Structural Insights into the Host-Guest Complexation between β-Cyclodextrin and Bio-Conjugatable Adamantane Derivatives

Understanding the host-guest chemistry of alpha-/beta-/gamma- cyclodextrins (CDs) and a wide range of organic species are fundamentally attractive, and are finding broad contemporary applications toward developing efficient drug delivery systems. With the widely used beta-CD as the host, we herein demonstrate that its inclusion behaviors toward an array of six simple and bio-conjugatable adamantane derivatives, namely, 1-adamantanol (adm-1-OH), 2-adamantanol (adm-2-OH), adamantan-1-amine (adm-1-NH2), 1-adamantanecarboxylic acid (adm-1-COOH), 1,3-adamantanedicarboxylic acid (adm-1,3-diCOOH), and 2-[3-(carboxymethyl)-1-adamantyl]acetic acid (adm-1,3-diCH(2)COOH), offer inclusion adducts with diverse adamantane-to-CD ratios and spatial guest locations. In all six cases, beta-CD crystallizes as a pair supported by face-to-face hydrogen bonding between hydroxyl groups on C2 and C3 and their adjacent equivalents, giving rise to a truncated-cone-shaped cavity to accommodate one, two, or three adamantane derivatives. These inclusion complexes can be terminated as (adm-1-OH)(2)subset of CD2 (1, 2:2), (adm-2-OH)(3)subset of CD2 (2, 3:2), (adm-1-NH2)(3)subset of CD2 (3, 3:2), (adm-1-COOH)(2)subset of CD2 (4, 2:2), (adm-1,3-diCOOH)subset of CD2 (5, 1:2), and (adm-1,3-diCH(2)COOH)subset of CD2 (6, 1:2). This work may shed light on the design of nanomedicine with hierarchical structures, mediated by delicate cyclodextrin-based hosts and adamantane-appended drugs as the guests.