No abstract

No abstract

Ultrasonic absorption measurements in dilute aqueous poly(a,L-glutamic acid)(PGA) have been carried out over the frequency range 40 kHz-35 MHz by the resonance, statistical reverberation, and pulse techniques. Data at the two PGA concentrations 2.7 X 10-3 and 1.1 X 10-3 (mol residue)/!, in 0.03 MNaCl at 37°exhibited relaxational absorption over the pH range 4.8-S.6. Both the relaxational amplitude (A) and relaxation time (t) passed through maximum values at pH 5.21, the midpoint of the helix-coil transition. The maximum values of A and r were found to be 100 X 10-17 sec2 cm-1 and 1.0 gsec, respectively. The value of the rate constant for helical growth in the Schwarz theory was estimated to be (8 ± 5) X 107 sec-1.

Relaxation measurements by the temperature-jump technique have been carried out in a number of indicator systems: alizarin yellow R (I), alizarin yellow G (II), tropaeolin O (III), Clayton yellow (IV), quinizarin-2sulfonic acid (V), phenolphthalein (VI), and p-xylenol blue (VII). Observed relaxation times ranged from 5 /¿sec to 0.3 msec and were interpreted in most cases on the basis of the hydrolysis reaction OH-+ HIn* (=5 In*-1 + H20. The smallest bimolecular rate constant (k¡) measured was 2.9 X 104 (for IV); the largest 3.0 X 109 (for VII). These values, in many instances several orders of magnitude lower than diffusion controlled, are interpretable on the basis of a finite chemical activation barrier resulting usually from internal hydrogen bonding. On a Brpnsted plot of log k vs. pA"a the data correlated well with other measurements in such systems. Several parallel curves were observed depending on the type of bonding present. The Brpnsted coefficient was found to be 1.0 in each case, indicating that the transition state is very similar to the products. The present data also support the generalization that the O-H• • N hydrogen bond is stronger than that in O-H• -O.

SynopsisIn a continuing study of the denaturation of DNA as brought about by Cu(I1) ions, results are presented for the dependence of T , and T (the terminal relaxation time) on ionic strength, pH, reactant concentrations, and temperature. Maximum stability of the double helix, as reflected by the longest relaxation times and highest T, values, was observed between pH 5.3 and 6.2. Outside this range both T, and T decreased sharply.A second, faster relaxation time was deduced from the kinetic curves. The apparent activation energies of the rapid and slow ("terminal") relaxations were found to be 12 and 55 kcal/mole, respectively. Several lines of evidence led to the conclusions that (1) the rate-determining step in DNA denaturation, when occurring in the transition region, is determined by chemical events and ( 2 ) the interactions which are disrupted kinetically in the rate-determining step are those which account for the major portion of the thermal (T,) stability of helical DNA.