JISCMail - CCP4BB Archives

Actually, UV fluorescence imaging appears to be a pretty reliable means of
discriminating protein crystals, provided your protein has Trp residue(s).

V. Nagarajan
JAN Scientific, Inc.
http://www.janscientific.com

-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Artem
Evdokimov
Sent: Tuesday, January 13, 2009 8:10 PM
To: [log in to unmask]
Subject: Re: [ccp4bb] offtopic__which crystals to harvest

Hi,

A few simple hints:

(Please note that I am aware of the inexact language in the statements below
but I don't have the time to write this up exactly - conversational English
would have to do. Caveat emptor.)

Most protein crystals will break or deform when poked with a steel needle.
Most inorganic salts/minerals won't deform from a simple poke, but will
break (often with an audible crack) when pushed hard against something solid
(like the surface of the glass slip etc.). Many, but not all protein
crystals can survive gentle prodding with a thin cat whisker. Nylon loops
are a bit trickier because they can exhert different forces depending on
their geometry, age of the loop, and user's manual aptitude.

To make matters more complicated - crystals of organic materials (i.e. not
salt but also not protein) can display properties similar to either (but
will more often than not tend to behave like salts).

Salt crystals sink very rapidly in most well solutions. Protein crystals
often take their time (lower density). Salt crystals often display Newton
rings (Newton rainbows) when viewed through a polarizer-analyzer pair (not
to be confused with relatively simple gradients of birefringence colors that
are also common to protein crystals!). I have seen a few crystals of
proteins that had distinct Newton rings and they were all exceptionally good
diffractors. Don't be confused by rainbow-like coloring that's often
associated with spherolites - the latter aren't likely to diffract X-rays in
a useful manner :-)

If in doubt - stick your crystals into an X-ray beam. Pretty much the best
way to resolve this ambiguity! The next best choice is to show the crystals
to an experienced crystallographer - oftentimes it's possible to guess just
by eyeballing the drops but it takes experience to learn the traits and
habits. Membrane crystals (or for that matter any crystals grown in the
presence of detergent) can be extremely tricky to identify correctly due to
the inherently soft and nasty nature of detergent crystals and the tendency
of the latter to form various quasi-crystalline artefacts.

Good luck,

Artem

________________________________

From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of
deliang
Sent: Tuesday, January 13, 2009 10:50 PM
To: [log in to unmask]
Subject: [ccp4bb] offtopic__which crystals to harvest

Hi there,

Since a lot of different forms of crystals shows, I am using a quick/simple
strategy to choose crystals by applying a force on the crystal against the
wall, with the nylon loop.

Some can never break apart, so they are salt crystals? The others can not
survive the force and lose their intact shape and sharp surface. It seems
these are protein crystals, but are they "bad" crystals? I just came to
this field, and welcome all your suggestions and experience.

Thanks a lot.

Deliang