The worldwide population is getting older, and cancer is often regarded as being one of the leading causes of death in the world. The development of new drug-targeted therapy will surely reduce the occurrence of cancer in the next few years. Nevertheless, the incidence of chronic diseases like cancer will keep rising. Thus, the quest for a more secure and affordable treatment is of critical importance (Sheikh et al. 2021). Recent work in the development and research of anticancer therapies that use natural products has resulted in the discovery of several terpenoids that inhibit cancer cell proliferation and metastasis through a variety of methods (Huang et al. 2012).

Terpenoids are the most abundant class of natural chemicals, serving as an enormous repository of potential therapeutic candidates. Terpenoids are classified into five subclasses depending on their structures: monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids, and tetraterpenoids (Huang et al. 2012). Much evidence has shown that the pentacyclic triterpenoids of the oleanane, ursane, lupane, and friedelane kinds (oleanolic, ursolic, betulinic, 18-glycyrrhetinic, asiatic acids, celastrol, lupeol, among others) have anti-cancer activities. These substances cause apoptosis in a variety of cancer cell types, including skin, breast, colon, and prostate tumor cells as well as inhibit tumor development and survival (Proshkina et al. 2020).

Lupeol ((3-beta)-Lup-20(29)-en-3-ol) (Fig. 1) is a pentacyclic triterpenoid that has captured the attention of medical practitioners, researchers, and pharmaceutical marketers due to its diverse pharmacological properties (Siddique and Saleem 2011). Lupeol’s chemical formula is C30H50O, with a melting point of 215–216 °C and a molecular weight of 426.7174 (g/mol). Lupeol’s infrared spectrum reveals the existence of a hydroxyl function and an olefinic moiety at 3235 and 1640 cm− 1, respectively (Sharma et al. 2020). Lupeol is present in a variety of plants and fruits, including tomato, white cabbage, cucumber, carrot, pea, pepper, bitter root, soybean, black tea, guava, strawberries, ivy gourd, red grapes, mulberries, figs, and date palm (Siddique and Saleem 2011). It is also frequently detected in plants such as Crataeva nurvala (Buch Ham), Betula platyphylla, the roots of Anemone raddeana, Hieracium pilosella, the bark of Gossampinus malabarica, Tamarindus indica, Arbutus unedo, Tipuana tipu, the latex of Leptadenia hastate, and Acacia mellifera (Chaturvedi et al. 2008). Several in vitro and preclinical animal investigations indicate that lupeol has the potential to serve as an antimicrobial, anti-protozoal, anti-invasive, anti-inflammatory, anti-proliferative, anti-angiogenic, and cholesterol-lowering agent (Wal et al. 2015). Lupeol has been shown to have extremely low toxicity levels. Lupeol taken orally at a dose of 2 g/kg body weight has been shown to have no deleterious effects on rats and mice, with no mortality after 96 h of observation (Chaturvedi et al. 2008). Additionally, lupeol reduces oxidative stress in the tissues of the eyes and helps rats with cataracts caused by selenite (Proshkina et al. 2020).

Chemical structure of lupeol (A)

Membranes that are as dynamic and complex as the cell itself define the boundaries of biological cells and organelles. The cell membrane is a complex mixture of proteins, lipids, and carbohydrates that performs a variety of tasks, such as enzymatic activity, receptor binding, molecularly specialized transport, and regulation of cell-cell interactions (Plant 1999). Due to the fact that biological membranes are too complicated to accurately characterize, analytical research employs simplified model systems (Hasan and Mechler 2017). Biomimetic lipid membranes have yielded detailed information on a variety of topics, including lipid phase transitions, bilayer structure, the effect of cholesterol on the structure and dynamics of lipid bilayers, and interactions with drugs, peptides, and proteins (Luchini and Vitiello 2021). Phosphatidylcholines (PC), the most prevalent phospholipids found in biomembranes, are usually the basis of model systems. PC is commonly found in neat, binary, or ternary mixes (Hasan and Mechler 2017).

Terpenoids’ biological activities have been studied extensively, but their molecular mechanisms remain unclear and in the literature, few publications exist on their membrane interactions. For example, Prades et al. (Prades et al. 2011) investigated the effects of oleanolic, maslinic, and ursolic as the most important free triterpenic acids found in orujo olive oil on phospholipid membranes. Different physicochemical techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), 31P nuclear magnetic resonance (NMR), and Laurdan fluorescence were used. Experimental data show that all three triterpenic acids change the structural properties of DPPC and DPPC-Chol (cholesterol) membranes. Calorimetric data on the effects of all three compounds on DPPC and POPC (1-palmitoyl-2-oleoyl sn-glycero-3-phosphocholine) membranes were considered in detail. It was observed that the pretransition temperature was not affected much depending on the concentration. The triterpenic acids caused the main phase transition peak to broaden, separate into two peak components, and decrease the main phase transition enthalpy at high concentrations. The DSC results of these three compounds with POPC bilayers also showed a similar profile to the results obtained with DPPC bilayers. It was determined that these compounds form two- or three-component phase transition peaks in cholesterol-enriched DPPC bilayers (DPPC: Chol 70:30), and thus these peaks may consist of multiple coexisting domains.

Lőrincz et al. (Lorincz et al. 2015) researched the influences of ursolic acid (UA), a plant-derived triterpenic molecule, on the structural and morphological characteristics of DPPC-water systems by using DSC, small and wide-angle X-ray scattering (SWAXS), freeze-fracture transmission electron microscopy (FF-TEM), and Fourier transformation infrared spectroscopy (FTIR) techniques. According to the DSC results, it was found that ursolic acid at a molar concentration of UA/DPPC = 0.01 caused a very small perturbation in the thermal behavior of the system. Domains rich in ursolic acid molecules present as a broad shoulder on the right side of the main phase transition, and this occurs more dominantly at increasing ursolic acid concentrations (UA/DPPC = 0.1, 0.2, and 0.3). Domains rich in ursolic acid show thermodynamically stable formations due to the shift of the main transition to the positive side. In the FTIR spectroscopy results, it was observed that a new band was formed in the region of approximately 1691 cm–1 due to the presence of ursolic acid in the system. It is thought that this new small band observed at UA/DPPC = 0.1, 0.2 and 0.3 molar ratios may result from the aggregation of a part of ursolic acid in dimer form. The temperature dependence of the wavenumber of the CH2 symmetric stretching mode of ursolic acid-DPPC liposomes is examined and it was determined that the wavenumber values of the DPPC system containing different ursolic acids in the gel phase shifted towards high values, whereas in the liquid crystal phase they remained at low wavenumber values.

Abboud et al. (Abboud et al. 2016) examined the effect of tetracyclic (cortisol, prednisolone, and 9-fluorocortisol acetate) and pentacyclic (uvaol and erythrodiol) triterpenes (TTPs) on the fluidity of DPPC membranes in detail by DSC, Raman spectroscopy, and fluorescence anisotropy. According to the DSC results, in liposomes containing various molar concentrations (DPPC: TTP = 100:1, 100:2.5, and 100:10) of cortisol (Co), prednisolone (PD), and 9-fluorocortisol acetate (9-FA), the main phase transition temperature shifts towards lower values, and the pretransition completely disappears. A shoulder occurred in the main phase transition of prednisolone and 9-fluorocortisol acetate at the concentration of DPPC: TTP = 100:10. From the Raman spectroscopy results, the presence of these compounds in the DPPC membrane system indicates an increase in the I2935/2880 intensity ratio, and therefore an increase in rotational disorders. It is seen that these compounds cause an increase in the I2844/2880 intensity ratio. It has been observed that the tetracyclic and pentacyclic triterpenes used increase the gauche/trans ratio and disrupt the lipid chain order, as the height intensity ratios of the peaks observed in the 1000–1200 cm–1 region allow direct comparison of order-disordered transitions between I1090/1130 liposome samples. Since the peak at 715 cm–1 observed in the 700–800 cm–1 region corresponds to the stretching vibration of the C–N band of the choline group of DPPC, it was determined that these triterpenes significantly reduced the intensity of the observed peak.

In this study, we investigated the effect of lupeol on 1,2‑dipalmitoyl‑sn‑glycerol‑3‑phosphocholine (DPPC) model membranes by using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. We chose DPPC and lupeol for two main reasons: (i) phosphatidylcholines (PC) are among the most abundant phospholipids found in mammalian cell membranes (ca. 45–55%), providing a simple model of a lipophilic environment to mimic cell membranes, even though DPPC itself is mainly present in lung and brain tissues, (ii) the mechanism of action of lupeol has not yet been elucidated in detail. The most popular calorimetric technique for examining the thermotropic phase behavior of liposomes and tracking the impact of host molecules on the gel-to-liquid crystalline acyl chain melting transition is differential scanning calorimetry. FTIR spectroscopy can quickly and label-freely produce a chemical fingerprint of the sample being studied. This vibrational technique, in particular, makes it possible to gather information on the structure and composition of complex biological systems, such as entire cells and tissues, in addition to the major isolated biomolecules (Mereghetti et al. 2014). DSC and FTIR data can be combined to study the type of interaction, depth of penetration in the bilayer, and conformational changes in the hydrophobic chain structure (Bonora et al. 2003). These interactions also affect the partitioning, direction, and conformation of terpenoids in bilayers, making them crucial for lipidic drug delivery systems (Mady et al. 2012).