 |
 |
Michael, It seems to me we have no disagreement, as we both say that it is *unusual* for protein crystals to be non-fragile. Furthermore, my objection is to "gelatin" characterization. I may be, as is my custom, wrong, but in terms of elasticity gels are purely entropic. Protein crystals, even the malleable ones you describe, have both enthalpic and entropic components to the deformation free energy (admittedly the entropic component is stronger than in "regular" materials). Your comment on the PEG-induced slow cross-linking is very interesting. What you describe fits perfectly the behavior of long rods of a deliberately cross-linked crystal - they bend and don't snap. Protein crystals (at least some) are also characterized by an unusually broad range of linear response to mechanical deformation, up to 10% http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2143058/ According to the theory in this paper though, you might be destroying diffraction by bending the crystal, since in crystal denaturation is likely irreversible. From what you are describing (60 degrees, 150x40x40 um), the linear deformation should reach almost 50%, definitely enough for partial "crystal denaturation". Cheers, Ed. On Fri, 2013-02-08 at 11:22 -0500, R. M. Garavito wrote: > Ed, > > > > Protein crystals are fragile but not soft. > > If your crystals are like gelatine it's unusual. > > > I hate to disagree with the disagreement, but there are many > exceptions to this rule. I have seen many protein crystals that are > quite malleable and bendable. One protein produced rod-shaped > crystals (150x40x40 um) that I could bend by almost 60 degrees and it > would slowly snap back. Mounting it old school was a real pain, and > their diffraction was mediocre. However, the majority of the crystals > I have worked with adhere to the general rule you describe. > > > Where the crystal physical behavior is anomalous, it is often when PEG > is used and/or there are multiple components that contribute to the > crystal's integrity (as in the case of membrane protein crystals with > detergent). > > > In the former case, crystals that sit in PEG solutions too long tend > to be cross-linked (most likely due to the aldehydes that can exist in > some batches of PEG). One could argue that the crosslinking adds > long-range elasticity and a resistance to fracturing. > > > In the latter case, I have observed large beautiful crystals of > membrane proteins that have the consistency and malleability of warm > butter. Sometimes optimization improved their integrity, and other > times a new crystal form is needed. > > > Regards, > > > Michael > > > > > > **************************************************************** > R. Michael Garavito, Ph.D. > Professor of Biochemistry & Molecular Biology > 603 Wilson Rd., Rm. 513 > Michigan State University > East Lansing, MI 48824-1319 > Office: (517) 355-9724 Lab: (517) 353-9125 > FAX: (517) 353-9334 Email: [log in to unmask] > **************************************************************** > > > > > > > On Feb 8, 2013, at 9:23 AM, Ed Pozharski wrote: > > > On Fri, 2013-02-08 at 14:53 +0400, Evgeny Osipov wrote: > > > Protein crystals behave rather as gelatine and not as solid > > > > I'd have to disagree on that. Protein crystals are fragile but not > > soft. If your crystals are like gelatine it's unusual. It has been > > demonstrated that elastic properties of protein crystals are similar > > to > > "organic solids". > > > > Cheers, > > > > Ed. > > > > -- > > "I'd jump in myself, if I weren't so good at whistling." > > Julian, King of Lemurs > > > -- Edwin Pozharski, PhD, Assistant Professor University of Maryland, Baltimore ---------------------------------------------- When the Way is forgotten duty and justice appear; Then knowledge and wisdom are born along with hypocrisy. When harmonious relationships dissolve then respect and devotion arise; When a nation falls to chaos then loyalty and patriotism are born. ------------------------------ / Lao Tse /