Diffraction anisotropy has nothing to do with beam size and its
relationship to the shape of the crystal. Diffraction anisotropy depends
only on microscopic order of the crystal and crystal symmetry. For
example, in cubic symmetries, diffraction is always isotropic.
Microscopic order is mostly a function of the type of packing molecular
contacts, so it could be quite variable even for the same protein, but
in different crystal packing arrangements. In my experience, I didn't
find much correlation between r-factors and anisotropy, but I use
anisotropic informativity cutoff of the data using HKL3000 automatic
corrections option, which some people also use. Other programs may use a
similar procedure but I am not familiar with them.
Zbyszek Otwinowski
On 2026-01-21 16:02, Martin Malý wrote:
Dear Purity,
One very common reason for diffraction anisotropy is a shape of crystal, in
combination with a beam size. If a crystal is a needle or a plate, then it's no
surprise that data with diffraction anisotropy are collected. But there are
also other reasons.
I would personally say - don't be so unhappy when comparing R-values of different structures/datasets. The R-values are only well comparable when they are calculated against the same data range (here it means the same set of reflections). I guess this criterion is not met in the cases you are referring to so you can probably relax.
Much more important is whether Rfree and Rfree-Rwork gap are getting lower and lower when
you do your iterations of model building and refinement against your particular dataset.
And it's also important whether you can see "density good enough" to build your
structure model. Staraniso can help there. You can also try Servalcat refinement against
intensities and carry out map sharpening e.g. in Coot.
Cheers,
Martin
On 21/01/2026 19:49, Ezennubia, Purity wrote:
Thank you, Gerard, for the document recommendation.
_Kelechi Purity Ezennubia_
_Graduate research assistant _
_Department of Chemistry & Biochemistry _
_Baylor University _
_Clinger lab C161R_
[email protected]_
-------------------------
From: Gerard Bricogne <[email protected]>
Sent: Wednesday, January 21, 2026 1:29:55 PM
To: Ezennubia, Purity <[email protected]>
Cc: [email protected] <[email protected]>; Star Aniso
<[email protected]>
Subject: Re: [ccp4bb] Anisotropy diffraction data user discussion
[You don't often get email from [email protected]. Learn why this is
important at https://aka.ms/LearnAboutSenderIdentification ]
Dear Purity,
A personalised answer will be forthcoming but in the meantime,
since you are asking about the causes of anisotropy in diffraction
data, you would certainly benefit from reading the document at
https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fstaraniso.globalphasing.org%2Fanisotropy_about.html&data=05%7C02%7CPurity_Ezennubia1%40BAYLOR.EDU%7C27c5ce1c69ac44cb2d7108de592377a3%7C22d2fb35256a459bbcf4dc23d42dc0a4%7C0%7C0%7C639046206077920452%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=xuRCFinfgQSFb0Ysfa6tuJsli%2BMFm6PjnANxnYi6Eug%3D&reserved=0
[1]
in which all technical terms used are explained in the accompanying
Glossary at
https://nam02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fstaraniso.globalphasing.org%2Fstaraniso_glossary.html&data=05%7C02%7CPurity_Ezennubia1%40BAYLOR.EDU%7C27c5ce1c69ac44cb2d7108de592377a3%7C22d2fb35256a459bbcf4dc23d42dc0a4%7C0%7C0%7C639046206077955875%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=JjXzau8KZJ1iSiWGEFN6lC6ZB0pyKu17GE3wiVBDG1s%3D&reserved=0
[2]
With best wishes,
Gerard.
--
On Wed, Jan 21, 2026 at 05:58:42PM +0000, Ezennubia, Purity wrote:
Hello everyone,
It's becoming almost a regular norm for my crystals to be anisotropic when I
look at the diffraction data. Its really unideal for me because when the data
is anisotropic my R-work and R-free values are usually high when compared to
some lucky data I got that was not anisotropic.
I would like to hear from other people in this forum what causes a crystal to
diffract anisotropically going to different resolution in the hkl axis.
Thank you,
Purity
Kelechi Purity Ezennubia
Graduate research assistant
Department of Chemistry & Biochemistry
Baylor University
Clinger lab C161R
[email protected]
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