pH-Tolerant Wet Adhesion of Catechol Analogs

The need for improved wet adhesives has driven research on mussel-inspired materials incorporating dihydroxyphenylalanine (DOPA) and related analogs of the parent catechol, but their susceptibility to oxidation limits practical application of these functionalities. Here, we investigate the molecular-level adhesion of the catechol analogs dihydroxybenzamide (DHB) and hydroxypyridinone (HOPO) as a function of pH. We find that the molecular structure of the catechol analogs influences their susceptibility to oxidation in alkaline conditions, with HOPO emerging as a particularly promising candidate for pH-tolerant adhesives for diverse environmental conditions.


Synthesis of N,N',N"-tris[Nα-Boc-Nε-Cbz-L-lysinyl]tris(2-aminoethyl)amine, 1.
Nα-Boc-Nε-Cbz-L-lysine (761 mg, 2.0 mmol) and N-hydroxysuccinimide (276 mg, 2.4 mmol) were dissolved in 15 mL of dry DMF under an argon atmosphere and cooled in an ice bath.Dicyclohexylcarbodiimide (495 mg, 2.4 mmol) was added at 0 ℃ and the flask was subsequently taken out of the ice bath and stirred at RT for four hours.Tris(2-aminoethyl)amine (74.9 µL, 0.5 mmol) and DIPEA (1044 µL, 6 mmol) were then added to the flask and the reaction was stirred overnight at RT.The following day the DCU byproduct was filtered off and the solvent was removed in vacuo.The crude reaction mixture was brought up in DCM and rinsed with sat.NaHCO3 (30 mL, x3) and brine (30 mL).The organic layer was concentrated and then loaded onto a silica column.Purification by flash chromatography using a gradient of 2 -4 % MeOH in DCM afforded 1 as a colorless solid.(76% yield). 1

Synthesis of N,N',N"-tris[Nα-2,3-di(benzyloxy)benzoyl-Nε-Cbz-L-lysinyl]tris(2-aminoethyl)amine (BnTren(Lys-3,4-DHB)3), 2.
Compound 1 (370 mg, 0.3 mmol) was added to a dry flask under argon and dissolved in 6 mL dry DCM.The flask was cooled in an ice bath and 4 mL of TFA was added.After stirring for 1.5 h at RT, full deprotection of the boc groups was observed by TLC.Volatiles were removed in vacuo and the pale yellow oil was brought up in 5 mL of dry DMF.In a separate flask, 3,4-dibenzyloxybenzoic acid (341 mg, 0.99 mmol), HATU (376 mg, 0.99 mmol), and DIPEA (627 µL, 3.6 mmol) were added to 5 mL of dry DMF under an argon atmosphere and stirred for 3 min at RT.The contents of the first flask were then transferred to the reaction mixture via syringe and the reaction was left to stir overnight at RT.The reaction mixture was concentrated, loaded onto a silica column, and then purified by flash chromatography using a gradient of 1 -3 % MeOH in DCM.Fractions were combined and concentrated to yield 2 as a white solid.(81% yield over 2 steps). 2 was immediately deprotected to yield 4 according to the procedure below.

Synthesis of N,N',N"-tris[Nα-1-(benzyloxy)-2-oxo-1,2-dihydropyridine]benzoyl-Nε-Cbz-Llysinyl]tris(2-aminoethyl)amine (BnTren(Lys-1,2-HOPO)3), 3.
Compound 1 (370 mg, 0.3 mmol) was added to a dry flask under argon and dissolved in 6 mL dry DCM.The flask was cooled in an ice bath and 4 mL of TFA was added.After stirring for 1.5 h at RT, full deprotection of the boc groups was observed by TLC.Volatiles were removed in vacuo and the pale yellow oil was brought up in 5 mL of dry DMF.In a separate flask, 1,2-HOPOBn (245 mg, 0.99 mmol), HATU (376 mg, 0.99 mmol), and DIPEA (627 µL, 3.6 mmol) were added to 5 mL of dry DMF under an argon atmosphere and stirred for 3 min at RT.The contents of the first flask were then transferred to the reaction mixture via syringe and the reaction was left to stir overnight at RT.The reaction mixture was concentrated, loaded onto a silica column, and then purified by flash chromatography using a gradient of 4 -10 % MeOH in DCM.Fractions were combined and concentrated to yield 3 as a white solid.(74% yield over 2 steps) 1

N,N',N"-Tris[2,3-dihydroxybenzoyl-L-lysinyl]tris(2-aminoethyl)amine (Tren(Lys-3,4-DHB)3), 4.
Compound 2 (376 mg, 0.2 mmol) was dissolved in 10 mL of 60% THF (aq.) + 0.5% acetic acid under an atmosphere of argon.10% Pd/C (100 mg) was carefully added, and a balloon of hydrogen attached to a three-way flushing adapter was fitted to the round bottom.The atmosphere was evacuated and back-filled with hydrogen four times and stirred under an atmosphere of hydrogen for 24 h at RT.The catalyst was then filtered off, rinsed with 25 mL of DMF, and concentrated to yield a dark-red oil.The crude reaction, deemed mostly pure by NMR, was further purified by semi-preparative HPLC on a YMC-Actus 20 x 250 mm C18 ODS-AQ column using a linear gradient of 5% MeOH in ddH2O (+0.1% trifluoroacetic acid) to 30% MeOH in ddH2O (+0.1 % trifluoroacetic acid) over 25 min.HPLC fractions were concentrated and subsequently lyophilized to yield 4 as a white solid.(65% yield) 1           Table S3.pKa values of selected compounds and functional groups.
Table S4.Average deprotonation fractions of functional groups in Tren(Lys-2,3-DHB)3 at pH 3, 7, and 10 calculated with the Henderson-Hasselbalch equation using the pKa values given in Table S3.The final column shows the total charge of Tren(Lys-2,3-DHB)3, calculated by summing the average charges of the six hydroxyl groups, three lysyl amines, and one tertiary amine.

Figure S8 .
Figure S8.Reversibility of the adhesion of Tren(Lys-3,4-DHB)3.(A) Plots of force/radius F/R vs micamica separation distance D. (B) Plots of adhesion force/radius -Fad/R and adhesion energy Ead vs contact time tcontact.Open circles correspond to approach; closed circles correspond to separation.

Figure S9 .
Figure S9.Range of repulsion of the surface primers, quantified by the difference in film thickness at different extents of compression.

Figure S12 .
Figure S12.Force-distance measurements of Tren(Lys-2,3-DHB)3 (left) and Tren(Lys-3,4-DHB)3(right). The surface primers were incubated at pH 10, and the solution was then adjusted to pH 3 prior to depositing the primers and measuring the adhesion force.The plots show that Tren(Lys-2,3-DHB)3 adsorbs to mica at pH 3 after exposure to pH 10, but does not mediate adhesion.By contrast, Tren(Lys-3,4-DHB)3 mediates an adhesion force similar to the force measured after cycling the pH from 3 to 10 to 3 (FigureS8).Insets show photos of the surface primers at pH 3 (left tube) and the primers after incubation at pH 10, followed by adjusting the pH 3 (right tube).Tren(Lys-2,3-DHB)3 showed a clear color change after incubation at pH 10, consistent with oxidation of catecholic compounds.

Table S3 .
Experimental counts for pH adhesion studies.

Table S4 .
Experimental counts for reversibility studies.