Dynamics of molecular organization in agarose sulphate

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Abstract
Nongelling solutions of structurally regular chain segments of agarose sulphate show disorder–order and order–disorder transitions (as monitored by the temperature dependence of optical rotation) that are closely similar to the conformational changes that accompany the sol–gel and gel–sol transitions of the unsegmented polymer. The transition midpoint temperature (Tm) for formation of the ordered structure on cooling is ∼25 K lower than Tm for melting. Salt‐induced conformational ordering, monitored by polarimetric stopped‐flow, occurs on a millisecond time scale, and follows the dynamics expected for the process 2 coil ⇌ helix. The equilibrium constant for helix growth (s) was calculated as a function of temperature from the calorimetric enthalpy change for helix formation (ΔHcal = −3.0 ± 0.3 kJ per mole of disaccharide pairs in the ordered state), measured by differential scanning calorimetry. The temperature dependence of the nucleation rate constant (knuc), calculated from the observed second‐order rate constant (kobs) by the relationship kobs = knuc(1 − 1/s) gave the following activation parameters for nucleation of the ordered structure of agarose sulphate (1 mg mL−1; 0.5M Me4NCl or KCl): ΔH* = 112 ± 5 kJ mol−1; ΔS* = 262 ± 20 J mol−1 K−1; ΔG*298 = 34 ± 6 kJ mol−1; (knuc)298 = (7.5 ± 0.5) × 106 dm3 mol−1 s−1. The endpoint of the fast relaxation process corresponds to the metastable optical rotation values observed on cooling from the fully disordered form. Subsequent slow relaxation to the true equilibrium values (i.e., coincident with those observed on heating from the fully ordered state) was monitored by conventional optical rotation measurements over several weeks and follows second‐order kinetics, with rate constants of (2.25 ± 0.07) × 10−4 and (3.10 ± 0.10) × 10−4 dm3 mol−1 s−1 at 293.7 and 296.2 K, respectively. This relaxation is attributed to the sequential aggregation processes helix + helix → dimer, helix + dimer → trimer, etc., with depletion of isolated helix driving the much faster coil–helix equilibrium to completion. Light‐scattering measurements above and below the temperature range of the conformational transitions indicate an average aggregate size of 2–3 helices.

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