using the equation d =
!wal
* €As
IPUI
Both signs must be included as only the absolute magnitude of the dipole moment can be measured. Since the maximum change in dipole moment observed was eitherol10or 102G D ,the value of d is found to be < 2.5 A. or 25 A. The result thus indicates that activation takes place near the center of the fibrinogen molecule. A potential source of doubt regarding the interpretation of the low Ap is the possibility that the charge pattern may readjust, through proton migration, to minimize Ap regardless of the site of loss of the negative charges.Id However, it seems unlikely that the result would be SO closely similar a t pH 7, where the migrating protons will be eychanging on histidine residue, and a t pFI 9 10 where they will be exchanging on tyrosiiie and lysine residues. The two peptides have nearly equal charges; therefore it could also be possible that tlic peptitles (?i) C 1 anlurd p r i v a t e
i~,iliiltiintcatiirri
Thermodynamic Properties of the Ammonium Ion BY AUBREYP. ALTSIIULLER RECEIVED FEBRUARY19, 1955
Structural and spectroscopic data 011 the ammonium ion recently have been available which make possible the statistical thermodynamic calculation of the thermodynamic functions C:, (11"- 1 l ~ ) . ' T , - ( F o - H,O)!T and Sofor XHHq+(g). The S-H internuclear distance in NH4+ has been obt:iined from both neutron diffractioiil-3 and nuclear niagnetic resonance meas~renients.~JThe most accurate determination of th,e N-.H distance5 gives a value of 1.0:E rt 0.00.5 A . Thus the moment of inertia, 1,has the value (4.75 f 0.051 X g. ciii.2. The vibrational freqiiencies of the spherical top NH4+(T,] symnietry is assunied for h"~+ in the gaseous state) have been found to be ~ ~ ( = 1 )3041 c111.-~; v , ( 2 ) = 1682 c171.-1; v&) = 33090 cn1.-' and yq(.?) = 1403 C I I I . - - ~ fronl infr:iretf nieasurenients on films of KH4Cl.6 The values of the therinodynamic functions ct, (11"- Ht):T,- (Fo- 11;) and S" for NH4+ (g) from 200 to 1000°K. as calculated by the rigid rotator-harmonic oscillator approximation are listed in Table I for the itleal gas state a t one atmosphere pressure. ( 1 ) G . H. (:oldiicltiiiid ancl D. C;. ITiirit, Phyr.. Re;'.,8 3 , 88 ( l ( L - J ) : 8 6 , 747 ( 1 9 5 1 ) . ( 2 1 H. A . 1,evy a n d S . W. Prtrrsoti ihitf , 86, 7tX (19.-t?). ('3) H. A . I.evy a n d S. LV, Prtrr>
