Photophysical modulations of biologically potent small molecules in biocompatible microheterogeneous environments created by cyclodextrins and lipid vesicles

254-264Small molecules having biological potency are applied in all the avenues of chemical, pharmaceutical, and biological research. Typically, the small molecules are delivered to the biological environment using a compatible host that acts as a carrier for the compound. The internal environments of the hosts are typically non-polar or less polar than the bulk aqueous environment so that the potent drugs, most of them being hydrophobic, can get encapsulated to be carried to the target. On incorporation, the photophysics of the compounds may change as compared to the external atmosphere. This may alter their inherent properties and mode of functioning. In this perspective article, the relevant progress in host-guest chemistry in biologically potent environment aided by cyclodextrins and lipid vesicles has been discussed

Rotamerisation and intramolecular proton transfer of 2-(2'-hydroxyphenyl)oxazole, 2-(2'-hydroxyphenyl)imidazole and 2-(2'-hydroxyphenyl)thiazole: a theoretical study

Semi-empirical (AM1-SCI) calculations have been performed on 2-(2'-hydroxyphenyl)oxazole (HPO), 2-(2'-hydroxyphenyl)imidazole (HPI) and 2-(2'-hydroxyphenyl)thiazole (HPT) to rationalise the photophysical behaviour of the compounds exhibiting intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The calculations reveal that there is a gradual variation in the properties from HPO to HPT through HPI so far as the existence of the rotational isomers in the ground state is concerned. While HPO gives rise to two stable rotamers (I and II) in all the common solvents, there is only one stable species for HPT in the S0 state. For HPI, rotamer II is possible only in the isolated state and/or in solvents of low polarity, but in high polar solvents it gives rise to the normal form (I) only. For all the molecules in the series, however, intramolecular proton transfer (IPT) takes place in the lowest excited singlet (S1) and the triplet (T1) states. Combination of the rotamerism and ESIPT gives rise to multiple fluorescence bands for the fluorophores. Theoretical assignments have been made for the excitation, fluorescence and phosphorescence bands. Simulated potential energy curves (PEC) in different electronic states reveal that the IPT process is feasible in either of the S1 and T1 states but not in the ground state. The ESIPT reaction has been found to be favoured both thermodynamically and kinetically in these electronic states compared to the ground state. However, quantum mechanical tunnelling has been proposed for the prototropic reaction to proceed in the excited states.http://www.sciencedirect.com/science/article/B6TGS-44HY5PX-9/1/f6f475f8cbcfdb79ee6df5446609e51

Theoretical modelling for the ground state rotamerisation and excited state intramolecular proton transfer of 2-(2'-hydroxyphenyl)oxazole, 2-(2'-hydroxyphenyl)imidazole, 2-(2'-hydroxyphenyl)thiazole and their benzo analogues

Two series of compounds, one comprising of 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzothiazole (HBT), and the other of 2-(2'-hydroxyphenyl)oxazole (HPO), 2-2'-hydroxyphenyl)imidazole (HPI) and 2-(2'-hydroxyphenyl)thiazole (HPT) are susceptible to ground state rotamerization as well as excited state intramolecular proton transfer (ESIPT) reactions. Some of these compounds show experimental evidence of the existence of two ground state conformers. Out of these two one undergoes ESIPT reaction leading to the formation of the tautomer. The two photophysical processes, in combination, result in the production of a number of fluorescence bands each one of which corresponding to a particular species. Semiempirical AM1-SCI calculations have been performed to rationalize the photophysical behaviour of the compounds. The calculations suggest that for the first series of compounds, two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the molecules of the other series existence of rotamers depends very much on the polarity of the environment. The potential energy curves (PEC) for the ESIPT process in different electronic states of the molecules have been generated theoretically. The simulated PECs reveal that for all these systems the IPT reaction is unfavourable in the ground state but feasible, both kinetically and thermodynamically, in the S1 as well as T1 states

2-(2’-hydroxyphenyl)imidazole, 2-(2’hydroxyphenyl)thiazole and Their Benzo Analogues

(HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) are susceptible to ground state rotamerization as well as excited state intramolecular proton transfer (ESIPT) reactions. Some of these compounds show experimental evidence of the existence of two ground state conformers. Out of these two one undergoes ESIPT reaction leading to the formation of the tautomer. The two photophysical processes, in combination, result in the production of a number of fluorescence bands each one of which corresponding to a particular species. Semiempirical AM1-SCI calculations have been performed to rationalize the photophysical behaviour of the compounds. The calculations suggest that for the first series of compounds, two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the molecules of the other series existence of rotamers depends very much on the polarity of the environment. The potential energy curves (PEC) for the ESIPT process in different electronic states of the molecules have been generated theoretically. The simulated PECs reveal that for all these systems the IPT reaction is unfavourable in the ground state but feasible, both kinetically and thermodynamically, in the S1 as well as T1 states

Photoluminescence of 2-(2'-hydroxy-5'-methylbenzoyl)-1,5-diphenylpyrrole in aqueous and β-cyclodextrin environments: manifestation of open and closed conformers

Steady-state fluorometric studies have been performed on 2-(2'-hydroxy-5'-methylbenzoyl)-1,5-diphenylpyrrole (HMBDPP) in aqueous and aqueous β-cyclodextrin (β-CD) environments at ambient temperature. The fluorophore mostly shows a single emission in aqueous solution. Addition of β-CD to the aqueous solution of the fluorophore results in the development of another emission band at higher energy. The difference in the fluorometric behaviour is assigned to a remarkable change in the polarity of the microenvironment within the supramolecular structural environment compared to that of the bulk aqueous phase. Semi-empirical calculation (AM1-SCI) rules out the possibility of intramolecular proton transfer reaction in any of the S<SUB>0</SUB>, S<SUB>1</SUB> and T<SUB>1</SUB> states of the fluorophore. It is proposed that HMBDPP exists mostly in the intermolecularly hydrogen-bonded form (open conformer) in aqueous solution while within β-CD environment, it is the intramolecularly hydrogen-bonded form (closed conformer) that predominates

Role of rotamerisation and excited state intramolecular proton transfer in the photophysics of 2-(2'-hydroxyphenyl)benzoxazole, 2-(2'-hydroxyphenyl)benzimidazole and 2-(2'-hydroxyphenyl)benzothiazole: a theoretical study

Semiempirical (AM1-SCI) calculations have been performed to rationalise the experimental findings in relation to the photophysics of 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzimidazole (HBI) and 2-(2'-hydroxyphenyl)benzothiazole (HBT). The calculations reveal that, while for HBO and HBI, two rotameric isomers are present in the ground state, there is only one stable species in the S<SUB>0</SUB> state of HBT. Excited state intramolecular proton transfer (ESIPT) reaction is, however, operative in the lowest excited singlet (S<SUB>1</SUB>) and triplet (T<SUB>1</SUB>) states for all the three molecular systems; resulting altogether three fluorescence bands for HBO and HBI and two for HBT. The excitation, fluorescence and phosphorescence bands have been assigned theoretically. The calculated results agree well with the existing experimental reports. The potential energy surfaces (PES) have been generated for the intramolecular proton transfer (IPT) reactions. The PES reflect that although the IPT process is not favourable in the ground state, the ESIPT process is feasible, both thermodynamically as well as kinetically, for all the three molecular systems in the S<SUB>1</SUB> as well as T<SUB>1</SUB> states

Photophysics of 1<i>H</i>-1,5-benzodiazepine in aqueous cyclodextrin environments

389-391The photophysics of 2, 3-dihydro-2,2,4-trimethyl-1 H-1,5-benzodiazepine (L) has been studied in aqueous cyclodextrin (CD) environments. Development of a new emission band for the CD-encapsulated moiety at the high energy side of the emission band in aqueous solution indicates that the fluorophore experiences a different microenvironment within the CD cavity, affecting the solvent-assisted excited state intramolecular proton transfer (SAESIPT) process. A difference in the relative enhancement of the blue emission compared to the other one has been rationalized on the basis of the size parameters of the fluorophore as well as the CD cavities

Application of Photoinduced Electron Transfer with Copper Nanoclusters toward Finding Characteristics of Protein Pockets

Proteins possess various domains and subdomain pockets with varying hydrophobicity/hydrophilicity. The local polarities of these domains play a major role in oxidation–reduction-based biological processes. Herein, we have synthesized ultrasmall fluorescent copper nanoclusters (Cu NCs) that are directed to bind to the different domain-specific pockets of the model protein bovine serum albumins (BSA). Potential electron acceptors, methyl viologen (MV) derivatives, were chosen such that they specifically reach the various domains following their hydrophobicity/hydrophilicity. Here, we have used MV²⁺, HMV⁺, and DHMV²⁺, possessing hydrophilic, intermediate, and hydrophobic specificities. Being electron acceptors, these derivatives draw electrons from the Cu NCs through photoinduced electron transfer (PET). The rate of PET varies at the different domains of BSA based on the local environment which has been analyzed. Here, PET is confirmed by steady state as well as time-resolved fluorescence spectroscopy. This study would provide a measurable way to identify the location of the different domains of a protein which is scalable by changing the superficial conditions without unfolding the protein