these values to calculate the average power E = P ave x t –Calculate the heat energy dissipated by the electrophoresis system in this process where t is 20 minutes converted into units of seconds Q =vc T –Calculate the energy absorbed by the electrophoresis system where v is the volume of the electrophoresis resistant components (buffer + gel), c is the specific heat capacity of the buffer + gel (estimated at 4 J/K*ml, slightly less than water…) and T is the difference between the initial and final temperate in degrees Kelvin (same as the difference in degrees Celsius) According to the law of conservation of energy, the energy dissipated by the resistor should equal the energy absorbed by the electrophoresis system. Do your findings support this? Compute the percent variation between the two values. Q =vc T –Or, assuming that all the energy generated by the system was dissipated as heat, calculate the specific heat capacity of the electrophoresis resistant components for differing buffering systems Do you find your results match “common knowledge” that TBE heats up less than TAE (has a higher heat capacity?)
Biotechnology Explorer™ | explorer.bio-rad.com 22 Challenge! From what you know about the chemistry and physics of the system –Design a new buffering system which Does not react with DNA Has a high heat capacity Has a high buffering capability –Design a new electrode system which Does not react electrochemically Does not cost a lot –Design a new molecular sieve system which Separates molecules of the appropriate size Separates molecules in a shorter time –Put it all together!!