I usually do not like to write about photoaffinity labeling reagents because of their indiscriminate nature. But this publication from the lab of Dr. Christina Woo in Harward. They were able to develop Cyclobutane Diazirine tags that can label Carboxylic Acids on the protein with variety of functionalities with pretty respectable efficiency (5-25%)! They used variable ring strain to tune the reactivity of the diazirines. Also, the most reactive amino-acid ended up being Cysteine, followed by glutamic acid and aspartic acid.
Well, if you are ever tired of Maleimides you can always try Cyclobutane Diazirines.
Special thanks to David Kamber, Ph.D. for pointing out this article.
You can read more about it here:
https://pubs.acs.org/doi/10.1021/jacs.2c08257?ref=PDF