Pyrenexulfonic Acids Useful in Fluorescent Lipid Probes

The invention is related to a new class of lipid probes that can be used in lipid chemistry and analytical biochemistry.

Fluorescent lipid probes have frequently been used in the last years in order to study membranes, micelles, vesicles and lipid-water interfaces (Fluorescent Probes, G. S. Beddard and M. A. West (ed.), Academic Press, New York, 1981). In essence these probes consist of a lipid chain to which a fluorophore is attached. The lipid chain provides the compatibility with the lipid phase, whereas the fluorophore acts as a reporter for the microenvironment. Thus from the effect of a changing microenvironment or pH on the spectral properties of the fluorophore, conclusions can be made as to the microenvironment that surrounds the fluorescent probe.

A number of lipid probes has so far been reported in the literature, but only a few have been found to be useful in practice. Typical examples include (9-anthroyloxy)stearic acids (L. Tilley et al. (1979) J. Biol. Chem. 254, 2592). Another useful probe is 1,6-diphenyl-1,3,5-hexatriene (DPH), which shows a polarity- and viscosity-dependent fluorescence (G. S. Beddard and M. A. West, loc. cit.). Another group comprises the (1-pyrene)fatty acids which have also found widespread use (C. Lovejoy et al. (1977) Biochemistry 16, 3668). Most of the probes reported so far have properties that limit their applications. Thus most probes have shortwave excitation and emission wavelengths so that the strong background fluorescence from biological materials can interfere heavily. Second, probes such as DPH are photolabile. And practically all fluorescent probes have inproper water solubility and suffer from fluorescence quenching by oxygen. We describe here a new group of fluorescent probes that have longwave excitation and emission, high fluorescence quantum yields, good water solubility and no sensitivity to oxygen. The probes are derived from fluoropohores whose chemical structure are given in structure I, II, and III. ##STR2## These fluorophores can be chemically linked to long alkyl chains to give a new class of lipid probes that is characterized by an apolar lipid chain and a highly polar fluorophore (structures IV, V, VI, and VII). ##STR3## The substituent R in structure IV, V, VI and VII can be an alkyl, alkenyl, arylalkenyl or arylalkyl group comprising 4-22 carbon atoms, which can be substituted by carboxy, alkoxycarbonyl, cyano, hydroxy, chlorine, bromine, or sulfonato groups. Typical substituents include the decyl, dodecyl, tetradecyl, hexydecyl, octadecyl group and their omega carboxy derivatives. Suitable cations in structure IV, V, VI, and VII are sodium, potassium, ammonium, or pyridinium. Sodium and pyridinium ions are prefered.

Another group of new lipid probes comprises compounds of chemical structure VIII, IX, and X ##STR4## where R and M have the same meaning as in structures IV, V, VI, and VII.

Compounds of general formula IV-VII can be obtained by reaction of proper alkyl halides of structure R--X, or dialkylsulfates of structure RO--SO.sub.2 --OR, or alkyltoluenesulfonates of structure RO--SO.sub.2 --C.sub.6 H.sub.4 --CH.sub.3, with phenols of structures I or II, or an amine of structure III. Reaction is preferentially performed in the presence of an inorganic or organic base at temperatures between 0.degree.-180.degree. C. in a solvent such as ethanol, dimethylformamide, or dimethylsulfoxide, or their mixtures with water. Prefered bases are pyridine, or dissolved or suspended alkali hydroxides or alkali carbonates.

Compounds of structures VIII, IX, and X are obtained from lipid probes of type IV, VI and VII by reaction with an alkali hydroxide at temperatures between 50.degree.-200.degree. C.

Probes IV, VI and VII can be obtained by sulfonation of pyrenes of structure XI, XII, or XIII ##STR5## where R has the same meaning as in compounds IV-VII, with a sulfonating agent such as sulfuric acid or oleum.

FIG. 1 shows the fluorescence maxima as a function of the detergent concentration c.

FIG. 2 shows the relative fluorescence intensity as a function of the detergent concentration c.

FIG. 3 shows the fluorescence intensity as a function of the MAD.sup.+ concentration.

The lipid probes of general formula IV-X have excellent water solubility because of the presence of polar sulfonato groups. Due to an unique combination of a polar fluorophore group and an apolar lipid chain the probes are most suitable for investigating lipid-water systems such as micelles, vesicles, membranes, and lipid-water interfaces. Usually the polar group is exposed to the aqueous phase and the alkyl chain is anchored in the lipid domaine. Typical probe applications include the determination of critical micelle concentration (cmc) and of lipid polarity and viscosity. FIG. 1 shows the effect of detergent concentration c (1, Triton X-100; 2, Brij-35) on the fluorescence maximum. The probe used in this experiment is 1-decyloxypyrene-3,6,8-trisulfonate. It is obvious, that at the cmc of Triton X-100 (2.2.multidot.10.sup.-4 M, curve 1) and Brij-35 (6.6.multidot.10.sup.-5 M, curve 2) a significant change in the emission maximum occurs. Similarly, the emission intensity, measured at 420 nm can be used to determine the cmc. FIG. 2 shows that there is a dramatic increase in relative fluorescence intensity when the concentration of detergents Triton X-100 (curve 1) or Brij-35 (curve 2 ) exceeds the cmc.

Aminopyrenes of structure VI and VII have applications similar to alkoxypyrenes IV and V but have even longer excitation and emission wavelengths (450/510 nm).

Probes of structure VIII-X are useful for determination of physiological pH values since the pK.sub.a of the hydroxy group is between 6.5 and 8.5. At pH values above the pK.sub.a value, they form a highly fluorescent phenolate anion with particular longwave excitation and emission (460/520 nm) that is highly pH-dependent. Because of the long chain alkyl substituent R the probes bind to the lipid membranes and therefore lends themselves to pH determination at membrane surfaces.

An unexpected observation is the fact that lipid probes IV-X suffer fluorescence quenching by cationic biomolecules such as nicotine adenine dinucleotide (NAD.sup.+) and its phosphate (NADP.sup.+), by vitamin B1 and other cations such as cetylpyridiniumchloride (CPC). A Stern-Volmer plot of the quenching of 1-octadecyloxypyrene-3,6,8-trisulfonate by NAD.sup.+ is shown in FIG. 3, where the ratio of I.sub.o /I (i.e., the fluorescence intensity of probe 1-octadecyloxypyrene-3,6,8-trisulfonate) is plotted versus the concentration of NAD.sup.+. I.sub.o is the fluorescence intensity of the probe in the absence of NAD.sup.+, and I is the intensity of the probe in the presence of a certain concentration [NAD.sup.+ ]. The curve can be described by a modified Stern-Volmer equation of type

as described by J. R. Lakowicz in: "Principles of Fluorescence Spectroscopy", Plenum Press, New York, 1983, p 266 ff. Here, I.sub.o and I have the same the same meaning as in FIG. 3, the quencher Q is NAD.sup.+ being presnt in a concentration [NAD.sup.+ ] (in millimol), K.sub.d is the dynamic quenching constant, and v in the exponential term accounts for static quenching. Solving the equation by using the experimental data shows that the dynamic quenching term K.sub.d is negligibly small (0.1), but that static quenching is predominant. The experimental value for v is as high as 2,480M.sup.-1.

Because of the unambiguous relation between fluorescence intensity and NAD.sup.+ concentration (FIG. 3), the probes are suitable for quantitative determination of NAD.sup.+ and other cationic species, and also for monitoring biochemical reactions during which these species are produced or consumed.

1-Decyloxypyrene-3,6,8-trisulfonic acid trisodium salt (structure IV, R=decyl, M=Na).

7.5 g (34 mmol) decylbromide are added to a mixture of 2.0 g (3.8 mmol) 1-hydroxypyrene-3,6,8-trisulfonic acid trisodium salt (compound I, M=Na) and 4.0 g (38 mmol) sodium carbonate in a mixture of 40 ml dimethylformamide and 10 ml water and heated under reflux for 6 h under stirring. Volatile solvents are removed in a vacuum at 100.degree. C. Then, 100 ml acetone are added. The grey material is filtered off, washed with acetone and recrystallized from 90% ethanol under addition of charcoal. The decylether is obtained in 75% yield (1.9 g) as yellowish needles that decompose at 245.degree. C.

In a similar fashion, the dodecylether (decomposition at 260.degree. C.) and the octadecylether (decomposition at 243.degree. C.) are obtained.

When 1,3-dihydroxypyrene-6,8-disulfonic acid disodium salt (compound II, M=Na) is used in place of compound I in the preceding procedure, lipid probes of structure V (M=Na) are obtained. Typical examples are the bis-dodecylether (structure V, R=n-dodecyl, M=Na) of melting point 202.degree. C. and the bis-octadecylether (structure V, R=n-octadecyl, M=Na) of melting point 234.degree. C.

1-Decyloxy-3-hydroxypyrene-6,8-disulfonic acid disodium salt (structure VIII, R=decyl, M=Na).

14.5 g Sodium hydroxide and 9.0 g water are heated to ca. 130.degree. C. and 8.53 g 1-decyloxypyrene-3,6,8-trisulfonate (structure IV, R=decyl, M=Na) are added to the melt in small portions under stirring. The temperature is raised within 30 min to 190.degree. C. The mixture is cooled, cautiously acidified with concentrated hydrochloric acid, and the resulting precipitate is filtered and recrystallized from a saturated aqueous salt solution. For further purification it is recrystallized from aqueous ethanol. Yield is 2.6 g of yellow crystals that decompose at above 232.degree. C. and which have a green fluorescence that is strongly pH-dependent in the pH 6-8 range.

N,N-Didodecyl-1-aminopyrene-3,6,8-trisulfonic acid trisodium salt (structure VII, R=dodecyl, M=Na).

2.0 g of N,N-Di-dodecyl-1-aminopyrene are added in small portions to a solution of 1.5 g sodium sulfate in 6.5 g concentrated sulfuric acid. To this mixture, 4.0 g of oleum (SO.sub.2 content 65%) is added dropwise at 60.degree. C. under stirring. After standing for 12-20 h, the mixture is carefully poured on ice and cautiously neutralized with concentrated sodium hydroxide. The solution is evaporated to dryness and the solid residue extracted with 150 ml methanol. After evaporating the methanol, the residue is crystallized, first from saturated salt solution, then from aqueous ethanol. Yield 1.7 g of brownish-yellow material that decomposes at 205.degree. C. When excited with 450 nm light, the probe displays a strong green fluorescence with a maximum at 508 nm.