Dimerization Equilibrium Constant Research Articles

Measurement of the Dimerization Equilibrium Constants of Enantiomers

The formation of homochiral and heterochiral dimers in solutions of enantiomers causes a number of remarkable phenomena, such as the enantiomeric enrichment of nonracemic mixtures on achiral chromatographic columns when chiral monomers and dimers exhibit different retention behavior. In this work, such effects on optical rotation and UV absorbance are analyzed theoretically and experimentally. In particular, it is demonstrated how optical rotation measurements can be used to estimate the dimerization equilibrium constants. It is shown how significant this is for polarimeter calibration, as well as for the determination of enantiomer concentrations in process streams, when optical rotation and UV absorbance measurements are combined. All experimental measurements refer to the enantiomers of the chiral 2,2‘-dihydroxy-1,1‘-binaphthyl in chloroform, at a temperature of 23 °C. The measured values of its dimerization equilibrium constants are 1.3 ± 0.5 and 3.1 ± 1.0 L/mol for the formation of homochiral and of heterochiral dimers, respectively.

Chemical equilibrium in supercritical fluids: Solvent effects on the dimerization equilibrium constant

We study dimerization equilibrium between two dilute solutes in a supercritical solvent. Nonlocal hypernetted chain integral equation theory is employed to calculate the dimerization equilibrium constant for a wide range of solvent thermodynamic conditions and model potential parameters. The equilibrium constant displays a characteristic three-regime density dependence along near-critical isotherms, where it initially increases at low densities, decreases in the medium-density near-critical region, and increases again at high densities. The three regimes in the behavior of the equilibrium constant are discussed in terms of the energy-dominated low-density, desolvation-dominated medium-density, and entropy-dominated high-density regions.

Dimerization of organic free radicals in solution.: 1. Temperature dependent measurements

Dimerization enthalpies and equilibrium constants have been determined for the radical anion of 2,3-dichloro-5,6-dicyano- p-benzoquinone (DDQ), the radical cations of N, N, N′, N′-tetramethyl- p-phenylenediamine, N, N-dimethyl- p-phenylenediamine, 2,3,5,6-tetramethyl- p-phenylenediamine, N, N, N′, N′-tetraethyl- p-phenylenediamine, N, N-diethyl- p-phenylenediamine and N, N, N′-trimethyl- p-phenylenediamine. Neutral radicals investigated are 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and galvinoxyl. Solvent used was acetone, EtOH/Et 2O-mixture (2:1 volume), propionitrile/butyronitrile-mixture (1:1 M) and dichloromethane. Measured dimerization enthalpies Δ H dim vary from −72.1 to −16.6 kJ/mol.

FT-IR study on interactions between solutes and entrainers in supercritical carbon dioxide

Fourier transform infrared (FT-IR) spectroscopy has been used to measure the molarities of hydrogen bonding species between carboxylic acids (acetic acid and palmitic acid) and water in supercritical CO 2. The equilibrium constants of dimerization for the carboxylic acids were determined in supercritical CO 2 with octane. Further, the interactions of propanol-d (1- and 2-propanol-d) or xylenol (2,5-, 2,6- and 3,4-xylenol) isomers with acetone in supercritical CO 2 were studied. Experiments were carried out at 308.2–313.2 K and 7.0–20.0 MPa. The molarities of hydrogen bonding species between the carboxylic acids and water in supercritical CO 2 increase with the increasing molarity of water. The carboxylic acids interact more easily with ethanol than water in supercritical CO 2. For supercritical CO 2+carboxylic acid+octane systems, the equilibrium constants between the carboxylic acid monomer and dimer increase with the increasing molarity of octane. The equilibrium constants of the carboxylic acids seem to approach to those in liquid paraffin according to addition of octane in supercritical CO 2. The amount of the interaction species between 1-propanol-d and acetone is larger than that between 2-propanol-d and acetone. The amount of acetone interacting with OH group for 3,4-xylenol is the largest among those for xylenol isomers. These differences among the isomers may be caused by the screen effects of methyl groups around hydroxyl group for the isomers.

The infrared spectra and enthalpies of strongly bound dimers of phosphinic acids in the gas phase. (CH 2Cl) 2POOH and (C 6H 5) 2POOH

Infrared spectra of phosphinic acids R 2POOH (R=CH 2Cl, C 6H 5 and CH 3) have been recorded in the gas phase. The equilibrium between monomers and dimers of R 2POOH is studied in the temperature range of 400–650 K. The broad absorption band in the region 3600–900 cm −1 is assigned to the ν(OH) stretching vibration of the very strongly bound cyclic dimer. The characteristic ABC structure of the ν(OH) band, typical for the spectra of strongly hydrogen-bonded complexes in the solid phase, is observed in the gas phase spectra of dimers. Unlike the case of solid-phase spectra, the intensity of components decreases in the sequence A, B, C. The ν(OH) bands of free R 2POOH molecules (effective half-width is about 35 cm −1) appear in the range 3660–3630 cm −1 at temperatures 420–500 K. The dimerization constants, the spectral moments of monomer and dimer bands, and their intensities have been determined. The H-bond energies are estimated from the temperature dependence of equilibrium constants of dimerization. The values obtained for phosphinic acids are found to be equal to 25–50 kcal mol −1 per dimer. The spectral and energetic features of phosphinic acid dimers are typical for very strong hydrogen bonds with a partial covalent character.

Aggregation studies of benzoporphyrin derivative

The photophysical properties of a ring B benzoporphyrin derivative 1,3-diene dimethylester were investigated in solution (water–DMSO mixtures). At low proportions of water in DMSO, the diester obeys Beer's law at all pH values and ionic strengths investigated. For solvent systems containing more than 30% water, the extent of deviation from Beer's law as a function of porphyrin concentration is attributed to formation of porphyrin dimers. Dimerization equilibrium constants (Kd) have been determined in different percentage of solvent mixtures, using absorption and fluorescence spectroscopies, and were found to exhibit a linear correlation between log (Kd) and the inverse of the dielectric constant (log (Kd) = 24.6 – 1060.4 × 1/εmix). This equation permits the calculation of Kd in mixtures of water–DMSO.Key words: porphyrins, dimerization, aggregation, fluorescence, benzoporphyrin derivative, photodynamic therapy, photosensitizers.

Computing molecular complexes in earth's and other atmospheres

There has been a considerable research interest in molecular aggregates, complexes or clusters, relevant to Earth's and other planetary atmospheres. Consequently, a considerable amount of data has been obtained in laboratory observations and also in computations. This report surveys our recent and ongoing computations of several such systems of atmospheric significancy, both systems with relatively weak and relatively strong bonding interactions. Among them, several types of homo- and hetero-dimers with importance to Earth's atmosphere: (H2O)2, (N2)2, N2O2, , (O2)2, or (O3)2. Dimer of carbon dioxide (CO2)2 is computed owing to its supposed significancy in the atmosphere of Venus. Systems with stronger bonding are represented by ClONO2H+ and 2,3,7,8-tetrachlorodibenzo-p-dioxin. The report also discusses computational tools, combining advanced quantum-chemical methods with statistical-mechanical treatments. The structure, energetics, and vibrations of the complexes are evaluated at correlated ab initio levels. The computations typically show several minimum-energy structures and their relative populations are sensitive to temperature. The computed dimerization equilibrium constants are of a special interest as a critical stability measure and an input information for evaluations of the altitude population profiles in the atmosphere. A special attention is paid to the temperature enhancement of clustering degree in saturated vapors. This interesting paradox represents a product of atmospheric studies though it is actually a phenomenon of a more general physico-chemical validity.

Coupled energetics of λ cro repressor self-assembly and site-specific DNA operator binding II: cooperative interactions of cro dimers

The bacteriophage λ relies on interactions of the cI and cro repressors which self assemble and bind the two operators (O R and O L) of the phage genome to control the lysogenic to lytic switch. While the self assembly and O R binding of cI have been investigated in detail, a more complete understanding of gene regulation by phage λ also requires detailed knowledge of the role of cro repressor as it dimerizes and binds at O R sites. Since dimerization and operator binding are coupled processes, a full elucidation of the regulatory energetics in this system requires that the equilibrium constants for dimerization and cooperative binding be determined. The dimerization constant for cro has been measured as a prelude to these binding studies. Here, the energetics of cro binding to O R are evaluated using quantitative DNaseI footprint titration techniques. Binding data for wild-type and modified O R site combinations have been simultaneously analyzed in concert with the dimerization energetics to obtain both the intrinsic and cooperative DNA binding energies for cro with the three O R sites. Binding of cro dimers is strongest to O R3, then O R1 and lastly, O R2. Adjacently bound repressors exhibit positive cooperativity ranging from −0.6 to −1.0 kcal/mol. Implications of these, newly resolved, energetics are discussed in the framework of a dynamic model for gene regulation. This characterization of the DNA-binding properties of cro repressor establishes the foundation on which the system can be explored for other, more complex, regulatory elements such as cI- cro cooperativity.

Quantitative characterization of desorptive stripping voltammograms complicated by surface dimerization reactions. Application to the reductive desorption of thiols from mercury

Abstract The influence of analyte adsorption and of a follow-up surface dimerization reaction on desorptive stripping voltammograms has been investigated. A modified version of the spline orthogonal collocation technique has been developed to compute the voltammetric response under Langmuirian reversible conditions. Simple analytical expressions derived for surface-confined redox processes are shown to reproduce the stripping voltammograms only in the presence of significant ( θ * R >0.01) analyte adsorption. In the absence of analyte adsorption, asymmetrical waves are obtained as an intrinsic characteristic of the stripping protocol. Empirical equations relating the stripping peak potential and the voltammetric half-height width with scan rate and adsorbed product coverage are presented. Voltammetric scan rate dependence is shown to provide a quantitative estimate of the analyte adsorption extent, whereas its dependence on adsorbed product coverage allows one to identify the presence of surface dimerization processes. Maximum surface excesses and dimerization equilibrium constants can readily be obtained by plotting the reciprocal half-height width versus the surface concentration of stripped species. In the absence of analyte adsorption, a complementary convolutive analysis is also proposed. Application is made to the cathodic stripping of three mercaptocarboxylic acids differing in their hydrocarbon chain-length. The extent of oxidized product dimerization is shown to display a strong dependence on the solution pH, so that higher peak current sensitivities are found in acidic solutions, where oxidized products are present as dimers.

Oligomerization and cis-trans isomerization equilibria in dichloropalladium(II) metallacrown ethers and a dichloropalladium(II) complex of 1,12-bis(diphenylphosphino)dodecane

Quantitative 31P{1H} NMR spectroscopic studies of monomer–oligomer and cis-trans equilibria in chloroform-d solutions of PdCl2{Ph2P(CH2CH2O)nCH2CH2PPh2-P,P′}n (n = 3, 4, 5) metallacrown ethers and of PdCl2{Ph2P(CH2)12PPh2-P,P′} are reported. The NMR data for all of the complexes can be adequately modeled using a single cis-trans isomerization equilibrium and two step-polymerization (dimerization, oligomerization) equilibria. As expected, both the dimerization and oligomerization equilibrium constants for the metallacrown ethers increase as n increases. However, the dimerization equilibrium constants of the metallacrown ethers are much smaller than are the oligomerization equilibrium constants. In contrast, the dimerization and oligomerization equilibrium constants for PdCl2{Ph2P(CH2)12PPh2-P,P′} are nearly identical and are significantly larger than are those of the metallacrown ethers. Kinetic studies of monomer–oligomer and cis-trans equilibria in solutions of cis-PdCl2{Ph2P(CH2CH2O)3CH2CH2PPh2-P,P′} indicate that the trans monomer and trans oligomers are formed at approximately the same rates and that the reactions follow reversible first-order kinetics. The very different dimerization constants for the metallacrown ethers and PdCl2{Ph2P(CH2)12PPh2-P,P′}and the similar rates of formation for the trans monomer and trans oligomers suggest that the isomerization and dimerization reactions have a common rate-determining step that involves cleavage of a palladium–phosphorus bond.

Correlation of Solubilities and Entrainer Effects for Fatty Acids and Higher Alcohols in Supercritical Carbon Dioxide Using SRK Equation of State with Association Model.

Solubilities of fatty acids (myristic acid, palmitic acid and stearic acid) and higher alcohols (cetyl alcohol, stearyl alcohol and arachidyl alcohol) in supercritical CO2, and the entrainer effect (ethanol or octane) on their solubilities, are correlated by the Soave-Redlich-Kwong equation of state with an association model. It is assumed that the system consists of supercritical CO2, solute monomer, solute dimer, hydrogen bonding species between solute monomer and entrainer, and entrainer. The mole fractions of these species are determined by the equilibrium constants for dimerization of palmitic acid, and those for hydrogen bonding species between palmitic acid and ethanol obtained from FTIR spectroscopic study. The calculated results agreed with the experimental data.

Structural properties of the metallointercalator cationic complex (2,2′:6′,2″-terpyridine)methylplatinum(II) ion

The complex [Pt(tpy)(Me)] +(BPh 4) − (tpy=2,2′:6′,2′′-terpyridine) crystallizes in the triclinic space group P1̄ with a=13.463(2) Å, b=13.618(3) Å, c=20.151(4) Å, α=73.59(2), β=74.56(3), γ=65.82(2)° and Z=4. The final conventional R factor is 0.035. In the unit cell a couple of weakly interacting dimers are formed by stack of two [Pt(tpy)(Me)] + cations in a head-to-tail fashion with intermolecular Pt⋯Pt distances of 4.437(1) and 4.931(1) Å, respectively. The absorption spectra of [Pt(tpy)(Me)] +(BPh 4) − in acetonitrile show bands assigned to π–π* and to MLCT transitions. The analysis of the dependence of the spectra on complex concentration gives a fairly low value for the dimerization equilibrium constant ( K d=180±52 M −1 at 298 K). These fluid solutions are not emissive. The room-temperature solid-state emission spectra of the salts [Pt(tpy)(Me)]X are strongly dependent on the counterion (X=BPh 4 −, Cl −, PF 6 −, ClO 4 −, CF 3SO 3 −). The cationic complex shows a considerable stability upon acidification and carbonilation in water.

Monomer-dimer equilibrium constants of RNA in the dimer initiation site of human immunodeficiency virus type 1.

The genome of the human immunodeficiency virus (HIV) exists as a dimer of two identical RNA molecules hydrogen bonded to each other near their 5' ends. The dimer, known to be important for viral infectivity, is formed by two monomers interacting through a stem-loop structure called the dimer initiation site (DIS). An initially formed intermediate, the "kissing" dimer, is unstable and rearranges to the stable, duplex form. In this report we use nondenaturing polyacrylamide gel electrophoresis to measure the monomer-dimer equilibrium constant of three RNA sequences, 41-, 27-, and 19-mers, located in the DIS of the MAL isolate of HIV-1. Experiments in which the RNA was equilibrated at various temperatures before electrophoresis revealed that interconversion is rapid for all the sequences, so that they reach equilibrium in the loading well of the gel at 5 degrees C before they enter the gel proper. However, interconversion kinetics in the gel are slow, so autoradiographic spot intensities can be used to measure the amounts of monomer and dimer present when the sample entered the gel. After correction for the amount of RNA added with the radiolabel and dilution of samples in the loading well of the gel, dimerization equilibrium constants were calculated from spot intensities. The calculated values of the dimerization constant K at 5 degrees C were approximately 10(5), approximately 10(6), and approximately 10(8) M(-1) for the 41-, 27-, and 19-mers, respectively, in solutions of ionic strength, I, of about 100 mM. The decrease in K by three orders of magnitude between the 19-mer and 41-mer is due in part to the change in rotational entropy of rodlike molecules on dimerization and in part to the increased conformational entropy of the monomers. As expected, increased ionic strength increases the dimerization constant for all three RNAs. For the 41-mer, however, K has a maximum value at I approximately 140 mM. The origin of the decrease in K for higher I is unknown but it may be due to formation of species (perhaps higher order oligomers) that do not enter the gel. The 41-mer exists in two dimeric forms assigned to the kissing and duplex dimers. The ratio of kissing to duplex form at 5 degrees C is 0.48 +/- 0.22 at I = 113 mM and 0.91 +/- 0.35 at I = 183 mM. The observed decrease in K with RNA length suggests that the dimerization constant of the packaging region of HIV-1 is small, < approximately 10(5) M(-1), implying that the nucleocapsid protein is important in promoting dimerization in the capsid of the virus.

Fluorescent Dimers of Merocyanine 540 (MC540) in the Gel Phase of Phosphatidylcholine Liposomes

Abstract— Fluorescence emission from merocyanine 540 (MC540) dimers was observed in dipalmitoylphosphatidylcholine (DPPC) vesicles. This unusual behavior was observed only for vesicles in the gel‐phase state. No dimer fluorescence was observed either in monopalmitoylphosphati‐dylcholine (C16PC) micelles or in liquid‐crystalline DPPC vesicles, indicating that dimer fluorescence efficiency increases in highly packed interfaces. The excitonic theory of Kasha was used to interpret the spectral features. The overall fluorescence quantum yield (φr) decreases with decreasing lipid: probe ratio, not only because of the presence of a weakly fluorescent dimer that absorbs a high fraction of the total absorbed light but also due to quenching of monomer emission. This suggests the existence of probe domains. The dimer fluorescence quantum yields (φm) were estimated in DPPC large unilamellar vesicles (LUV) and DPPC multilamellar vesicles. The dependence of φr with probe concentration is compatible with values of φm lower than 0.05. The dimerization equilibrium of MC540 in C16PC micelles and DPPC‐LUV was also studied. Apparent dimerization equilibrium constants, Kdapp and dimer absorption spectrum were calculated in C16PC micelles for the first time. The dimerization equilibrium constant in DPPC‐LUV was calculated and discussed in terms of the fraction of volume occupied by the lipid phase.

Structure-Reactivity Studies in Copper(II)-Catalyzed Phosphodiester Hydrolysis.

We have investigated the ability of a series of three related copper complexes Cu([9-11]aneN(3))X(2) to hydrolyze the activated phosphodiesters bis(4-nitrophenyl) phosphate (BNPP) and ethyl 4-nitrophenyl phosphate (ENPP). The compound Cu([10]aneN(3))Br(2) crystallizes in the monoclinic space group C2/c, a = 20.693(4) Å, b = 11.429(2) Å, c = 20.138(4) Å, beta = 104.78(3) degrees, V = 4605(2) Å(3), and Z = 16. The compound Cu([11]aneN(3))Br(2) crystallizes in the orthorhombic space group Pnma, a = 13.7621(10) Å, b = 8.7492(13) Å, c = 10.0073(10) Å, V = 1205.0(2) Å(3), and Z = 4. The structure of Cu([9]aneN(3))Cl(2) was previously reported (Inorg. Chem. 1980, 19, 1379). The crystal structures of the three complexes show a progression in geometry from square pyramidal to distorted trigonal bipyramidal as the size of the macrocycle increases. Larger macrocycles also result in the copper ion being pulled closer to the plane defined by the three ligand nitrogens, which in turn results in an increase in the sum of the three N-Cu-N angles from 249 degrees to 257 degrees to 278 degrees along with a concomitant decrease in the X-Cu-X angle. Significantly, the rate constant for the hydrolysis of BNPP by Cu([9-11]aneN(3))X(2) increases by nearly an order of magnitude as the ligand size increases from a nine-membered to an 11-membered ring. Correlations have been made between the catalytic ability of Cu([9-11]aneN(3))X(2) and the structural and electronic properties of the complexes. All three catalysts exist in a monomer-dimer equilibrium in solution, with the monomer being the catalytically active species. As the ligand size increases, the dimer formation constant (K(f)) decreases due to steric constraint, thereby increasing the concentration of active species and hence the rate of hydrolysis. The contributions of Lewis acidity and steric constraint on both substrate binding (K(2)) and P-O bond cleavage (k(3) or k(cat)) are less important than the dimerization equilibrium constant in determining the rate of the reaction.

Medium effects on the dimerization of coproporphyrin-I free base

Equilibrium constants of dimerization of the tetraanion of coproporphyrin-I free base were determined spectrophotometrically in aqueous solution at pH 9.0 in the presence of nine electrolytes, including salts of alkali metal, tetraalkylammonium and Ca(II) cations. Additions of all electrolytes promoted dimerization and the effect increased in the order tetraalkylammonium ≪ alkali metals ≪ Ca(II) salts. The Effects of alkali metal salts at concentrations below 0.1 M were quantitatively described in terms of Debye–Hückel model assuming an effective charge of the porphyrin monomer of −3. At higher salt concentrations salting-out/in effects must be taken into account. Addition of tetraalkylammonim salts to solutions containing 0.1 M NaCl produced the monomerization effect attributed to strong salting-in behavior of these salts. The effect of Ca(II) was described by complexation-induced dimerization of the porphyrin. Addition of organic co-solvents (methanol, ethanol, acetone, dioxane) to aqueous electrolyte solutions of the porphyrin produced a strong monomerization effect. Logarithms of dimerization constants extrapolated to the zero ionic strength in each mixed solvent roughly correlated with inverse dielectric constant with a negative slope considerably exceeding, however, that expected on the basis of a purely electrostatic model. Correlations with empirical solvent parameters (δH, ETN, SP) were less satisfactory than the correlation with­1/ε. A satisfactory description of solvent effects on dimerization constants was found in terms of a specific solvation model, which takes into account association of porphyrin monomer and dimer with several co-solvent molecules. Logarithms of the association constants were linear functions of the partial molar volume of the hydrocarbon portion of co-solvent molecules. Copyright © 1999 John Wiley & Sons, Ltd.

Solvent-induced frequency shifts in the infrared spectrum of acetone in organic solvents

The solvent induced frequency shifts observed in the infrared spectrum of acetone as a dilute solute are reported for 23 organic solvents, both polar and non-polar. Both the C2O stretching frequency and the asymmetric stretch of the molecular skeleton show strong correlations with the acceptor number of the solvent, a measure of its Lewis acidity. A detailed study of the absorption in the 1715 cm-1 region for acetone–carbon tetrachloride and acetone–nitrobenzene mixtures reveals the presence of three bands, one due to the acetone monomer and two due to dimers. These data are analyzed to obtain estimates of the dimerization equilibrium constant.

Glycosylated derivatives of tetraphenyl porphyrin: photophysical characterization, self-aggregation and membrane binding

A series of glucose- or galactose-residue-bearing tetraphenyl porphyrins (TPPs) has recently been synthesized with the aim of studying the structural dependence of porphyrin cellular localization and efficiency in photodynamic therapy (PDT). For the present investigation, four derivatives with different (spherical or planar) configurations and/or different hydrophobicity have been selected. As a first step, singlet-state spectroscopic properties such as spectral characteristics, lifetime and quantum yield in different solvents are determined. It is found that besides the solvent composition, the configuration of the molecule critically influences the singlet-state spectroscopic properties. Possible self-aggregation processes are investigated at room temperature in phosphate buffer (pH = 7.4). Dimerization equilibrium constants are determined. These values are found to be dependent on the chemical structure of substituents on the tetraphenyl groups and only slightly on the conformation of the whole molecule. The association of non-aggregated forms of porphyrins to unilamellar liposomes, modelling the lipid bilayer of a biological membrane, is studied by fluorescence spectroscopy at neutral pH. On mixing with liposomes, amphiphilic porphyrin derivatives exhibit an increase in their fluorescence intensity and lifetime. The monomer-liposome binding constants are determined for these derivatives. The localization of liposome-bound dyes is studied by fluorescence labelling of (a) the lipid region in interaction with both lipid chains and headgroups or (b) the carbohydrate chain region of lipids. Alterations in fluorescence intensities of porphyrin derivatives in the presence of liposomes and changes in the lifetime and fluorescence excitation anisotropy of fluorescent markers in liposomes are detected only for unsymmetrically substituted amphiphilic porphyrin derivatives. Our results suggest that the presence of at least one apolar substituent on tetrapyrrolic ring is required for the localization in the lipid phase.

Voltammetric Studies of t ‐ BuC60 Anion

Electrochemical studies of the reduction and oxidation of have been conducted at platinum electrodes in tetrahydrofuran and dimethyl sulfoxide. The formal potentials for the first three steps of reduction are reported and compared to those of . The fact that these potentials are displaced in the negative direction from those of has been interpreted in terms of the dominance of electrostatic effects in determining the potentials of fullerenes and their derivatives. Oxidation of gives the corresponding radical which takes part in a reversible dimerization reaction. The cyclic voltammetric behavior as a function of scan rate and concentration is consistent with this mechanism and simulations of the data have provided limits on the dimerization equilibrium and rate constants. Specifically, the equilibrium constant falls in the range of 1.9 to and the rate constant for dimerization is 2 to .

Synthesis of a New Cationic Pyropheophorbide Derivative and Studies of its Aggregation Process in Aqueous Solution

Abstract— A new water‐soluble cationic pyropheophorbide was prepared from pyropheophorbide a by a Curtius rearrangement. The self‐aggregation process of this chlorhydrate of aminopyropheophorbide was investigated from the changes observed in its spectroscopic properties when passing from ethanol to aqueous solutions. The formation of aggregates was characterized in the absorption spectrum by a broadening of the Qy band, which is slightly shifted to the red, a significant increase in signals of circular dichroism and a drastic quenching of the fluorescence intensity at 674 nm. Analysis of the nonlinear variation of the fluorescence intensity with the dye concentration in Tris‐buffered solutions shows that dimerization is the dominant aggregation process. The dimerization equilibrium constant(at 25AoC, pH 7.5 in 5 mM Tris buffer, 10 mM NaCl) was determined to be 3.106M‐l. NMR measurements performed in DMSO at various dye concentrations showed that only the chemical shifts of some atoms are affected by these changes, indicating the part of the ring involved in the overlap of the two molecules in the dimer.A molecular modeling study of the packing arrangement of the aminopyropheophorbide molecules in aggregates performed by both semi‐empirical quantum chemical PM3 and force field MM2 methods provided a structure consistent with these spectroscopic data.