Methoxy-Substituted Nitrodibenzofuran-Based Protecting Group with an Improved Two-Photon Action Cross-Section for Thiol Protection in Solid Phase Peptide Synthesis

Abstract

Photoremovable caging groups are useful for biological applications since the deprotection process can be initiated by illumination with light without the necessity of adding additional reagents such as acids or bases that can perturb biological activity. In solid phase peptide synthesis (SPPS), the most common photoremovable group used for thiol protection is the o-nitrobenzyl group and related analogues. In earlier work, we explored the use of the nitrodibenzofuran (NDBF) group for thiol protection and found it to exhibit a faster rate towards UV photolysis relative to simple nitroveratryl-based protecting groups and a useful two-photon cross-section. Here we describe the synthesis of a new NDBF-based protecting group bearing a methoxy substituent and use it to prepare a protected form of cysteine suitable for SPPS. This reagent was then used to assemble two biologically relevant peptides and characterize their photolysis kinetics in both UV-and twophoton-mediated reactions; a two-photon action cross-section of 0.71 -1.4 GM for the new protecting group was particularly notable. Finally, uncaging of these protected peptides by either UV or two-photon activation was used to initiate their subsequent enzymatic processing by the enzyme farnesyltransferase. These experiments highlight the utility of this new protecting group for SPPS and biological experiments.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

“…4,170,171 Nitrodibenzofuran 8a (NDBF; Figure 1), introduced by Ellis-Davies and co-workers, is an exceptional red-shifted oNB derivative that releases LGs efficiently. 172 The photolysis of ether, 172 thioether, 151 and phosphoester 173,174 LGs caged with this group reportedly proceeded with Φ r values of 0.5−0.7, although lower quantum yields were obtained in some cases (0.04−0.2). 175−177 The tail absorption of 8a in the visible range (398−440 nm) was sufficient to promote the photoreaction.…”

“… Absorption spectra of selected nitroaryl PPGs. Green line, a 2-nitrobenzyl derivative (LG = thymidine); 149 purple line, an α-methyl-(6-nitropiperonyloxymethyl) derivative (LG = thymidine); 150 red line, a nitrodibenzofuran derivative (LG = Fmoc-Cys-OH); 151 blue line, an o -nitro-2-phenethyl derivative (LG = Boc-glutamate); 152 black line, a π-extended 2-nitrobenzyl derivative (LG = GABA). 153 …”

“… 175 , 178 Introducing electron-donating groups (EDG) at the 7-position of NDBF ( 8b and 8c ) led to a bathochromic shift in λ max abs but also reduced its photouncaging quantum efficiency ( Table 4 ). 151 , 174 The low quantum yield of 8c was attributed to a charge-transfer transition following photoexcitation that competes with LG release. 174 , 179 Ball and co-workers recently reported that derivatives of 8a and 8c undergo efficient photocleavage of C(sp 2 )–N bonds.…”

“… However, due to the reduction in quantum efficiency, the uncaging cross section (Φ r ε­(λ irr )) of the latter group tends to be comparable to that of the parent oNB 5a . ,, Nitrodibenzofuran 8a (NDBF; Figure ), introduced by Ellis-Davies and co-workers, is an exceptional red-shifted oNB derivative that releases LGs efficiently . The photolysis of ether, thioether, and phosphoester , LGs caged with this group reportedly proceeded with Φ r values of 0.5–0.7, although lower quantum yields were obtained in some cases (0.04–0.2). − The tail absorption of 8a in the visible range (398–440 nm) was sufficient to promote the photoreaction. , Introducing electron-donating groups (EDG) at the 7-position of NDBF ( 8b and 8c ) led to a bathochromic shift in λ max abs but also reduced its photouncaging quantum efficiency (Table ). , The low quantum yield of 8c was attributed to a charge-transfer transition following photoexcitation that competes with LG release. , Ball and co-workers recently reported that derivatives of 8a and 8c undergo efficient photocleavage of C­(sp 2 )–N bonds .…”

“…The photolysis of ether, thioether, and phosphoester , LGs caged with this group reportedly proceeded with Φ r values of 0.5–0.7, although lower quantum yields were obtained in some cases (0.04–0.2). − The tail absorption of 8a in the visible range (398–440 nm) was sufficient to promote the photoreaction. , Introducing electron-donating groups (EDG) at the 7-position of NDBF ( 8b and 8c ) led to a bathochromic shift in λ max abs but also reduced its photouncaging quantum efficiency (Table ). , The low quantum yield of 8c was attributed to a charge-transfer transition following photoexcitation that competes with LG release. , Ball and co-workers recently reported that derivatives of 8a and 8c undergo efficient photocleavage of C­(sp 2 )–N bonds . To explain this, a mechanism was proposed involving hydrogen-atom abstraction followed by selective nucleophilic attack of a solvent molecule on the resulting extended conjugated system.…”