Protonation-induced Changes Research Articles

Insights of the pH-dependent emission of carbon dots: a density functional theory study

The mechanism of luminescence in carbon dots has long been a topic of controversy, as accurately characterizing their structure has proven difficult. This has made it challenging to guide the design and synthesis of carbon dots, hindering the development of highly efficient fluorescent varieties. Understanding the mechanism and regulatory mechanisms of carbon dot luminescence at the molecular level is important not only from a theoretical perspective but also has practical value. Previous studies have suggested that the fluorescence emission wavelength of carbon dots can be altered by changing the pH of the carbon dot solution, but the mechanism behind this modulation is not yet fully understood. In this study, we used density functional theory calculations and excited state simulations with 2, 3-Diaminophenazine, a representative luminescent molecule found in molecular state luminescent carbon dots, to investigate the effect of protonation on the absorption and fluorescence emission spectra of carbon dots. We analyzed the electronic structure and spatial distribution of the luminescent molecules before and after protonation and found that the change in molecular dipole moment caused by protonation was responsible for the significant red shift in the fluorescence spectrum. To further verify our findings, we modified the substituted functional groups with different electron-pulling abilities in the corresponding luminescent molecules to modulate the molecular dipole. The results showed that the larger the dipole moment in the luminescent molecules with different substituted functional groups, the longer the fluorescence emission wavelength. Therefore, the protonation-induced change in molecular dipole is the main reason for the pH-dependent modulation of luminescence in molecular state luminescent carbon dots.

A General Twisted Intramolecular Charge Transfer Triggering Strategy by Protonation for Zero-Background Fluorescent Turn-On Sensing

The exploration of organic fluorescent sensing materials and mechanisms is of great significance, especially for the deep understanding of twisted intramolecular charge transfer (TICT). Here, the electron-donating ability of a chemically protonated amino group and the corresponding excitation primarily ensure the occurrence of excited-state intramolecular proton transfer. Due to the hybridization of the amino group from sp3 to sp2, the steric hindrance effect and conjugative effect together boost the rotation efficiency of the TICT process and the complete elimination of the background fluorescent signal. Furthermore, a sharp turn-on fluorescent detection of trace nitrite particulate with a diameter of 0.44 μm was realized. In addition, this protonation-induced change in the amino group configuration was verified through around nine categories of compounds. We expect this modulation of the photochemical activity path of the TICT process would greatly facilitate the exploration of novel fluorescent sensing mechanisms.

Protonation-induced change in the conformation, crystal structure and property of triarylmethyl carbocation radical

We report a new triarylmethyl carbocation radical which was easily synthesized via ⿿one-pot⿿ approach. Interestingly, this ⿿cis⿿ triarylmethyl carbocation radical converted to the ⿿trans⿿ one upon protonation. Such an obviously conformational change is triggered by the intermolecular hydrogen bond interaction involving one exocyclic ⿿keto⿿ oxygen atom and the protonated pyridinium nitrogen atom. Two triarylmethyl carbocation radical crystals were characterized by the elemental analysis and single crystal X-ray diffraction. A change in conformation also brings about change in crystal structure, phosphorescence and magnetic susceptibility properties.

Protonation-induced changes in the macroorganization of LHCII monolayers

The major light-harvesting complex of photosystem II (LHCII) is an important regulatory protein in photosynthetic membranes. In vivo LHCII forms stable trimers and is found either associated to photosystem II or in LHCII-only containing domains. It was suggested that in native thylakoid membrane LHCII changes its conformation and macroorganization upon switching from light-harvesting to photoprotective state. Herein we have analyzed LHCII Langmuir monolayers at different subphase salt composition and in two different states – partly deprotonated (LHCII), at low basic pH 7.8, and highly protonated (p-LHCII), at pH 5.2, mimicking the functional light-harvesting and light-protective states of the protein, respectively. We have found strong difference in the supramolecular organization of the protein in these two functional states, the protonated monolayer exhibiting higher order of organization and significantly higher stability compared to the partly deprotonated one. Both LHCII and p-LHCII monolayers are composed of trimers self-assembling in aggregates with different packing density – loosely packed compiling homogeneous well-ordered monolayer areas and tightly packed organized in heterogeneous disordered phase. These two types of macroorganization are found in different proportions in protonated and partly deprotonated LHCII monolayers, the p-LHCII monolayer being much more heterogeneous than LHCII one.

Protonation-Induced Changes in the Photophysical Properties of a Phthalocyanine and a Covalently-Linked, Phthalocyanine-C60 Fullerene Dyad

The effect of the protonation on the photophysical properties (i.e., ground state and excited state properties) of Zn(II)phthalocyanine (Zn(II)Pc) 2 and covalently-linked, electron donor-acceptor Zn(II)Pc-C60 1 have been investigated. On one hand, important changes in the absorption features of these two systems have been seen upon addition of trifluoroacetic acid due to protonation of some meso nitrogens of the Pc moiety. On the other hand, significant changes in the lifetime of the singlet excited states – in the case of 1 and 2 – and of the charge-separated state – in the case of 1 – were also observed. We reckon that this study, unprecedented in term of the study of the time-resolved excited state features of a protonated Pc-C60 conjugate, will contribute in terms of shedding light toward a better understanding of photoinduced processes in Pc-based systems.

Isomer-selective detection of microsolvated oxonium and carbenium ions of protonated phenol: infrared spectra of C6H7O+-Ln clusters (L=Ar/N2, n

Infrared photodissociation (IRPD) spectra of clusters composed of protonated phenol (C(6)H(7)O(+)) and several ligands L are recorded in the O-H and C-H stretch ranges using a tandem mass spectrometer coupled to a cluster ion source. The C(6)H(7)O(+)-L(n) complexes (L=Ar/N(2), n=1-6) are generated by chemical ionization of a supersonic expansion. The IRPD spectra of mass selected C(6)H(7)O(+)-L(n) clusters obtained in various C(6)H(7)O(+)-L(m) fragment channels (m<n) display the unambiguous fingerprints of at least two different C(6)H(7)O(+) nucleation centers: the oxonium ion (5) and the carbenium ion(s) corresponding to protonation of phenol in ortho and/or para position (1/3). These two classes of C(6)H(7)O(+)-L(n) isomers show very different fragmentation behavior upon IR excitation, facilitating the assignment of the observed vibrational transitions. The vibrational frequency shifts as a function of cluster size reveal that the microsolvation of 1/3 and 5 in Ar and N(2) begins with the formation of intermolecular hydrogen bond(s) to the acidic OH group(s) and proceeds by the formation of intermolecular pi-bonds to the respective six-membered rings. The analysis of photofragmentation branching ratios yields estimated ligand binding energies of the intermolecular OH- and pi-bonds for solvation of the different C(6)H(7)O(+) isomers. The effects of microsolvation on the properties of 1/3 as reactive intermediates in electrophilic aromatic substitution reactions are discussed. Comparison of clusters of protonated phenol with those of neutral phenol reveals the drastic protonation-induced changes in the topology of the intermolecular potential of aromatic molecules interacting with a nonpolar solvent. Moreover, the results show that the IRPD process can be used to selectively generate a spectroscopically clean ion beam of either 1/3 or 5 with some control over their internal energies.

Solvent accessibility of the adenosine 5'-triphosphate catalytic site of sarcoplasmic reticulum CaATPase.

The CaATPase of rabbit skeletal sarcoplasmic reticulum was labeled at or near the ATP catalytic site with fluoresceinyl isothiocyanate (FITC), and the accessibility of the attached probe to the bulk solvent was determined by I- quenching of its fluorescence. The quenching of free FITC was also measured. In both cases, the quenching was of the Stern-Volmer type and collisional quenching rate constants were obtained over the pH range 5-8 in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and with added Ca2+, vanadate, or phosphate. The fluorescence intensity and susceptibility to quenching by I- of free FITC were insensitive to the added ligands. In all cases, the intensity decreased with pH, as predicted from the known properties of FITC mono- and dianions. The collisional quenching rate constants increased at lower pH, as expected for I- quenching of a molecule with decreasing negative charge due to protonation. When FITC was attached to the CaATPase, the FITC fluorescence intensity and I- collisional quenching rate constants were sensitive to ligand binding as well as pH. The changes in fluorescence intensity with acidity, when compared to the results for free FITC, indicated the pKa of the FITC was reduced 0.6 unit when it was attached to the CaATPase. Excited-state lifetime measurements indicated that ligand effects at constant pH were not due to protonation-induced changes in FITC quantum yield but to conformational changes of the CaATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

The Use of Protonation Induced Changes in Carbon-13 NMR Chemical Shifts to Investigate the Solution Microscopic Structure of Partially Protonated Polybasic Molecules

Abstract Changes in the NMR chemical shift of carbon-13 nuclei upon protonation of a nearby basic center are shown to be a useful method of probing the microscopic site of protonation of molecules containing multiple basic centers, such as diethylenetriaminepenta-acetic acid (DTPA). This approach is also shown to support the predominance of the pyridine amine protonated tautomer in the 4-pyridone ↔ 4-hydroxypyridine equilibrium.