the formation of aggregates in polymer matrices does not require diffusion and aggregation due to attractive forces. Rather, these aggregates can form simply because statistically there is no alternative. Consequently, these aggregates may not be crystallites with the structure of the pure crystal. The structure may reflect the statistical distribution perturbed by the presence of and interaction with the polymer. This agreement also indicates for this case that the spectral changes are not a sensitive probe of the nonstatistical aspects of aggregate formation; consequently, the spectral data do not contain information about these aspects, and more sophisticated models of aggregate formation are not needed to account for the observations.

Abstract— Model studies including quantum chemical calculations and the measurement of infrared and ultraviolet spectra are presented as contributions to the elucidation of the nature of the photochemical step of vision. The importance of the hydrogen bond in which the protonated nitrogen of the retinal Schiff base is involved is stressed as well as that of the perturbation of the β‐ionone ring by negative groups. It is suggested that by combining these two perturbations the low excitation energy of rhodopsin can be obtained without actual protonation of the Schiff‐base prior to photon absorption. The variation of rhodopsin's color from one species to another could also be related to this. Protonation could be a consequence of photonabsorption and the higher basicity of the excited state. This, in turn, leads to the suggestion that the protonated species is actually bathorhodopsin, not rhodopsin. Comments are made on the identity of the (ππ*) state which is involved.

The Fourier‐transform infrared spectra of CHCl3 solutions containing a non‐conjugated imine, R1HC=NR2 and an acid of the series HCl, HBr, HI, trichloro‐, dichloro‐, monochloro‐, monobromo acetic acids and propionic acid have been recorded. Both the N+H and double bond stretching regions are examined. The results show that HCl, HBr and HI protonate the imine completely, trichloroacetic acid to about 90‐95%, the other haloacetic acids to 55‐80% and propionic acid to <10%. Considered as model studies for the problem of protonation in rhodopsins our results lead to the conclusion that with the relatively weak acids available in the pigments full protonation requires a stabilization mechanism which should be further investigated.

Abstract— The Fourier‐transform infrared spectra of chloroform‐d solutions of conjugated imines CH3CH=CHCH=NCH(CH3)2 and CH3CH2CH=CHCH=CHCH=NCH(CH3)2 and the related protonated species with HCl, HBr, HI, trichloro, dichloro, monobromo and monochloroacetic acids or propionic acid are presented. The effects of conjugation and protonation are examined. The results show that conjugation slightly increases the basicity of the Schiff bases. HCl, HBr and HI protonate the Schiff bases completely. The carboxylic acids protonate partially depending on their pKa, values. When the Schiff base contains two (or more) C=C bonds conjugated with C=N, the main C=C stretching band undergoes a strong intensification showing that sizeable dipole moment variations occur along the conjugated chain.

. 64, 225 1 (1986).The photochemical step in the functioning of visual and bacterial rhodopsins entails cis-trans or trans-cis isomerization and changes in the state of protonation of the retinylidene Schiff base chromophore. In this review our present knowledge on these two events is discussed as well as the role of interactions between the chromophore and the surrounding protein, opsin. The relation between protonation and hydrogen bonding at the Schiff base nitrogen, the problems of stabilization of the proton bridge and charge separation are also discussed. A new proposal is made which implies Schiff base to counter-ion proton translocation with concomitant isomerization and reprotonation of the chromophore.