RESEARCH ARTICLE Copyright © 2015 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Bionanoscience Vol. 9, 1–7, 2015 The Interaction of Biosynthesized Gold Nanoparticles with Casein Enzyme Hydrolysate Swarup Roy ∗ and Tapan Kumar Das Department of Biochemistry and Biophysics, University of Kalyani, Kalyani 741235, West Bengal, India The interaction between Aspergillus foetidus mediated biosynthesized gold nanoparticles (AuNP) and casein enzyme hydrolysate-Type I have been carried out using multi-spectroscopic techniques. UV-vis spectroscopic data suggested the formation of ground state complex between casein enzyme hydrolysate and AuNP. The biosynthesized AuNP have a strong ability to quench the intrinsic flu- orescence of casein enzyme hydrolysate by dynamic quenching process. The number of binding sites ‘n’ and binding constants ‘K ’ were 239 × 10 2 L/mol and ∼1 respectively, determined based on fluorescence quenching data. Based on the Förster’s non-radiation energy transfer (FRET) theory, the average binding distance (r ) between casein enzyme hydrolysate and AuNP was estimated to be 4.67 nm. Both synchronous fluorescence and circular dichroism (CD) spectra also indicated the interaction as mentioned which reflected in the conformational change of casein in presence of AuNP. Keywords: Casein Enzyme Hydrolysate, AuNP, Biosynthesis, Spectroscopy. 1. INTRODUCTION Gold nanoparticles have unique physical and chemical characteristics, which has been studied extensively in many areas. 1–11 Colloidal gold has several applications like in self-assembled monolayers, 4 optical absorption study, 5 resonance light scatting spectroscopy, 6 and immunoassay. 7 Among all nanostructured materials, AuNP have attracted particular interest due to their high stability, biocompatibil- ity, surface plasmon resonance effect, and unique catalytic activities. 12 Owing to the unique optoelectronic proper- ties with their controlled size and morphology, AuNP find significance in the field of bio-nanotechnology 13 as biomarkers, 14 biosensors, 15 cancer diagnostic 16 and vehi- cles for drug delivery. 17 Casein is a phosphoprotein having phosphate groups attached to some of the amino acid side chains and the attachment involved the hydroxyl groups of the serine and threonine moieties. Casein is a proline rich globular pro- tein which is readily available in milk and it has both hydrophilic and hydrophobic domains. 17 Casein is a mix- ture of several proteins, which is present to the extent of 3 to 4% in the cow’s milk. In milk, casein micelles exist as colloidal particles of 100–300 nm in diameter. They are composed of 93.3% casein and 6.6% inorganic constituents. The casein constituents,  s1 -,  s2 -, -, and ∗ Author to whom correspondence should be addressed. -casein, exist in proportions of approximately 3:0.8:3:1 by weight. Neither  nor  casein is soluble in milk, singly or in combination. If  casein is added to either one, or to a combination of the two, however, the result is a casein complex that is soluble owing to the formation of the micelle. The non-protein component of the micelles, expressed as low molecular weight ions, include calcium (37.5%) and phosphates (50%), and smaller amounts of citrate (7.5%) and magnesium (2%). 18–21 The main phys- iological role of casein in the milk system was widely accepted to be a source of amino acids required by growth of the neonate. However, the dominant physiological fea- ture of the casein micelle system has more recently been proven to be the prevention of pathological calcification of the mammary gland. 22 Due to the excellent functional properties and natural abundance, casein proteins (or their hydrolysates) found to be a crucial tool for the food indus- try. Casein has been used as glue since the days of ancient Egypt, Greece, Rome, and China and now a day’s casein glue also widely used as a binder, in leather, in match, in paper industries and in wood and laminated beams. Casein glue has been also used in leather industry as a dressing and finishing agent. Recently casein has been also used for the green synthesis of graphene 23 and the report suggested aspartic acid and glutamic acid residue of casein molecule are responsible for the graphene oxide reduction. Studies of interaction between biomolecules and nanoparticles are becoming a key area now a days. Already J. Bionanosci. 2015, Vol. 9, No. xx 1557-7910/2015/9/001/007 doi:10.1166/jbns.2015.1332 1