Drug therapy is always accompanied by undesirable side effects, because the drug spread out and affect to healthy tissues and organs as well. One of the ways to reduce the side effects is to develop a system that deliver a desired amount of the drug to a diseased part directly. The system is known as drug delivery system (DDS). The construction of the DDS requires the technology to encapsulate the drug and release it around the diseased part. Lipid bilayer vesicles, so-called liposomes, are typical DDS materials. However, liposomes are not very stable because all the lipids are connected via van der Waals interactions. As a result of this low stability, sometimes liposomes cannot reach the diseased part intact. There is a pressing need to develop more stable liposomes capable of encapsulating the drug. Recently, erasomes, i.e., liposomes featuring surfaces reinforced with a siloxane bond network, have been developed. It has been shown that a cerasome can encapsulate the drug and is much more stable than liposomes. Thus, cerasomes are promising materials for DDS. However, the improvement in stability is accompanied by a difficulty in opening the capsule. Breaking the chemical bonds of a cerasome surface requires information about its microstructure and electronic states, although the microscopic properties of cerasomes remain unclear.

In the previous work, we construct a simple cerasome model (figure) to investigate the electronic structures of its surface. Comparing the electronic structures of the membrane model with that of the molecule, we reveal that there are mid-gap states in the membrane model that are not in the molecule. These mid-gap states consist of antibonding states of Si-C bonds that connect the siloxane network and organic parts of the lipid. The existence of the antibonding states in the gap indicates that there is a possibility to break cerasomes with some electronic excitations to the system. However, the previous model is extremely simple, we assume perfect siloxane network, although there are many O^- and OH groups on the surface of actual cerasomes. The purpose of this study is to investigate effects of surface substituent groups on the electronic structures of the cerasome model.

We introduce some O^- and OH groups in the surface of the model and calculate the electronic strucutres. All calculations are carried out using first-principles scheme based on density functional theory. It is revealed that the energy of the mid-gap states is modulated around 0.2 eV, but still in the HOMO-LUMO gap of the lipids. This result indicates that we can expect that the mid-gap states valid for the realistic cerasomes.