9614
The Journal of Physical Chemistry, Vol. 95, No. 23, I991
The rota-microconstants for protonation of the carboxylate group are essentially identical for the three rotamers of the (COO-, NH3+, YH) form of cysteine and selenocysteine, whereas for serine, the rota-microconstant for rotamer h is significantly larger than that for rotamer t. This large difference for serine is presumably due to stabilization of rotamer h by formation of an intramolecular hydrogen bond that involves both the COOH and OH groups. Since thiol and selenol groups show much less tendency to form hydrogen bonds, this source of stabilization is not available to rotamer h of the fully protonated forms of cysteine or selenocysteine. The rota-microconstants kS and kNSfor protonation of the thiolate group of rotamer h of the (COO-, NH2, S-)and (COO-, NH3+,S-)microspecies of cysteine are significantly larger than those for rotamer t, due to increases in the population of rotamer h upon protonation of the thiolate group. The population of rotamer h increases, presumably because protonation eliminates the electrostatic repulsion between the COO- and S-groups, which favors rotamer t. A greater apparent basicity is also found for
Additions and Corrections the selenolate group of rotamer h of the (COO-, NH3+, Se-) form of selenocysteine. In contrast, the rota-microconstants for protonation of the amino group of rotamers t and h of the (COO-, NH2, S-)microspecies are the same within experimental error. The same is also observed for the (COO-, NH2, SH), (COO-, NH2, SeH), and (COO-, NH2, OH) microspecies of cysteine, selenocysteine, and serine, which suggests that N H 2 and NH3+ groups are similar in their effects on rotamer populations for this series of amino acids.
Acknowledgment. This research was supported by National Institutes of Health Grant GM37000. B.N. gratefully acknowledges a fellowship from the Soros Foundation. The NMR instrumentation was supported in part by BRSG 2 SO7 RR 0701 0-20 awarded by Biomedical Research Resources, National Institutes of Health. Registry No. H-Ser-OH, 56-45-1; H-Cys-OH, 52-90-4; selenocysteine, 3614-08-2.
ADDITIONS AND CORRECTIONS 1991, Volume 95
J. L. Tilson and J. F. Harrison*: Electronic and Geometric Structures of Various Products of the Sc+ + H 2 0 Reaction. Page 5 100. Figure 6 is in error. The corrected figure is shown below. Sc+('D)+%O +Produels 214.7
Sc'('D)+O+ZH*
168.2 +SKI ( 3 ~ 3 + 2 ~ .
14R.J
'ScO ('A )+2H* 117.1
S~+(~D)+OH+H' S&('D)+O+H,
0.0
1
+ H.sco&+)+H.
"D)IHIO\
\
-36.2 ~ ) . . . ~ , ~
*sC( 3
D. Balasubramanian* and C. A. Rodley: Incorporation of a Chemical Oscillator into a Liquid Crystal System. Page 5 148. The legend to Figure 2 should read as follows: Plots of the change in period with time for various BZ/LC and related water-only BZ oscillators: (a) BZ/LC with [H2S04] = 0.75, [KBrO,] = 0.09, [MA] = 0.175; (b) BZ/LC with [H2S04] = 0.75, [KBrO,] = 0.09 [MA] = 0.1 15; (c) BZ/LC with [H2S041 = 0.75, [KBr03] = 0.09, [MA] = 0.06; (d) BZ/LC with [H&304] = 0.75, [KBrO,] = 0.18, [MA] = 0.1 15; (e) water-only BZ with [H&304] = 0.75, [KBr03] = 0.18, [MA] = 0.1 15; (0 water-only BZ with [H2S04]= 0.75, [KBr03] = 0.09, [MA] = 0.06. In all cases, [Mn2+] = 0.005. All concentrations are in molarity.
