3 Kapodistrian Uni ver sity of Ath ens, Greece Pro teins of ten crys tal lize in microcrystalline pre cip i tates. The pro tein mol e cules are then sur rounded by sol vent and their pack ing ar range ment is re tained by lim ited inter-molecular con tacts. A change in the crys tal en vi ron ment first af fects the bulk sol vent that fills the inter molecular space, with re sult ing changes in the crys tal struc ture. In liter a ture it is re ported that pro tein crys tals in con trolled humid

more »

... ty en vi ron ments show a large change in unit-cell pa ram e ters when the hu mid ity is de creased [1-2]. When a pro tein crys tal is care fully de hy drated, it is in a metastable state in which the crys tal ini tially still re tains the orig i nal pack ing struc ture [2]. Fur ther de hy dra tion may cause the col lapse of the crys tal lat tice: the crys tal no lon ger maintains its pack ing struc ture be cause of the loss of a large amount of bulk sol vent. How ever in some crys tals, the de-hy dra tion in duces a mo lec u lar ar range ment change re sult-ing in a new crys tal struc ture. This has been al ready re ported for hen egg-white (HEW) lysozyme [3]. While de hy dra tion can in duce struc tural changes, this is also likely to hap pen upon hydration of the same crys tals. Here, we pre sent our re sults of micro crys tal li ne tetra go nal and mo noc li nic HEWL sam ples on a la bo ra to ry X-ray pow der diffrac to me ter inclu ding in situ me a su re ments under va ri a ble re la ti ve hu mi di ty con di ti ons. The ob ser ved gra du al structu ral chan ges as well as phase tran si ti ons upon de hyd rati on and hyd rati on of HEWL are ana ly zed in the relative humidity range 50%-95%. (Figure 1.) De hy dra tion and hydration pro cesses are re vers ible in hu mid ity cy cles in the range of 95% rH to 75% rH. With out sta bi liz ing PEG the lower limit for de hy dra tion of tetragonal HEWL is around 75% rH. With PEG the tetragonal HEWL sam ples re main crys tal line be low 75% rH, but show phase tran si tions and larger vari a tions of the cell pa ram e ters. Be low 75% rH a new tetragonal polymorph was dis cov ered.