[3dem] Liposomes in ice (Reza Khayat)

Fri May 12 13:01:33 PDT 2017

Hi, Reza,

With Hideki Shigematsu we’ve done a lot of imaging of liposomes, and here are some more observations to add.

1.  The particular lipid makes a big difference.  PC makes small liposomes (20-30 nm), PE makes big ones (1000 nm, obvious from the milky-looking suspension).  Use lipids above their transition temperature.  We like POPC, and even better, POPC+cholesterol.
2.  The way you form them makes a difference.  We dry the lipids, then hydrate them overnight (yields big, multilamellar structures) then sonicate (this gives SUVs) but then we solubilize in detergent and later remove detergent because we’re reconstituting protein too.
3.  Extrusion doesn’t work quite as expected.  If we make big PE liposomes and extrude them, either 30nm or 50 nm filters give us 100 nm and larger liposomes.  We prefer controlling size with lipid composition.  Dynamic light scattering can give a rough measure of size.
4.  We use a thin carbon film spanning holes for liposomes to adsorb to.  In the final prep applied to the grid the overall lipid concentration is a few mM.

Fred Sigworth

Professor
Cell Molec Physiology
Molec Biophys and Biochem
Yale University

On May 12, 2017, at 3:39 PM, Kenneth Goldie <k.goldie at unibas.ch<mailto:k.goldie at unibas.ch>> wrote:

Dear Reza,

a quick indication with having a good concentration of liposomes for making cryo grids with a reasonable dispersion is to have a solution that looks pretty milky. As white as milk is good! Adsorb undiluted. If not try to get the concentration up. Best results work with liposomes of 100nm or less. Ideally they should be in water or low salt buffer. Avoid buffers with any additives as much as possible. No ethanol for example.
My feeling is the liposomes sit nicely dispersed in the buffer drop when you add it to a glow discharged grid, and float there like balloons and do not settle as easily as say single particle protein type samples. The key is to adsorb for much longer times than "usual". I usually use 5 minutes adsorbing on-grid in tweezers on a bench and not in the cryo-plunger chamber.  If you still have problems or cannot increase the sample concentration you can adsorb for an hour or more or even overnight as long as you keep the grid "wet". This is easily done by using a large petri dish that you can "humidify" by wetting several pieces of round filter paper in the bottom with water and add a lid. Place your grid with sample in there for the desired time (also sometimes helps for low concentrations of protein samples, filaments or planar crystals). Then take it out and cryo-plunge. Being able to plunge from the backside (opposite the side you adsorbed on) can also up the concentration sometimes b
y pulling the liposomes down onto the surface and sometimes producing a filter effect to get more in holes. They may tend to stick and pull off on the filter paper if you blot from the sample adsorbed side.
Holey films will result in more sample in holes as there is less background carbon (same goes for proteins!) for them to stick to. Even in low magnification, you should see the liposome spheres if you defocus a lot which helps you pinpoint them.

Best of luck

Ken

-----------------------

Ken Goldie, PhD
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Today's Topics:

  1. Re: Liposomes in ice (Terje Dokland)

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Message: 1
Date: Fri, 12 May 2017 12:00:20 -0500
From: Terje Dokland <dokland at uab.edu<mailto:dokland at uab.edu>>
To: Reza Khayat <rkhayat at ccny.cuny.edu<mailto:rkhayat at ccny.cuny.edu>>
Cc: 3dem <3dem at ncmir.ucsd.edu<mailto:3dem at ncmir.ucsd.edu>>
Subject: Re: [3dem] Liposomes in ice
Message-ID: <842FF5C0-9F1E-43BF-9110-E7D234E089D8 at uab.edu<mailto:842FF5C0-9F1E-43BF-9110-E7D234E089D8 at uab.edu>>
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Maybe you didn't get that many liposomes in the first place? We have had similar problems with extruded liposomes. It seemed that a lot of the lipid did not form liposomes at all, but remained in micellar form, contaminating the background.

On May 12, 2017, at 9:30 AM, Reza Khayat wrote:

Hi,

We are preparing extruded liposomes (200nm). Problem is we're getting very few liposomes in ice. The starting lipid concentration (DOPG:https://avantilipids.com/product/840475/) is 4mg/ml.

Reza

Reza Khayat, PhD
Assistant Professor
City College of New York
Department of Chemistry
New York, NY 10031
From: Ludtke, Steven J <sludtke at bcm.edu<mailto:sludtke at bcm.edu>>
Sent: Friday, May 12, 2017 10:27 AM
To: Reza Khayat
Subject: Re: [3dem] Liposomes in ice

You'll have to define exactly what you mean by liposomes. Extruded SUVs are one of our favorite test/training specimens because they are so easy to prepare and freeze. When most people say liposome, they mean something much larger. You'll need to give some details if you want useful advice. What are the lipid constituents, how large are the liposomes, how are they prepared, etc.   Would also be useful to see a representative image or two.

On May 12, 2017, at 9:18 AM, Reza Khayat <rkhayat at ccny.cuny.edu<mailto:rkhayat at ccny.cuny.edu>> wrote:

Hi,

We're trying to visualize liposomes in ice but are having some serious difficulty. Can anyone suggest some tricks that may be helpful? Thanks.

Best wishes,
Reza

Reza Khayat, PhD
Assistant Professor
City College of New York
Department of Chemistry
New York, NY 10031

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