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Dynamics of molecular organization in agarose sulphate
Author(s) -
Norton I. T.,
Goodall D. M.,
Austen K. R. J.,
Morris E. R.,
Rees D. A.
Publication year - 1986
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360250604
Subject(s) - chemistry , enthalpy , crystallography , nucleation , relaxation (psychology) , optical rotation , reaction rate constant , metastability , thermodynamics , analytical chemistry (journal) , kinetics , chromatography , organic chemistry , psychology , social psychology , physics , quantum mechanics
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 ( T m ) for formation of the ordered structure on cooling is ∼25 K lower than T m 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 (Δ H cal = −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 ( k nuc ), calculated from the observed second‐order rate constant ( k obs ) by the relationship k obs = k nuc (1 − 1/ s ) gave the following activation parameters for nucleation of the ordered structure of agarose sulphate (1 mg mL −1 ; 0.5 M Me 4 NCl or KCl): Δ H * = 112 ± 5 kJ mol −1 ; Δ S * = 262 ± 20 J mol −1 K −1 ; Δ G * 298 = 34 ± 6 kJ mol −1 ; ( k nuc ) 298 = (7.5 ± 0.5) × 10 6 dm 3 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 dm 3 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.