Modern chemical biology oriented research frequently relies on the ability to site-selectively label macromolecules such as peptides, carbohydrates and oligonucleotides with fluorophores. Many strategies exploit the nucleophilic functionalities naturally present in biomolecules, providing efficient but less selective labeling. The need for more orthogonal strategies can be met by the introduction of unnatural amino acids. In continuation of previous work conducted in our lab in the area of furan-mediated nucleic acid crosslinking,[1] our group developed a method for the labeling of peptides by incorporation of non-natural 2-Furylalanine into a peptide and its subsequent conversion into an alfa,beta-unsaturated aldehyde by selective oxidation of the furan moiety. Next, a simple reductive amination allows introducing the desired fluorophore.[2] The furan moiety has further shown to be a useful partner in Diels-Alder reactions with commercially available maleimides as dienophile. Also, the more recently described 1,2,4-triazole-3,5-diones present an excellent alternative.[3]
We further investigated singlet oxygen (1O2) mediated furan-based peptide labeling in physiological aqueous solutions. Furan-containing peptides were subjected to standard oxidative conditions (air, light, photosensitiser) so that the reactive electrophilic species were generated. These reactive intermediates were intercepted by α-effect nucleophiles to form stable conjugates. Incorporation of nucleophilic fluorophores through a cascade reaction sequence, led to the efficient construction of site-selectively labeled fluorescent peptides.[4] Recently, the methodology could be further applied for the efficient ligation of peptide fragments.[5]