Most of the sequences were from Lulu Qian’s sequences, designed as described in . Modifications were added using a computer program NUPACK, and staple sequences for the rectangular origami were taken from Rothemund’s . All oligonucleotides were ordered from Integrated DNA Technologies (IDT) Inc. Most of the DNA molecules were ordered standard desalted and unpurified, and modified molecules are ordered HPLC purified. Staple strands were ordered in wet plate form at 150μM in 1x TE buffer. Dried DNA was reconstituted in an appropriate amount of ddH2O, and quantified with the biophotometer.
1x TE/Mg2+ (TE=Tris-EDTA, Tris 40 mM, EDTA 2 mM, and Magnesium acetate, 12.5 mM, pH 8.0) buffer was used for all experiments.
Gel Electrophoresis/ Gel Purification
5% polyacrylamide gels of 1 mm in thickness were used. Native gel was polymerized by mixing 3.75 ml 19:1 40% polyacrylamide, 1 mL 10x TAE/Mg++, 5.25 ml ddH2O, 60 μL 10% APS, 6 μL TEMED. Native gels were run at 150V at 25 ºC. 1x TAE/Mg2+ was used as a running buffer. Gels were stained with SybrGold for 15 min and then scanned with Quantity 1 FX scanner. Some complexes were annealed using PCR machine and gel purified using bigger gel. Same recipe was used to make native gel. Glycerol was used as a loading dye for the modified strands. Under UV light, the bands were cut and DNA was obtained by diffusion in 1x TE/Mg2+ buffer for 48 hours.
DNA origami formation
Rectangular DNA origami consists of M13 viral DNA and 202 DNA staples. Staples at the side of the origami were not included in the 202 staples to avoid the stacking problem. Sometimes, a probe at the walker start site was intentionally left out, making a hole in the middle of the origami. M13 scaffold DNA and staple strands were mixed in 1x TE/Mg2+ buffer with molar ratio of 1:4 between the M13 and staple strands. For the speed of assembly, origami were annealed at 90nM, and diluted into different final concentration for the purpose of various experiments; 15 nM of M13 were used for SPEX experiment, and 10 nM, 5 nM, 1 nM for AFM experiment. Origami was annealed by heating up to 90 ºC then cooling down to 20 ºC at 1 ºC/min using an Eppendorf PCR machine. After annealing, tracks were added to the annealed origami at 1:6 ratio of M13 to tracks. Walker goal and a walker start complex which is a preannealed complex “walker – walker inhibitor – track1 – probe for track 1” were also added to the origami at 1:1 ratio of M13 to strands. Mixed solution was incubated at 37 ºC overnight.
Atomic force microscopy (AFM)
5 μL of sample was deposited onto a freshly cleaved mica, and 20 μL of 1x TE/Mg2+ buffer was added. 40+ μL of 1x TE/Mg2+ buffer was added to the fluid cell and the sample was scanned in a tapping mode with DNP tips. When streptavidin was used, 1 μL of streptavidin was added to the mica, and we waited 15 or more minutes before imaging to ensure the streptavidin and biotin could bind. The exception to this was when we used the Walker Lock-Down Protocol.
Walker Lock-Down Protocol
To see what this protocol was used for see the AFM experiments page. (When we tried this protocol, the concentration of origami was too high, so we tried using 2 μL of sample instead of 5 μL, but in the future we will dilute the sample before using it, and we will stick to using 5 μL of sample.) The sample, buffer, and streptavidin are mixed on mica as above, but after waiting 15 minutes, we do the following:
1. add 20 μL of 1x TE/Mg2+ buffer to mica, then remove as much liquid as possible (40+ μL) from the mica without touching the mica.
2. add 40 μL of 1x TE/Mg2+ buffer to mica, then remove as much liquid as possible. Repeat this 3 times. (In the future we may repeat this more times to remove as much excess streptavidin as possible.)
3. add 20 μL of 1x TE/Mg2+ buffer to mica.
4. add biotinylated† walkers at a 1-to-1 ratio to the tracks or higher (excess should not affect the results, and when implemented a 1-to-1 ratio would require a volume that is too small to be pipetted, so excess is likely necessary.)
5. wait for 15+ minutes before imaging sample.
†We have only seen the walkers when 5'-end biotinylated walkers were used, but 3'-end biotinylated walkers were only tried at a low concentration.
Based on our preliminary results using this protocol, streptavidin binds to mica, and if it is ever lifted off of the mica, it can bind to walkers on origami. Thus, we will try using the above protocol, but using 1x TE/Mg2+ with 100 μM Na+ instead to minimize the amount of streptavidin that binds to mica.
After cuvettes are cleaned with ddH2O and 70% ethanol, 10 nM - 1 uM sample was loaded in 1.5mL 1x TE/Mg2+ buffer into each cuvette for kinetics experiment in solution. For random walking experiment on origami, 200 ul of 15 nM samples were used. Fluorescent level of each cuvette was observed in real time.
Normalizing SPEX Data in MATLAB