Mechanism of ionic strength dependence of crystal growth rates in a subtilisin variant

An orthorhombic crystal form of a subtilisin variant exhibits a systematic variation in growth rates of its three unique faces, resulting in pronounced morphology variations, depending on ionic strength. Several common salts cause a concentration-dependent change in the crystal habit from thin plates to isometric bars. Analysis of the four crystal contacts in the 1.8 A resolution X-ray structure leads to models of the morphological growth fronts and reveals a short (2.5 A) hydrogen bond between two carboxyl groups in one of the contacts. Cation screening of this interaction in the nascent contact is proposed to play a key role in making the growth rates ionic strength sensitive.