RESEARCH ARTICLE Elucidation of the interaction of human serum albumin with anti‐cancer sipholane triterpenoid from the Red Sea sponge Mohd. Sajid Ali 1 * | Musarat Amina 2 * | Hamad A. Al‐Lohedan 1 | Nawal M. Al Musayeib 2 1 Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Arriyadh, Saudi Arabia 2 Department of Pharmacognosy, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Arriyadh, Saudi Arabia Correspondence Mohd. Sajid Ali, Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia. Email: smsajidali@gmail.com Musarat Amina, Department of Pharmacognosy, College of Pharmacy, King Saud University, P.O. Box 2457, Riyadh 11451, Saudi Arabia. Email: musarat.org@gmail.com Abstract A sipholane triterpenoid, named sipholenone A, with anti‐cancer properties was isolated from the Red Sea sponge Siphonochalina siphonella and characterized by proton and carbon‐13 nuclear magnetic resonance ( 1 H NMR and 13 C NMR) spectroscopies. The goal of this study was to visu- alize the binding of this triterpenoid with human serum albumin (HSA) and to determine its bind- ing site on the biomacromolecule. The interaction was visualized using fluorescence quenching, synchronous fluorescence, far‐ and near‐UV circular dichroism (CD), UV–visible and Fourier transform‐infrared (FT‐IR) spectroscopies. UV–visible spectroscopy indicated the formation of a ground‐state complex as a result of the interaction. Sipholenone A quenches the fluorescence of HSA via a static quenching mechanism. A small blue shift in the fluorescence quenching pro- files suggested the involvement of hydrophobic forces in the interaction. Sipholenone A binding takes place at site I of subdomain II A with a 1:1 binding ratio, as revealed by displacement bind- ing studies using warfarin, ibuprofen and digitoxin. Far‐UV CD and FT‐IR studies showed that the binding of sipholenone A to HSA also had a small effect on the protein's secondary structure with a slight decrease in the α‐helical content. Several thermodynamic parameters were calculated, along with Forster's radiative energy transfer analysis. KEYWORDS fluorescence quenching, human serum albumin, interaction, sipholenone A, spectroscopy 1 | INTRODUCTION Marine sponges are one of the major groups of biological organisms that provide significant numbers of natural products and secondary metabo- lites with pharmacological properties that have led to the formulation of novel drugs. [1] More than 5300 different products are known from sponges and their associated microorganisms and, every year, hundreds of new compounds are being discovered. [2–4] Most active constituents of sponges have been categorized as either anti‐tumour, anti‐viral, immuno‐ or neuro‐suppressive, anti‐fouling or antibiotic, anti‐malarial or anti‐inflammatory agents, thereby resulting in an amazing chemical diversity of sponge by‐products. In addition to rare nucleosides com- pounds, bioactive terpenes, steroids, peptides, alkaloids, peroxides, fatty acids, as well as the derivatives of amino acid (commonly halogenated), have been reported as derived from sponges. [5] The Red Sea sponge Siphonochalina (= Callyspongia) siphonella, is a grey colonial tube‐like sponge, commonly found in the Gulf region of Aqaba and Suez. [6] It is a rare sponge identified to make squalene‐ derived cyclic ethers triterpenes, consisting of two separate polycyclic systems connected by different types of linkers. [7] From S. siphonella, to date, around 30 triterpenoids that are mainly made‐up of sipholane, siphonellane, neviotane or dahabane frameworks have been reported. [8–10] In the aforementioned four triterpenoids, the most sig- nificant ones are sipholane triterpenoids that are comprised of sipholenol A, sipholenone E, sipholenol L and siphonellinol D respec- tively. Among these, sipholenol A and sipholenone A are the most important sipholane triterpenoids and contain a perhydrobenzoxepine (rings ‘A’ and ‘B′) and a [5,3,0] bicyclodecane system (rings ‘C′ and ‘D’), coupled with each other via an ethylene bridge. [11,12] Sipholenone A (Figure 1) or 15‐sipholen‐10,19‐diol‐4‐one, has been found to exhibit potential anti‐cancer effects and has the ability to reverse P‐gp‐ mediated MDR in some cancer cells. [13,14] A literature survey on sipholenone A has revealed that the molecule, along with other members of sipholane triterpenoids, is found to be a potent reversal of multidrug resistance in cancer cells that over‐express P‐glycopro- tein. Moreover, these compounds are found to improve the cytotox- icity of numerous P‐glycoprotein substrate anti‐cancer drugs and therefore considerably reverse the multidrug resistance phenotype Received: 14 February 2016 Revised: 13 April 2016 Accepted: 16 May 2016 DOI 10.1002/bio.3172 Luminescence 2016; 1–8 Copyright © 2016 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/bio 1