Computers in Biology and Medicine 36 (2006) 157 – 166 www.intl.elsevierhealth.com/journals/cobm Isoelectric point determination of proteins and other macromolecules: Oscillating method Antonio Sillero ∗ , Andres Maldonado Facultad de Medicina, Departamento de Bioquímica, Instituto de Investigaciones Biomédicas Alberto Sols, CSIC/UAM, Arzobispo Morcillo 4, 28029 Madrid, Spain Received 5 April 2004; accepted 7 September 2004 Abstract A program written in Visual Basic has been developed to calculate the isoelectric point of proteins and other macromolecules bearing acid–basic residues. The pI value can be theoretically calculated with the precision re- quired. The computer automatically supplies a representation of the charge of the protein versus pH values. The corresponding values can also be obtained, on command, in the form of table.  2004 Elsevier Ltd. All rights reserved. Keywords: Electric charge; pH; Proteins; pI Theoretical determination; Visual Basic; Acid base residues 1. Introduction Part of our investigation has been focused on the theoretical determination of the isoelectric point of proteins and other macromolecules, a point at which negative and positive charges of their acid–base residues are equal. Acid–base residues have been classified as belonging to the P or N types, depending on whether they are protonated or neutral, respectively, when not dissociated (Table 1) [1–5]. As deduced from the Henderson–Hasselbach equation, the charge of a specific acid–base group depends on the pH ∗ Corresponding author. Tel.: +34 91 4975413; fax: +34 91 5854401. E-mail address: antonio.sillero@uam.es (A. Sillero). 0010-4825/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compbiomed.2004.09.006