skin, GG (Gelzan CM Gelrite, Mw 1000 g
mol–1), spermidine trihydrochloride (SPD), sucrose,
adipic dihydrazide (ADH), carbodihydrazide (CDH), dimethyl sulfoxide
(DMSO), ethylene glycol, 1-ethyl-3-[3-(dimethylamino)-propyl]-carbodiimide
(EDC), hydroxylamine hydrochloride, N-hydroxybentzotriazole
(HOBt), 4-hydroxybenzaldehyde, deuterium oxide (99.9 atom % D, contains
0.05 wt % 3-(trimethylsilyl)-propionic-2,2,3,3-d4 acid, sodium salt),
hydrochloric acid (HCl), sodium hydroxide (NaOH), sodium chloride
(NaCl), and sodium periodate (NaIO4) were purchased from
Sigma-Aldrich (St. Louis, MO, USA). Dialysis membrane (Spectra/Por
12–14 kDa) was purchased from Spectrum Laboratories (Rancho
Dominguez, CA, USA).