Wednesday, January 31, 2007

Proteins through a glass darkly

There is a long-standing idea that proteins undergo something like a glass transition at around 200-220 K: above this temperature the peptide chains can diffuse, while below it they are trapped and exhibit harmonic vibrations. It has also seemed to be the case that this behaviour is intimately connected to that of the hydration water, which apparently undergoes a similar transition. (See E. W. Knapp, S. F. Fischer & F. Parak, J. Am. Chem. Soc. 86, 5042-5047 (1982); B. F. Rasmussen, A. M. Stock, D. Ringe & G. A. Petsko, Nature 357, 423-424 (1992); R. F. Tilton, J. C. Dewan & G. A. Petsko, Biochemistry 31, 2469-2481 (1992); I. V. Borovykh, P. Gast & S. A. Dzuba, J. Phys. Chem. B 109, 7535-7539 (2005); V. Reat, R. Dunn, M. Ferrand, J. L. Finney, R. M. Daniel & J. C. Smith, PNAS 97, 9961-9966 (2000); A. R. Bizzarri, A. Paciaroni & S. Cannistraro, Phys. Rev. E 62, 3991-3999 (2000); C. F. Wong, C. Zheng & J. A. McCammon, Chem. Phys. Lett. 154, 151-154 (1989); C. Arcangeli, A. R. Bizzarri & S. Cannistraro, Chem. Phys. Lett. 291, 7-14 (1998); A. L. Tournier, J. Xu & J. C. Smith, Biophys. J. 85,1871-1875 (2003)). Bizzarri and Cannistraro (J. Phys. Chem. B 106, 6617-6633; 2002) have speculated that the dynamics of the protein and solvent are so strongly coupled that they ‘should be conceived as a single entity with an unique rough energy landscape.’ In other words, the protein motions are not simply ‘slaved’ to those of the solvent, but ‘the very topological structure of the protein energy landscape could be deeply altered by the spatial organization, as well as by the dynamical behaviour of the hydration water.’

But is this truly glassy behaviour that we're seeing in the proteins and the hydration sphere? Jürgen Köhler at the University of Bayreuth and his colleagues don't think so. They've looked at spectral diffusion in individual molecules of the light-harvesting LH2 protein complex in purple bacteria (see paper here) at 1.4 K, and say that the dynamics don’t fit the standard two-level-system model used to understand spectral diffusion in glasses.

Gene Stanley at Boston University and his coworkers don't think so either. In a paper published in Phys. Rev. Lett. towards the end of last year they argued that the dynamical transition in proteins is in fact driven by a change in the diffusivity of the hydration water, which is itself caused not by a glass-like transition but by a crossover in dynamics related to the critical point of a liquid-liquid phase transition in water, predicted to occur at around this temperature (200 K) but at high pressure. Beyond the critical point (that is, in the one-phase region of the liquid), a 'ghost' of the first-order transition remains in the form of a 'Widom line' where the response functions of the liquid are maximal. That would certainly provide a reasonable explanation for why DNA seems to show the same kind of dynamical change at much the same temperature (around 247 K) – in both cases, the change in macromolecular behaviour is driven by a change in the solvent. Thus, says Gene, the protein glass transition is not a transition, not a glass, and not protein.

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