The pressure tunning Raman spectral studies of the bilirubinIXα and neutral calcium bilirubinate at high external pressure

Abstract

The bilirubinIXα and its neutral calcium bilirubinate were studied using Raman spectroscopy at high external pressure. The results showed that the bilirubinIXα has two pressure-induced phase transitions (15–18 and 30–36 kbar) and three pressure phase areas. Its pressure sensitivities in the low-pressure phase are very low. It is believed that the four internally hydrogen bonds in bilirubinIXα molecule cause the atoms to attract each other tightly in the bilirubinIXα molecule. Therefore, the low pressure is not strong enough to shorten the bonds significantly. The pressure sensitivities in the middle-pressure phase are much higher than those in the low-pressure phase, but those in the high-pressure phase are slightly lower than in the middle-pressure phase. There is only one pressure-induced phase transition (25–34 kbar) in the neutral calcium bilirubinate. The pressure sensitivities in the low-pressure phase are higher than those in the high-pressure phase as usually.