High-density information storage in an absolutely defined aperiodic sequence of monodisperse copolyester

Synthesis of a polymer composed of a large discrete number of chemically distinct monomers in an absolutely defined aperiodic sequence remains a challenge in polymer chemistry. The synthesis has largely been limited to oligomers having a limited number of repeating units due to the difficulties associated with the step-by-step addition of individual monomers to achieve high molecular weights. Here we report the copolymers of α-hydroxy acids, poly(phenyllactic-co-lactic acid) (PcL) built via the cross-convergent method from four dyads of monomers as constituent units. Our proposed method allows scalable synthesis of sequence-defined PcL in a minimal number of coupling steps from reagents in stoichiometric amounts. Digital information can be stored in an aperiodic sequence of PcL, which can be fully retrieved as binary code by mass spectrometry sequencing. The information storage density (bit/Da) of PcL is 50% higher than DNA, and the storage capacity of PcL can also be increased by adjusting the molecular weight (~38 kDa).

Characterization 1 H NMR spectra were recorded on a Varian 500 MHz using CDCl3 as solvent. 13 C NMR spectra were recorded on an Agilent 400-MR DD2 Magnetic Resonance System using CDCl3 as solvent. Gel permeation chromatography (GPC) was performed on an Agilent 1260 Infinity equipped with a PL gel 5 μm mixed D column and differential refractive index detectors. THF was used as an eluent with a flow rate of 0.3 mL min -1 at 35 °C. A polystyrene standard kit (Agilent Technologies) was used for calibration. Automated column chromatography was performed on a Biotage SP1 flash chromatography purification system equipped with a silica column cartridge (KP-Sil 100g and 50g). n-hexane and ethyl acetate were used as eluent. Differential Scanning Calorimetry (DSC) was performed on a TA Instruments Q10 from -40 °C to 140 °C with a scan rate of 10 °C min -1 under N2 atmosphere. The instrument was calibrated before measurements by measuring the melting temperature of indium (Tm = 429.75 K). Mass spectra of PAHs were measured on a Bruker Ultraflex III TOF-TOF mass spectrometer equipped with a Nd:YAG laser (355 nm). The instrument was operated in a positive ion mode. External calibration was conducted using the following peptides and proteins as reference: Bradykinin fragment 1-7 (757.3997 Da), Angiotensin Ⅱ (1046.5423 Da), P14R (1533.8582 Da), ACTH fragment 18-39 (246518-39 ( .1989, Insulin oxidized B chain (3494.6513 Da), Insulin (5735 Da), Cytochrome c (12362 Da), Apomyoglobin (16952 Da), Aldolase (39212 Da) and Albumin (66430 Da)). The ion is [M+H] + ion. 2-(4-Hydroxyphenylazo)benzoic acid (HABA) or trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2propenylidene]malononitrile (DCTB) was used as a matrix.
General procedure for the convergent growth of poly(α-hydroxy acid)s (PAHs) General procedure for deprotection of the benzyl group by hydrogenation A compound protected with tert-butyldimethylsilyl (TBDMS) and benzyl groups was dissolved in ethyl acetate. Palladium on activated charcoal (10% Pd/C, 0.03~0.2 eq.) was added to the solution, and the suspension was purged with argon for 15 minutes. The argon atmosphere was then replaced with hydrogen atmosphere, and the reaction mixture was stirred at room temperature. The reaction was monitored by thin layer chromatography (TLC) analysis. Upon completion of the reaction, the suspension was filtered through a Celite cake to remove Pd/C. The product was obtained by removing the solvent from the filtrates under reduced pressure.
General procedure for deprotection of the TBDMS group with fluoride A compound protected with TBDMS and benzyl groups was dissolved in dry dichloromethane (DCM). The solution was cooled to 0 o C on an ice bath, and boron trifluoride diethyl etherate (BF3•Et2O) was added dropwise. The reaction mixture was stirred at room temperature for 4 h. The reaction was monitored by TLC analysis. Upon completion of the reaction, the reaction was quenched with saturated NaHCO3 followed by dilution with water. The organic layer was separated and washed with brine. The combined organic layer was dried over MgSO4, and the solvent was removed under reduced pressure. The crude product was purified by automated column chromatography.
General procedure for esterification reactions Alcohol and carboxylic acid (1.05 eq.) were dissolved in dry DCM, and the mixture was cooled to 0 o C on an ice bath. To the mixture, 4-(dimethylamino)pyridine (DMAP, 0.2 eq.) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC•HCl, 1.4 eq.) were added. The reaction mixture was stirred overnight at room temperature, and the reaction was monitored by TLC analysis. Upon completion of the reaction, the reaction mixture was washed with water and brine. The combined organic layer was dried over MgSO4, and the solvent was removed under reduced pressure. The crude product was purified by automated column chromatography. Products with 32 repeating units or more were purified by preparative-size exclusion chromatography (prep-SEC) with a series of columns using chloroform as an eluent.
Subsequently, the product (6 g, 25.82 mmol) was dissolved in dry THF (280 ml) and was cooled to 0 o C on an ice bath. An aqueous LiOH solution (1.2M, 100 mL) was added dropwise over the next 15 minutes. The ice bath was removed and the reaction was stirred for 4 h. When the reaction was complete, water was added to the reaction flask and the solvent was removed under reduced pressure. The resulting mixture was extracted with Et2O to remove the starting material. The organic layers were discarded. The combined aqueous layer was then acidified to pH 3~4 using 1.0 M HCl, then was extracted with Et2O. The combined organic phase was dried over MgSO4 and was concentrated under reduced pressure to give the title compound. A white solid (2.71 g, 51 %); 1 H NMR (500MHz, CDCl3): Synthesis of poly(α-hydroxy acid) (PAH) TBDMS-PAm-COOH was prepared by the hydrogenation of PAm according to the described general procedure. HO-PAn-Bz was prepared by removing the TBDMS group from PAn according to the described general procedure. An equimolar mixture of TBDMS-PAm-COOH and HO-PAn-Bz was reacted with EDC•HCl in the presence of DMAP (5 mol%) in dry DCM. The reaction time varied from 5 h to 24 h depending on m and n. The desired product PA(m+n) was isolated by automated column chromatography using n-hexane/EA (19:1 to 4:1 v/v) as an eluent. PPA with 32 repeating units or more was purified by size exclusion chromatography with a series of columns. Chloroform was used as an eluent.