Orthopyridyl disulfide (OPSS) PEGs are thiol-specific PEGylation reagents that react with free sulfhydryls (-SH) at a wide pH range (pH 3-10) through disulfide exchange. The newly formed disulfide (S-S) bond is unstable in a reducing environment. Thus, the bond can be cleaved to reverse the pegylation and release free thiols. The biotinyl functionality is used to attach avidin or streptavidin containing substrates. Pegylation by binding of biotinyl PEG to avidin and streptavidin is very specific and fast.