What type of compounds can hydrazides target for chemical cross-linking?

Hydrazides are chemical compounds that contain a hydrazine functional group, which is particularly reactive towards carbonyl groups. Among the options provided, aldehydes are the primary targets for hydrazides in chemical cross-linking due to the presence of a reactive carbonyl carbon that can form a hydrazone linkage with hydrazides.

When a hydrazide reacts with an aldehyde, the nucleophilic nitrogen in the hydrazine portion of the hydrazide attacks the electrophilic carbon of the aldehyde, resulting in the formation of a stable hydrazone. This type of interaction is significant in various biochemical applications, including drug development and biomaterials.

While alcohols, esters, and ketones contain functional groups that can participate in chemical reactions, they do not form the same types of stable cross-links with hydrazides as aldehydes do. For instance, alcohols lack a carbonyl group needed for this specific reactivity, and although ketones can form hydrazones as well, aldehydes are generally more reactive due to their inherent properties and sterics in chemical reactions. Therefore, aldehydes are the most suitable and relevant targets for hydrazide-derived cross-linking reactions.