Preparation, Properties And Application Of Hydroxypropyl-β-cyclodextrins

A series of hydroxypropyl-β-cyclodextrin(HPCD) samples were prepared by an optimized process.The effects of degree and distribution of substitution on the complex forming ability,thermal stability and decomposing property of HPCDs were explored based on the structure investigation of HPCDs.The application of HPCD in forming inclusion complex with astaxanthin was also studied.HPCD,a hydroxyalkyl derivative ofβ-cyclodextrin,with improved water solubility properties and may be slightly more toxicologically benign,was considered a potential alternative ofβ-cyclodextrin.HPCDs have widely applications in food,agriculture and pharmaceutical field.In this study,the monofactorial tests and response surface method were performed to optimize the preparation process of HPCD.The following optimal parameters were obtained:reaction time of 16h,reaction temperature of 31℃and dialysis time of 7.4h. The degree of substitution(DS) and recovery of the production prepared under optimal parameters respectively were 4.02 and 61.80%,respectively.Under the optimized process,A, B two groups of HPCD were prepared,each had 4 types of samples with different DS and substituent distribution.The samples had higher purity and their physicochemical properties fulfilled the requirement of next step investigation.The infrared spectrums showed that bothβ-cyclodextrin and HPCDs displayed the characteristic absorption bands of saccharide,3400 cm^-1(O-H),2930 cm^-1(C-H) and 1640 cm^-1(C=O).HPCDs showed absorption bands at 2960 cm^-1 for methyl antisymmetric vibration,it proved the existence of hydroxypropyl group.The absorption bands of HPCDs prepared in the lab were in accordance with that of standard,therefore,they were proved to be the same substance,and the prepared HPCDs had higher purity.The improved reductive-cleavage method and methylation analysis were performed to determine the DS and distribution of substitution of HPCDs through analyzing the total ion current and mass spectrums of HPCDs.In group A,the DS value of O-6 was the highest,however,there also had many substituents at O-2 and O-3 positions.The ratio of DS(2+3) to DS(6) is close to 1. The distribution of substituents on primary(DS(6)) and secondary hydroxyl groups(DS(2+3)) was even.In group B,substituents concentrated at O-2.The DS value of secondary hydroxyl groups was about three times as much as that of primary face.The configurantion of reference was similar to that of group B HPCDs,especially to that of number 6.It was obviously that the quality and configuration of prepared HPCDs were equivalent to that of reference.The introduction of hydroxypropyl groups changed the crystalline structure ofβ-cyclodextrin, madeβ-cyclodextrin an amorphous compound containing a large mumber of chemically individual.These conduce to improve water solubility and dissolving speed of HPCDs and their inclusion complexes.A test was designed to determine the complex forming ability of HPCDs,based on the change of absorbance of the guest before and after complex forming.The choosed guests were phenolphthalein as a model for "larger spheriform" guest,methyl red as a model for "middling linear" guests and ethyl benzoate as a "smaller" model.Consider the effect of DS,distribution of substitution and guest molecules,the following conclusion should be drown:the stability constants of the complexes of HPCDs with the "larger spheriform" guests were all lower than that ofβ-cyclodextrin,no matter what DS and distribution of substitution.Steric hindrance was the dominant factor.The stability constants of the complexes of HPCDs with the "middling linear" guests were higher than that ofβ-cyclodextrin."Solubility increasing" was the major factor under low DS values.The HPCD products with substituents distributed evenly could reach higher stabilities,especially which the DS value was about 3.5.For the "smaller" guests,the complex forming ability of HPCDs was far higher than that ofβ-cyclodextrin.Hydroxypropyl group showed obvious "solubility increasing" effect.Because this type of molecules can only situate at the larger end of HPCDs,the products with substituents concentrared on the larger end had better stabilities,especially that the DS value was about 4.5.No matter what kind of guestes,the HPCDs which had too higher or lower DS values could not reach the best complexes forming result.The DS values between 3 and 5 were commended;however,the effect of substituent distrbution should be determined by the guests.The thermal stability of prepared HPCDs were determined,decomposition temperature of HPCDs was between 300 and 400℃,the peak value was about 367℃.Thermal stability of HPCDs was excelled thanβ-cyclodextrin,their decomposition temperature enhanced about 32℃.The thermal decomposition kinetics was investigated,the activation energy E_A is 149.15kJ/mol and the pre-exponential factor k₀ is 2.19×10¹⁰min^-1.The biodegradation of HPCDs in soils was studied according to ISO 17556(2003).All HPCDs were found to be more or less biodegradable.Increasing the DS had negative effect on the biodegradation rate of HPCDs.The substitution pattern affected the biodegradation,too. The soil characteristics might also slightly affect the biodegradation of HPCDs,however,the effect was lower than that of DS and substitution pattern.The degradation rate of HPCDs in fertile soil was a little higher than that in sandy soil.The biodegradation speed ofβ-cyclodextrin in the contaminated soil was rapid,which was degradated by 74.7%at the end of the 70 days test.The final biodegradation ratio of HPCD was 40.9%.Both were higher than that obtained in the uncontaminated soils.The contamination removing ability of cyclodextrins was highly affected by their own biodegradation fate in soil.The experiments in the contaminated soil indicated that bothβ-cyclodextrin and HPCD enhanced the bioavailability of contaminants in soil.HPCDs were better because they were more endurable in soil.Astaxanthin is a high value carotenoid pigment with strong antioxidant properties.It can easily be decomposed by light and oxygen,which can cause the loss of antioxidant properties. Furthermore,the poor aqueous solubility limits its use in aqueous phase antioxidant.The inclusion of astaxanthin with HPCD should improve the solubility and stability of astaxanthin and be helpful to store,carry and application.In this study,the HPCD/astaxanthin complex was prepared.The preparation process was optimized,the optimized factors were:HPCD concentration of 0.03mol/L,molar ratio of host-guest of 60,reaction temperature of 20℃, mixture speed of 1000rpm and the forecasted inclusion ratio was 54.0%(±5.91%).The real inclusion ratio was 51.6%under the optimized process.Therefore,the optimized process was credible.The formation of HPCD/astaxanthin complex was confirmed and the configuration was speculated by UV-Vis,IR and NMR.The configuration of HPCD/astaxanthin complex was that the hexacyclic ring end of astaxanthin entered the cavity of HPCD,and the barycenter of the hexacyclic ring located around the H-5 of HPCD cavity.The TG/DTA result indicated that the formation of complex improved the thermal decomposition start temperature by at least 40℃,reached up to 290℃.The light/thermal test showed that inclusion increased the stability of astaxanthin,and regulated the release of astaxanthin.The complex also had high storage stability.The antioxidant ability of HPCD/astaxanthin complex was investigated in vitro.The deoxidization ability of the complex was higher than that of astaxanthin and ascorbic acid.The ability of eliminating DPPH free radical of the complex was far higher than that of astaxanthin and ascorbic acid at a low concentration,however,the elimination rate decreased along with the increase of the concentration.The ability of eliminating hydroxy free radical of the complex was a little lower than that of astaxanthin but higher than that of ascorbic acid.The superoxide anion radical scavenging activity of the complex was lower than that of astaxanthin but far higher than that of ascorbic acid.In conclusion,the antioxidant ability of the complex became a little lower due to the effect of inclusion.However,it had better results in hydrophilic systems.