Covalently linked diubiquitin (diUbq) is known to adopt specific interfacial interactions owing to steric hindrance induced by the covalent tether. K48-linked diUbq preferentially forms hydrophobic interfacial interactions between the two 144 faces under physiological conditions, whereas K63-linked diUbq preferentially forms electrostatic interfacial interactions. Here, we show using collision-induced unfolding ion mobility-mass spectrometry that the recently reported noncovalent dimer of ubiquitin exhibits structural preferences and interfacial interactions that are most similar to that of K48-linked diUbq.