## 1 Introduction  
Proteins are the main biomolecular forces that are involved in controlling most biological processes in living cells, tissues, and organisms. They exhibit their biological functions through selective interactions with other molecules, which could also be proteins and/or DNA. The most complex protein interactions are interactions between proteins and their receptors, which are proteins or complexes of proteins that selectively drive specific biological pathways. Currently, the selectivity of interactions between proteins and their receptors are investigated mostly using 3D matching between interacting proteins, which is not explicit enough to explain the high specificity of these interactions. Experimentally, protein-receptor interactions are investigated by a number of techniques including X-ray, MRI, spectroscopy, etc. However, all these techniques are very expensive and time consuming. Thus, there is a need for a biophysical approach that can investigate protein-receptor interactions with more specificity than 3D matching.  
Here, we present the ability of the Resonant Recognition Model (RRM) [1; 2; 3; 4; 5; 6; 7], which is a biophysical model based on the finding that certain periodicities (frequencies), in distribution of free electron energies along the protein and corresponding electromagnetic radiation, are critical for protein biological function and selective interaction with receptors. The RRM model has already been tested in numerous examples including fibroblast growth factor (FGF) and its receptor [4], insulin and its receptor [5], tumor necrosis factor (TNF) and its receptor [6], and interleukin-12/23 and their receptors [7]. Here, we present the RRM concept for analysis of protein receptor recognition on an example of leptin and leptin receptor.  
Leptin is a hormone that regulates energy balance within the organism by inhibiting hunger by acting on cell receptors in the brain. Leptin is potentially identified to sustain weight loss and could be used in treatment of obesity and overweight. Overweight and obesity are huge health problems within modern society. Obesity is medical condition in which excess body fat has accumulated to such an extent that it may have a negative effect on health and lifestyle [8]. Obesity could be involved in the development of various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, osteoarthritis, and depression [8; 9]. Obesity is commonly caused by a combination of excessive food intake, lack of physical activity, and genetic factors [10]. Thus, there is a need to find out how to control and overcome factors that cause obesity. There are many possible mechanisms involved in the development and maintenance of obesity, and one of the approaches to solve this problem is to be able to control the feeling of hunger and subsequently control food intake. This could be achieved by controlling the protein cascade that controls hunger and energy balance within the body. Leptin is a crucial protein in such a cascade, which is involved in limiting hunger. Thus, by using leptin or mimicking leptin activity, the appetite of obese and overweight individuals can be controlled. Leptin acts on leptin receptors located in the hypothalamus to regulate appetite to achieve energy homeostasis [11]. Leptin also binds to neuropeptide Y (NPY) in NPY neurons, decreasing activity of NPY neurons. In addition, neuropeptide Y is inhibited by activated leptin receptors. Thus, NPY neurons are also the key element in the regulation of hunger [12].  
We used the Resonant Recognition Model (RRM) to analyze lectins, leptin receptors, and neuropeptides Y, with the aim to find out which parameter (electromagnetic resonant frequency) characterizes leptin main activity of recognition and interaction with leptin receptors and neuropeptide Y. Such a parameter can be used for developing treatments to achieve hunger control and subsequent energy balance (homeostasis) within the body.