Photocleavage studies of-aminobutyric acid ( GABA ) conjugates

Polycyclic aromatic labels such as naphthalene and pyrene were coupled to the Cterminus of N-benzyloxycarbonyl protected -aminobutyric acid (GABA). The photophysical properties of the corresponding fluorescent conjugates were evaluated, as well as their behaviour towards photocleavage by irradiation in MeOH/HEPES buffer solution (80:20), in a photochemical reactor at different wavelengths (254, 300, 350 and 419 nm), followed by HPLC/UV monitoring.


Introduction
Photochemically removable protecting groups represent a very important tool in both synthetic and biological chemistry, as cleavage only requires light, which is a soft deprotection strategy and usually compatible with base or acid sensitive groups.Photolabile goups have found extensive application in organic synthesis, particularly that involving polyfunctional molecules, 1 photoactive precursors of neurotransmitters 2 and in time-resolved studies in cell biology. 3 recent years, photoreleasable groups based on 2-nitrobenzyl, 4 benzyl, 5 benzoin, 6 phenacyl, 7 cynnamyl, 8 vinylsilane 9 groups and their derivatives, have been developed and applied in the protection of several classes of compounds, with 2-nitrobenzyl derivatives being the most extensively studied.Also, the use of polycyclic aromatics, namely anthraquinon-2ylmethoxycarbonyl, 10 anthraquinon-2-ylethyl-1',2'-diol, 11 pyren-1-ylmethoxycarbonyl, 12 phenanthren-9-ylmethoxycarbonyl, 10 anthracene-9-methanol, 13 and oxobenzopyrans (trivially known as coumarins) 14 has been reported for the protection of alcohols, amines, phosphates, carboxylic acids, aldehydes and ketones.Polycyclic aromatics usually display fluorescence and fluorescent phototriggers are more interesting than non-fluorescent protecting groups, since they may act as temporary fluorescent labels, during the course of reaction, and also allow tracing of the location of caged molecules inside living cells by fluorescent techniques

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as well as the visualisation of processes during in situ synthesis of oligonucleotides and peptides. 15e significance of photorelease applications in neurological sciences, for studying the chemical mechanisms and the kinetics of synaptic transmission, 16 has grown steadily in recent years.Considering the role played by amino acids in neuronal communication at the central nervous system (CNS), -aminobutyric acid (GABA) has a strong inhibitory function within the mammalian CNS and is one of the most extensively studied neurotransmitter amino acids.
Taking these facts into consideration in connection with our research interests in the development of new fluorescent heterocyclic compounds and their applications as photoreleasable protecting groups, 17 we now report the use of fluorophores of aromatic nature, namely naphthalene and pyrene, in the preparation of fluorescent conjugates of GABA, with the aim of undertaking a comparative study of their performance as photolabile groups.

Results and discussion
1-Hydroxymethyl-4-methoxy-naphthalene 1a was obtained by reduction of the formyl group of 4-methoxy-naphthaldehyde, with sodium borohydride.1-Chloromethylpyrene 1b was commercially available.Fluorophores 1a,b will be designated in this report by a three letter code for simplicity of naming the fluorescent conjugates, as indicated in Table 1.
Our purpose being the investigation of compounds 1a,b as fluorescent photocleavable protecting groups for neurotransmitter amino acids, namely -aminobutyric acid (GABA), we synthesised the corresponding conjugates in order to do a comparative study of the behaviour to photolysis conditions of the ester linkage between fluorophores 1a,b and the carboxylic function of GABA.
Derivatisation at the C-terminus of N-benzyloxycarbonyl-protected GABA with labels 1a,b was carried out in DMF, at room temperature, with the aid of N,N'-dicyclohexylcarbodiimide (DCC) assisted by 1-hydroxybenzotriazole (HOBt) under standard conditions 18 (for 1a, 17% yield) or by using potassium fluoride 19 (for 1b, 98% yield), yielding fluorescent GABA conjugates 2a,b (Scheme 1).All conjugates were characterised by IR, 1 H and 13 C NMR spectroscopy and elemental analyses or high resolution mass spectrometry.The UV/Vis absorption and emission spectra of degassed 10 -5 -10 -6 M solutions in absolute ethanol of compounds 2a,b were measured, absorption and emission maxima, molar absorptivities and fluorescence quantum yields are also reported (Table 1).Fluorescence quantum yields were calculated using 9,10-diphenylanthracene as standard ( F = 0.95 in ethanol). 20The wavelength of maximum absorption and emission was red-shifted for the conjugate bearing the pyrene label, due to more extensive conjugation.Labelled GABA 2a,b exhibited moderate quantum yields (0.15 < F < 0.20), and moderate Stokes' shift (33 to 44 nm).The plots of peak area of the starting material versus irradiation time were obtained for each compound, at the considered wavelengths.Peak areas were determined by HPLC, which revealed a gradual decrease with time, and were the average of 3 runs.The determined irradiation time represents the time necessary for the consumption of the starting materials until less than 5% of the initial area was detected (Table 2).
For each compound and based on HPLC data, the plot of ln A versus irradiation time showed a linear correlation for the disappearance of the starting material, which suggested a first order reaction, obtained by the linear least squares methodology for a straight line.Concerning the influence of the wavelength of irradiation on the rate of the photocleavage reactions of GABA conjugates 2a,b in methanol/HEPES buffer 80:20 solution, it was found that the most suitable was 300 nm, resulting in shorter irradiation times.Cleavage at 419 nm lead to a very large increase in the irradiation time (ca 68 h in the case of compound 2b, and higher for compound 2a) which is not useful for practical applications.
Taking into consideration the influence of the structure of the conjugates on the photocleavage rates, it was found that the irradiation times were comparable at 254 nm (ca.20 min), while that at 300 nm, compound 2a cleaved ca. 3 times faster (7 min).However at 350 nm, this trend was reversed (2b cleaved approximately 27 times faster).
As reported before, 17a the N-blocking group was stable in the tested conditions, no cleavage being detected.The photochemical quantum yields were calculated as previously described 15c,17c and are given in Table 2.Although the efficiency of the photocleavage process was not high, nevertheless and considering the low irradiation times, these fluorescent labels can be considered as suitable photocleavable protecting groups in organic synthesis.

Conclusions
By using general synthetic methods, fluorescent -aminobutyric acid ester conjugates 2a,b were prepared through reaction of hydroxymethylnaphthalene or chloromethylpyrene and the C-terminus of N-benzyloxycarbonyl-protected GABA.The photophysical studies showed that the labels are appropriate fluorogenic reagents for the derivatisation of non-fluorescent molecules, due to their fluorescence properties.
Regarding the photocleavage studies of the fluorescent conjugates, in methanol/ HEPES buffer solution (80:20), at 254, 300 and 350 nm, it was possible to conclude that the irradiation time depended on the structure of the label.Regarding the stability to radiation of the analyte, a choice of the protecting group can be made based on the wavelength of irradiation, i.e. at 300 nm, naphthalene (1a) and at 254 and 350 nm, pyrene (1b).Irradiation times at 419 nm were too long and not convenient for pratical applications.

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General photolysis procedure: A 1 × 10 -4 M MeOH/HEPES buffer (80:20) solution of compounds 2a,b (5 mL) were placed in a quartz tube and irradiated in a Rayonet RPR-100 reactor at the desired wavelength.The lamps used for irradiation were of 254, 300, 350 and 419 ± 10 nm.Aliquots of 100 µL were taken at regular intervals and analysed by RP-HPLC.The eluent was acetonitrile/water, 3:1, at a flow rate of 1.0 (for 2a) or 1.2 (for 2b) mL/min, previously filtered through a Millipore, type HN 0.45 µm filter and degassed by ultra-sound for 30 min.The chromatograms were traced by detecting UV absorption at the wavelength of maximum absorption for each compound (retention time: 2a, 5.7; 2b, 8.8 min.).

Table 1 .
UV/Vis and fluorescence data for GABA ester conjugates 2a,b in absolute ethanol.HEPES buffer 80:20 solution were irradiated in a Rayonet RPR-100 reactor, at 254, 300, 350 and 419 nm, in order to determine the best cleavage conditions.The course of the photocleavage reaction was followed by reverse phase HPLC with UV detection.
Considering that the main goal of this research was to compare the performance of compounds 1a,b as fluorescent photocleavable protecting groups, photolysis studies of GABA conjugates 2a,b were carried out.Solutions of the mentioned compounds in