Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1380–1383 Modelling heating effects in cryocooled protein crystals James Nicholson a, *, Colin Nave a , Khalid Fayz b , Barry Fell b , Elspeth Garman c a Synchrotron Radiation Department, Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK b Engineering Department, Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK c Laboratory of Molecular Biophysics, Oxford University, Oxfordshire, OX1 3QU, UK Abstract With the application of intense X-ray beams from third generation synchrotron sources, damage to cryocooled macromolecular crystals is being observed more commonly [1]. In order to fully utilize synchrotron facilities now available for studying biological crystals, it is essential to understand the processes involved in radiation damage and beam heating so that, if possible, action can be taken to slow the rate of damage. Finite Element Analysis (FEA) has been applied to model the heating effects of X-rays on cryocooled protein crystals, and to compare the relative cooling efficiencies of nitrogen and helium. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 61.10.Nz; 61.82.Pv Keywords: Synchrotron; Modelling; Protein; Crystal; Radiation; Damage 1. Introduction Protein crystals are routinely frozen in fibre loops [2], with the crystal surrounded by a thin film of frozen cryoprotectant. A schematic diagram of the typical set-up is shown in Fig. 1. On second generation sources, such as the SRS at Daresbury, freezing in this way effectively negates radiation damage due to beam heating and free radical diffusion for the duration of data collection [3,4]. However, the extremely intense X-ray beams from third generation sources, such as the ESRF and the proposed new UK source DIAMOND, cause noticeable damage to frozen crystals. This can have serious detrimental effects on both the quantity and quality of data collected, especially in MAD experiments where it may be necessary to collect several datasets from a single crystal. At present there is insufficient information available about radiation damage and beam heating, and their relationship to incident dose, dose rate, cooling medium and wavelength. A better understanding of these processes is vital so that, if possible, evasive action can be taken to slow the damage rate. Some anecdotal evidence exists about the temperature fluctuations in irradiated protein crystals. However, these experi- ments tend to be based on measurements with metal temperature probes which are not accurate models for protein crystals. 2. Modelling protein crystals If accurate parameters for protein crystals, such as density, thermal conductivity and specific *Corresponding author. Tel.: +44-1925-603-905; fax: +44- 1925-603-124. E-mail address: j.nicholson@dl.ac.uk (J. Nicholson). 0168-9002/01/$-see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0168-9002(01)00735-5