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Increased efficiency in biomimetic Lewis acid–base pair catalyzed monoacylation of diols by acyl phosphate monoesters

Publication: FACETS
5 September 2017

Abstract

Acyl phosphate monoesters are biomimetic acylation reagents that require coordination to metal ions to react with cis-diol substrates in water. With lanthanide catalysts, outcomes are compromised by (1) the competitive lanthanide-promoted hydrolysis of the acyl phosphate reagents as well as by (2) the high affinity of lanthanum ions for the phosphate monoester by-product. Based on analysis of the mechanism of the process, optimizing reaction conditions can selectively inhibit the lanthanum-promoted hydrolysis of acyl phosphate monoesters. Furthermore, using zinc salts and lead salts in place of lanthanides enhances the reactivity of the reactants and causes less complexation of the metal ion with the by-products.

Graphical Abstract

Introduction

The utility of acyl phosphates esters as biomimetic reagents for selective monoacylation of diols is a subject of continuing interest with important applications, including kinome interrogation (Patricelli et al. 2007; Nordin et al. 2015) and aminoacylation of the 3′-terminus of tRNA (Tzvetkova and Kluger 2007; Duffy and Dougherty 2010). We previously reported that lanthanide ions can serve as the chelated core to bind and activate the reacting species toward acyl transfer (Cameron et al. 2004; Tzvetkova and Kluger 2007; Her and Kluger 2011). Bidentate coordination of a cis-1,2-diol to a Lewis acid facilitates ionization of one of the coordinated diols while enhancing the activity of the bidentate-coordinate acyl phosphate (Scheme 1). The bis-bidentate array of coordinated Lewis base (diol) and Lewis acid (acyl phosphate) promotes the ultimate combination that produces the acylated diol along with a phosphate monoester as a by-product. The arrangement can utilize the potential energy made available by the separation of the Lewis acid–base pair. Utilizing acyl phosphate monoesters as metal-coordinated acyl donors at the same Lewis acid where the ionized diol is coordinated promotes the reaction without involvement of the solvent. With a similar approach, regioselective monoacylation of carbohydrates in water can be based on the geometry of adjacent hydroxyl groups (Gray and Kluger 2007; Dhiman and Kluger 2010). However, although the system is set up for efficient acylation, the formation of the correct combination of ligands in the chelate for reaction is subject to competing homologous formation of the chelates as well as interference due to coordination to the lanthanide of the phosphate monoester product (see below).
Scheme 1.
Scheme 1. Lanthanum-promoted monoacylation of the diol in a nucleotide via a chelate formation.
An important potential use of the approach is for biomimetic aminoacylation of tRNA for the ribosomal introduction of unnatural amino acids into new proteins, which requires an aminoacylated reagent whose reactive functionality parallels that of the biological agents, aminoacyl adenylates (Schimmel 1987). However, complications arise from the competing effectiveness of the lanthanum ion in the hydrolysis of phosphate derivatives that are the acylation reagents (Hendry and Sargeson 1989; Kluger et al. 1997). Furthermore, transfer of the acyl group, whether to the hydroxyl or to water, releases ethyl phosphate, forming an insoluble complex with the metal ions that prevents further catalysis (Tzvetkova 2008).
In the present study, we assessed the effects of other metal ions for reactions in water that are reported to be effective in nonaqueous systems (Hikawa et al. 2014). We also assessed aqueous reaction conditions with added solvents and at lower reaction temperatures. In some cases, these provided significant improvements for the overall process.

Experimental

Preparation of N-deprotected phenylalanyl ethyl phosphate

BOC-PheEtP was formed in the DCC-mediated coupling of BOC-Phe and ethyl phosphate. PheEtP was produced by removal of the BOC protecting group using a minimal amount of trifluoroacetic acid. The N-deprotected product was precipitated from ice-cold dry acetone as a white powder (Kluger et al. 1996). ESI-MS [M − H+] calculated m/z = 272.0693 and found m/z = 272.0711.

Analysis of the extent of PheEtP hydrolysis

A mixture of PheEtP (50 mmol/L) and lanthanum triflate (50 mmol/L) was incubated in an aqueous MES buffer (200 mmol/L, pH 6). We assessed the effects of conducting the reaction in a range of DMSO/buffer mixtures, as follows: 0/1, 2/3, 3/2, 9/1, 19/1, and 39/1 (v/v). The effect of temperature on product yield was studied by conducting reactions at 20 and 4 °C. Samples were analyzed after periodic additions of 20% volume of saturated EDTA solution until no further changes were observed. Hydrolysis of PheEtP was then tracked by the integrated peak area from UV-HPLC at 260 nm.

Monoaminoacylation of ethylene glycol

A mixture of ethylene glycol (60 mmol/L), PheEtP (50 mmol/L), and metal salt (La(OTf)3 = Zn(NO3)2 = Pb(NO3)2 = 50 mmol/L) was stirred in an MES buffer (200 mmol/L, pH 6) at 4 °C for 24–72 h. Samples were analyzed after periodic additions of 20% volume of saturated EDTA solution until no further changes were observed. Products were isolated by reversed phase HPLC and lyophilized. A colorless solid was collected. The yield was estimated from the integration of the peak areas from UV-HPLC at 260 nm. ESI-MS [M + H+] calculated m/z = 210.11 and found m/z = 210.1.

Monobenzoylation of ethylene glycol

A mixture of ethylene glycol (60 mmol/L), BMP (50 mmol/L), and metal salt (La(OTf)3 = Zn(NO3)2 = Pb(NO3)2 = 50 mmol/L) was stirred in an EPPS buffer (200 mmol/L pH 8) at 25 °C for 24–72 h (Cameron et al. 2004). Samples were analyzed after periodic additions of 20% volume of saturated EDTA solution until no further changes were observed. Products were isolated by reversed phase HPLC and lyophilized. A colorless solid was collected. The yield was estimated from the integration of the peak areas from UV-HPLC at 230 nm.

Preparation of 5′-phospho-2′-deoxyribocytidylylriboadenosine

dCA was produced following published methods (Noren et al. 1989). ESI-MS [M + H+] calculated m/z = 637.1167 and found m/z = 637.1173.

Aminoacylation of nucleotides

AMP (50 mmol/L) and dCA (1 mmol/L) were reacted with PheEtP (50 mmol/L) and metal salts (50 mmol/L) as described for the reaction with ethylene glycol. ESI-MS [M + H+] for AMP-Phe: calculated m/z = 493.1242 and found m/z = 493.1197; for dCA-Phe: calculated m/z = 784.2000 and found m/z = 784.2000.

Results and discussion

An aminoacyl phosphate monoester, PheEtP, was used as the acylation agent for the evaluation of catalysts in this study. This reagent is a good model for the general approach to the formation of amino acids esters at the 3′-terminal of tRNA. We found that conducting reactions in mixtures of DMSO and water reduced the rate of hydrolysis of PheEtP in the presence of lanthanum ion compared with the reaction in water alone. In a 95/5 DMSO/aqueous buffer (v/v), the t ½ of PheEtP was about 30 min, whereas the t ½ in water was <2 min. Nonetheless, the yield of acylated products did not increase with the reduced hydrolysis rate of the reagent. Surprisingly, the yield of ester derivatives that were formed at the 2′- or 3′-ribosyl positions of AMP with PheEtP remained near 30% after 24 h, regardless of the amount of DMSO in the aqueous reaction mixture. This suggests that the reactants are locally solvated exclusively by water. The polarity of the reaction partners and their ability to form hydrogen bonds is consistent with this outcome. It is likely that the ionization of water coordinated to lanthanum ions is essential for the activation of PheEtP, whereas the reactivity of the hydroxyl group of diol toward the carboxyl carbon is increased as it is deprotonated by the lanthanum-coordinated hydroxyl (Kluger and Cameron 2002). Lowering the relative amount of water reduces the extent of ionization of the hydroxyl of the diol moiety, reducing the reactivity in the coordinated ligands.
The problematic competing hydrolysis of PheEtP occurs by the addition of non-chelated water. This will have a higher enthalpic barrier if it avoids the coordination sphere of the metal ion. Lowering the reaction temperature should reduce the rate of the reaction with water to a greater extent than it will reduce the desired acylation from reaction with the bis-bidentate coordinated chelate. As expected, at 4 °C, hydrolysis occurred at a significantly slower rate than at room temperature. At the lower temperature, 15% of the initial PheEtP remained after 2.5 h, corresponding to a t ½ of 1 h under these conditions. Applying the same conditions to the lanthanum ion-promoted aminoacylation of AMP increased the yield from around 30% to 40%. For aminoacylation of dCA, a dinucleotide used by Robertson et al. (1989) in the chemical aminoacylation of tRNA, the combined yields of 2′ and 3′ esters doubled from 6.8% to 15% after 12 h of reaction. With PheEtP at its saturating concentration, the reaction produced the desired esters at a 40% yield after 12 h at 4 °C (Table 1). This establishes the importance of using lower reaction temperatures, consistent with the expected effect of selectively slowing the competing hydrolysis.
Table 1.
Table 1. Yields for aminoacylation of AMP and dCA by PheEtP in the presence of Lewis acids.
SaltNucleotideTemperature (°C)Combined yield
La(OTf)3AMP2529%
  436%
 dCA256.8%
  415% (40%)
Zn(NO3)2AMP431%
 dCA419%
Pb(NO3)2AMP431%
 dCA413%

Note: AMP, adenosine monophosphate; dCA, 5′-phospho-2′-deoxyribocytidylylriboadenosine; PheEtP, phenylalanyl ethyl phosphate.

With this useful outcome, we optimized conditions for acylation with metal salts and determined the extent of the competing hydrolysis of PheEtP. The t ½ for catalyzed hydrolysis of PheEtP at 4 °C in the presence of zinc or lead salts were 60 and 5 min, respectively. The reaction in the presence of cupric salts is not effective, as it selectively promotes the hydrolysis of PheEtP. Based on these observations, we anticipated that Zn2+ and Pb2+ should catalyze aminoacylation of diols at or near 0 °C. We tested this approach by following the aminoacylation of ethylene glycol by PheEtP at 4 °C (Table 2).
Table 2.
Table 2. Summary of metal-mediated monoacylation reaction of ethylene glycol with PheEtP and BMP.
Acyl phosphateSalt[Salt] (mmol/L)Yield
PheEtPLa(OTf)35064%
 Zn(NO3)25023%
  15024%
 ZnSO45022%
 Pb(NO3)25067%
  15063%
BMPLa(OTf)3503.5%
 Zn(NO3)2503.6%
 Pb(NO3)2507.5%

Note: PheEtP, phenylalanyl ethyl phosphate; BMP, benzoyl methyl phosphate.

In the presence of Zn2+ or Pb2+, the reactions of ethylene glycol and PheEtP produced a mixture of the ester and the hydrolysis product, phenylalanine, within 1 min after addition to the solution. The Pb2+-catalyzed reactions occurred with a half-life for PheEtP of <1 min. The yields are comparable with those from reactions conducted in the presence of La3+. The Zn2+-catalyzed reactions were slower and their overall yields were lower than with Pb2+ and La3+.
We also investigated the reactions of ethylene glycol with BMP (Scheme 2). The t ½ for hydrolysis of BMP in the presence of Zn2+ and Pb2+ were 24 and 1 h, respectively. The Pb2+-catalyzed monobenzoylation reactions of ethylene glycol gave the best yield of the monoester after 3 h, whereas reactions with Zn2+ required about 12 h to reach their maximum yield.
Scheme 2.
Scheme 2. Lanthanum/lead/zinc-mediated monobenzoylation of ethylene glycol.
We also evaluated the effectiveness of Zn2+ and Pb2+ as catalysts for reactions of PheEtP with a mononucleotide and a dinucleotide. Both AMP and dCA were separately incubated with PheEtP in the presence of the metal ions at 4 °C, followed by analysis that used reverse-phase HPLC to separate the reactants and products. The reactions gave 2′ and 3′ nucleotidyl esters in each case. In reactions with dCA, the yield was 19% with Zn2+ and 13% with Pb2+ (Figures 1 and 2). These did not improve on the previously reported 15% yield with La3+. We also monitored the reactions for an additional 72 h during which the distribution of products remained unchanged, establishing that the esters are stable under these conditions and that the reactions are self-limiting.
Fig. 1.
Fig. 1. Amionacylation reaction with adenosine monophosphate (AMP) and phenylalanyl ethyl phosphate (PheEtP) catalyzed by La3+, Pb2+, and Zn2+.
Fig. 2.
Fig. 2. Aminoacylation reaction with 5′-phospho-2′-deoxyribocytidylylriboadenosine (dCA) and phenylalanyl ethyl phosphate (PheEtP) catalyzed by La3+, Pb2+, and Zn2+.
Reports of previous studies indicate that Pb2+ is among the most effective catalysts for site-specific cleavage of RNA (Huff et al. 1964; Brown et al. 1983). Lead hydroxide is an active catalyst for RNA hydrolysis (Brown et al. 1983) and for transesterification of a phosphate diester (Morrow et al. 1992). In reactions at pH around 7, where Pb2+-promoted aminoacylation occurs, lead hydroxides are present (Perera et al. 2001). Lead hydroxides could assist in deprotonation of the incoming hydroxyl, further promoting acylation.
As noted earlier, a major problem with La3+-promoted acylation is the high affinity of La3+ to phosphate monoesters. The resulting precipitate removes the La3+ catalyst from solution. As a result, La3+-promoted acylation reactions require a large excess of the added metal ion (Gray and Kluger 2007). Reactions with Zn2+ and Pb2+ reduce the formation of insoluble metal phosphates, providing alternative catalysts for acylation reactions via acyl phosphates that can operate at lower concentrations.

Conclusions

We have demonstrated that lowering the reaction temperature improves lanthanide-ion-promoted aminoacylation reactions by selectively slowing the competing hydrolysis of activated amino acyl phosphates. Catalysis by Zn2+ and Pb2+ is effective, producing the desired aminoacylation products under conditions where the acyl phosphate reagent has improved stability.

List of abbreviations

AMP
adenosine 5′-monophosphate
BMP
benzoyl methyl phosphate
BOC
tertbutyl oxycarbonyl
dCA
5′-phospho-2′-deoxyribocytidylylriboadenosine
DCC
N, N′-dicyclohexylcarbodiimide
DMSO
dimethyl sulfoxide
EDTA
ethylenediamine tetraacetic acid
EPPS
N-(2-hydroxyethyl)piperazine-N′-(3-propane) sulfonic acid
ESI-MS
electrospray ionisation mass spectrometry
MES
2-(N-morpholino)ethanesulfonic acid
PheEtP
phenylalanyl ethyl phosphate
tRNA
transfer RNA
UV-HPLC
UV/Vis spectroscopic detection of output of high performance liquid chromatograph

Acknowledgements

We thank NSERC Canada for support through a Discovery Grant.

References

Brown RS, Hingerty BE, Dewan JC, and Klug A. 1983. Pb(II)-catalysed cleavage of the sugar-phosphate backbone of yeast tRNAPhe—implications for lead toxicity and self-splicing RNA. Nature, 303(5917): 543–546.
Cameron LL, Wang SC, and Kluger R. 2004. Biomimetic monoacylation of diols in water. Lanthanide-promoted reactions of methyl benzoyl phosphate. Journal of the American Chemical Society, 126(34): 10721–10726.
Dhiman RS, and Kluger R. 2010. Magnesium ion enhances lanthanum-promoted monobenzoylation of a monosaccharide in water. Organic & Biomolecular Chemistry, 8(9): 2006–2008.
Duffy NH, and Dougherty DA. 2010. Preparation of translationally competent tRNA by direct chemical acylation. Organic Letters, 12(17): 3776–3779.
Gray IJ, and Kluger R. 2007. Chelation-controlled regioselectivity in the lanthanum-promoted monobenzoylation of monosaccharides in water. Carbohydrate Research, 342(14): 1998–2002.
Hendry P, and Sargeson AM. 1989. Metal ion promoted phosphate ester hydrolysis. Intramolecular attack of coordinated hydroxide ion. Journal of the American Chemical Society, 111(7): 2521–2527.
Her S, and Kluger R. 2011. Biomimetic protecting-group-free 2′, 3′-selective aminoacylation of nucleosides and nucleotides. Organic & Biomolecular Chemistry, 9(3): 676–678.
Hikawa H, Hamada M, Yokoyama Y, and Azumaya I. 2014. Benzoyl methyl phosphate as an efficient reagent for the selective monobenzoylation of N-Bz-FTY720. RSC Advances, 4(44): 23131–23136.
Huff JW, Sastry KS, Gordon MP, and Wacker WEC. 1964. The action of metal ions on tobacco mosaic virus ribonucleic acid. Biochemistry, 3(4): 501–506.
Kluger R, and Cameron LL. 2002. Activation of acyl phosphate monoesters by lanthanide ions: enhanced reactivity of benzoyl methyl phosphate. Journal of the American Chemical Society, 124(13): 3303–3308.
Kluger R, Li X, and Loo RW. 1996. 1996 Bader Award Lecture Aminoacyl ethyl phosphates. Biomimetically activated amino acids. Canadian Journal of Chemistry, 74(12): 2395–2400.
Kluger R, Loo RW, and Mazza V. 1997. Biomimetically activated amino acids. Catalysis in the hydrolysis of alanyl ethyl phosphate. Journal of the American Chemical Society, 119(50): 12089–12094.
Morrow JR, Buttrey LA, and Berback KA. 1992. Transesterification of a phosphate diester by divalent and trivalent metal ions. Inorganic Chemistry, 31(1): 16–20.
Nordin BE, Liu Y, Aban A, Brown HE, Wu J, Hainley AK, et al. 2015. ATP acyl phosphate reactivity reveals native conformations of Hsp90 paralogs and inhibitor target engagement. Biochemistry, 54(19): 3024–3036.
Noren CJ, Anthony-Cahill SJ, Griffith MC, and Schultz PG. 1989. A general method for site-specific incorporation of unnatural amino acids into proteins. Science, 244(4901): 182–188.
Patricelli MP, Szardenings AK, Liyanage M, Nomanbhoy TK, Wu M, Weissig H, et al. 2007. Functional interrogation of the kinome using nucleotide acyl phosphates. Biochemistry, 46(2): 350–358.
Perera WN, Hefter G, and Sipos PM. 2001. An investigation of the lead (II)-hydroxide system. Inorganic Chemistry, 40(16): 3974–3978.
Robertson SA, Noren CJ, Anthony-Cahill SJ, Griffith MC, and Schultz PG. 1989. The use of 5′-phospho-2 deoxyribocytidylylriboadenosine as a facile route to chemical aminoacylation of tRNA. Nucleic Acids Research, 17(23): 9649–9660.
Schimmel P. 1987. Aminoacyl tRNA synthetases: general scheme of structure-function relationships in the polypeptides and recognition of transfer RNAs. Annual Review of Biochemistry, 56: 125–158.
Tzvetkova S. 2008. Biomimetic aminoacylation. Ph.D. thesis, University of Toronto, Toronto, Ontario. 104 p.
Tzvetkova S, and Kluger R. 2007. Biomimetic aminoacylation of ribonucleotides and RNA with aminoacyl phosphate esters and lanthanum salts. Journal of the American Chemical Society, 129(51): 15848–15854.

Information & Authors

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Published In

cover image FACETS
FACETS
Volume 2Number 2January 2017
Pages: 682 - 689
Editor: Maxim V. Berezovski

History

Received: 26 April 2017
Accepted: 6 July 2017
Version of record online: 5 September 2017

Data Availability Statement

All relevant data are within the paper.

Key Words

  1. biomimetic acylation
  2. Lewis acid catalysis
  3. acyl phosphate esters
  4. zinc and lead ion catalysts
  5. cis-diol substrates
  6. monoesters

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Authors

Affiliations

Yuyang Li
Davenport Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada
Ronald Kluger [email protected]
Davenport Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada

Author Contributions

All conceived and designed the study.
YL performed the experiments/collected the data.
All analyzed and interpreted the data.
RK contributed resources.
All drafted or revised the manuscript.

Competing Interests

RK is currently serving as a Subject Editor for FACETS, but was not involved in review or editorial decisions regarding this manuscript.

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1. Lead-Catalyzed Aqueous Benzoylation of Carbohydrates with an Acyl Phosphate Ester

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