Surface chemistry and spectroscopy of the b-galactosidase Langmuir monolayer Nicholas F. Crawford a , Miodrag Micic b,c , Jhony Orbulescu b , Daniel Weissbart d , Roger M. Leblanc a,⇑ a Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146, United States b MP Biomedicals LLC, 3 Hutton Center, Santa Ana, CA 92707, United States c Department of Engineering Design Technology, Cerritos College, 11110 Alondra Boulevard, Norwalk, CA 92650, United States d MP Biomedicals SAS, Parc d’innovation-Rue Geiler de Kaysersberg, Illkirch-Graffenstaden 67402, France graphical abstract article info Article history: Received 25 February 2015 Accepted 29 April 2015 Available online 7 May 2015 Keywords: b-Galactosidase Langmuir monolayer Spectroscopy Photophysical properties abstract The changes of interfacial properties of b-galactosidase introduced into different pH environments are investigated through surface chemistry and in situ spectroscopy. Conditions for an optimal Langmuir monolayer formation were firstly obtained by varying the subphase salt concentration and the surface-pressure area isotherm was used to extrapolate the limiting molecular area of the enzyme mono- layer to be around 42,000 Å 2 molecule 1 . Surface pressure stability measurements held at 20 mN/m for 90 min along with compression–decompression cycles revealed no aggregate formation at the air–water interface. Consistent with the data obtained from the isotherm, in situ UV–Vis and fluorescence spec- troscopy shows a steep rise in absorbance and photoluminescence intensity correlating to with a switch from a liquid-expanded to a liquid-condensed phase. A decrease in subphase pH increased the electro- static repulsion as the enzyme was protonated, leading to an expanded monolayer. Infrared absorp- tion–reflection spectroscopy demonstrates that the enzyme adopts mainly b-sheet conformation at the air–water interface before and during the compression. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction b-Galactosidase (E.C. 3.2.1.23) is an important family of hydro- lase exoglycosidase enzymes [1,2]. Its primary function is to cleave b-glycosidic bond formed between a galactose and organic http://dx.doi.org/10.1016/j.jcis.2015.04.063 0021-9797/Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: rml@miami.edu (R.M. Leblanc). Journal of Colloid and Interface Science 453 (2015) 202–208 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis