Properties of B‐ring analogues of colchicine

Abstract

Absorption spectra of colchicine and its analogues are affected by the presence of the B‐ring, although it is not part of the chromophore (C‐ring). Thus, 2‐methoxy‐5‐(2′,3′4′‐trimethoxyphenyl)tropone has absorption maxima at 341 nm, whereas that of desacetamidocolchicine is at 353 nm. A similar red shift in the λmax of colchicine, desacetamidocolchicine and 2‐methoxy‐(2′,3′,4′‐trimethoxyphenyl)tropone also occurs when they are immobilized in the binding site to tubulin or in pure glycerol. We also observed that the B‐ring of colchicine alone or with substituent does not affect the UV‐induced rearrangement of colchicine to lumicolchicine. However, in the absence of the B‐ring, as in the case of 2‐methoxy‐5‐(2′,3′4′‐trimethoxyphenyl)tropone, the rearrangement reaction of the C‐ring slows down significantly.

Colchicine photosensitizes covalent tubulin dimerization

“…This was observed in more than a dozen experiments and was true for the formation of dimers (Figure 5) and trimers and tetramers (data not shown). While the absence of the B-ring may contribute to this greater potency, we are inclined to the view that the much slower photoisomerization to the corresponding "lumi" derivative (Maity & Bhattacharyya, 1987) was largely responsible for the greater cross-linking efficiency.…”

“…For example, MTC attains maximum yields of oligomers that are as much as 50% greater than colchicine (Figure 5). We believe this to be due, at least in part, to the greater stability of MTC to photoisomerization (Maity & Bhattacharyya, 1987); similar stability considerations apply to the cross-linking promoted by isocolchicine (Wilson & Friedkin, 1966).…”

UDP‐galactose 4‐epimerase from Kluyveromyces fragilis: existence of subunit independent functional site

“…While epi‐D could be reversibly refolded and reactivated to between 80% and 90% after denaturation by 8 M urea [20–22], it was of interest to verify whether epi‐M could reactivate efficiently under identical conditions. It was ensured that under the stated denaturing conditions, of the monomer was reduced to 15% indicating almost complete loss of its secondary structure.…”

“…Refolding/reactivation was initiated after 20‐fold dilution of the denaturant with the same buffer containing 1 mM of extraneous NAD. These conditions were identical to reversible refolding of epi‐D from 8 M urea induced unfolded state [20–22]. Equilibrium unfolded states were generated after incubation of 0.1 mg/ml of epi‐M with 0–8 M urea in 20 mM K‐phosphate, pH 7.5, containing 2 mM 2‐ME for 16 h at 25 °C.…”