A Computational Study Of The Ground And Excited State Acidities Of Synthetic Analogs Of Red Wine Pyranoanthocyanins

Natural pigments are a kind of pigments that are derived from the natural resources such as roots,stems,leaves,flowers,fruits of plants and animals or microorganisms.Anthocyanins are a kind of natural pigments found in almost all tissues of plants and are water-soluble in the vacuole.In nature,anthocyanins accumulation provides the colors that could attract a wide variety of animal pollinators in flowers,while the same colors could help seed transmission by attracting herbivores to edible fruits with these red,blue,or purple colors in fruits.Anthocyanins are sensitive to pH values,so they can be used as a pH indicator.In addition,anthocyanins can convert light energy into electricity,so they can be used as the solar cell.Anthocyanins are widely used,but due to their unstable chemical properties,they are easily affected by temperature,acidity and other conditions,these has restricted the applications of anthocyanins.Pyranoanthocyanins can be obtained from anthocyanins during the maturation of red wines.Their chemical properties are similar to anthocyanins,but pyranoanthocyanins have higher pH stability.However,Isolation and purification of pyranoanthocyanins from red wine are difficult and this became the limiting factor for studies of pyranoanthocyanin chemistry,spectroscopy and photophysics.Therefore,synthetic routes for the preparation of analogs of pyranoanthocyanins(pyranoflavylium cations)are improved and now available.Pyranoflavylium cations have the basic chromophoric unit analog of pyranoanthocyanins.The acidities researches of compounds have important implications for many aspects of practical chemistry and biological chemistry.Therefore,precise quantitative studies of acidities in gas and/or condensed phase are very important.The acidity of the compound can be represented by the acid dissociation constant(pKa)value.There are many experimental methods to determine the pKa value but accurate experimental measurements of acidities are not always straightforward.Consequently,it is of great significance to develop a computational protocol that can consistently predict acidities in the gas and condensed phases.Quantum chemical calculation methods have been proved to be of great significance in the determination of pKa values.In previous studies of calculations of pKa values,there are many quantum chemistry methods for the calculations of pKas at the ground state among which the DFT method is the most widely used.Meanwhile,some studies also tend to calculate the pKa values at the excited singlet state.However,there are few studies on the pKa calculations of excited triplet state.Our work is based on the exist calculation methods for calculation pKa values of the ground state,and we calculated the pKa values for the ground state,excited singlet state and excited triplet state.These calculations provide a solid theoretical basis for studing the reliable methodology for calculating the pKa values at the excited state especially the excited triplet state.In the present work,we employed TD-DFT with the B3-LYP functional and the def2-TZVP basis set,combined with estimation of solvation free energies via COSMO,to estimate a priori the pKa values of the ground state and the lowest excited singlet and triplet states of a series of seven substituted hydroxypyranoflavylium cations.For the ground state and the first excited singlet state,the quantum chemical results compare favorably to experimental values for most of these compounds.Although there are currently no experimental data for the lowest excited triplet state,the acidity is predicted to be more similar to that of the ground state than to the excited singlet state,as is generally found experimentally for photoacids.The adequacy of the present theoretical framework for estimating the ground and excited state acidities of pyranoflavylium cations indicates that it could provide a better understanding of the ground and excited state acidity of the naturally occurring pyranoanthocyanins in mature red wines as well.Therefore,our work provides a significant theoretical basis for the photoacid study of anthocyanins compounds,and the developed method is of great significance for applications of anthocyanins in solar cell,promoting the production of plant foods,as well as in biology and pharmacology.