Selective binding and analysis of macromolecules

A method for analysing one or more proteins, protein functions and/or peptides in one or more samples comprising the steps of:

• providing a binding reagent having the formula
• X—NH—C(═NH)—OR
• or
• X—L—NH—C(═NH)—OR
• where X is an affinity label that selectively binds to a capture reagent, R is a residue group, and L is a linker moiety;
• introducing the binding reagent to the one or more samples so
• as to effect a guanidination reaction between the binding reagent and proteins and/or peptides having a lysine functionality, thereby producing one or more affinity label containing homoarginine derivatives;
• optionally modifying the affinity label containing homoarginine derivatives to produce further affinity label containing homoarginine derivatives;
• capturing affinity label containing homoarginine derivatives using the capture reagent that selectively binds X;
• and
• performing an analysis of affinity label containing homoarginine derivatives.

A reagent for selectively binding molecules having a lysine functionality having the formula

• X—NH—C(═NH)—OR
• or
• X—L—NH—C(═NH)—OR
• where X is an affinity label that selectively binds to a capture reagent, R is a residue group, and L is a linker moiety.