DISCRIMINATION OF DOXYLPALMITIC ACIDS BY GAMMA-CYCLODEXTRIN - ESR STUDY OF STABILITY, CONFORMATION, AND DYNAMICS

Six palmitic acids labeled at different methylene positions by paramag netic doxyl moiety were used as guests in order to examine molecular d iscrimination using gamma-cyclodextrin (gamma-CD). The DPA inclusion c omplexes using gamma-CD in an aqueous solution were characterized acco rding to stability, conformation, and dynamics at 25 degrees C, mainly based on ESR spectral analyses and supported by force-field calculati ons. The stability was equal among all DPA complexes, as represented b y the formation constant K-s = 8.0 (+/- 0.5) x 10(2) M-1 (M = mol dm(- 3)), corresponding to the conformation constant when the palmitate cha in penetrates a CD cavity and the doxyl ring locates inside the cavity . Both overall molecular motion and chain segmental motion of DPA, rep resented by the parallel (R-zz) and perpendicular components (R-xx = R -yy) Of the rotational diffusion tensor, respectively, were significan tly restricted in inclusion state. The magnitude of restriction of seg mental motion increased towards the chain terminals from the center, i ndicating that faster segmental motion was restricted at a higher rate . Thus, a series of DPAs were discriminated by gamma-CD in terms of ch ain segmental motion but not in terms of stability, to present a typic al example of dynamic molecular discrimination.