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VISUAL PIGMENTS‐V. GROUND AND EXCITED‐STATE ACID DISSOCIATION CONSTANTS OF PROTONATED ALL‐TRANS RETINAL SCHIFF BASE AND CORRELATION WITH THEORY *
Author(s) -
Schaffer Arnold M.,
Yamaoka Tsuguo,
Becker Ralph S.
Publication year - 1975
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1975.tb06675.x
Subject(s) - chemistry , protonation , ground state , isomerization , schiff base , excited state , cndo/2 , photochemistry , imine , singlet state , dissociation (chemistry) , computational chemistry , crystallography , molecule , atomic physics , organic chemistry , ion , physics , catalysis
— The acid dissociation constants of protonated all‐trans retinal Schiff base ( SB‐H + ) in a 50% water‐methanol solution at 0°C is 6–95 for the ground state and nominally 16–65 for the first excited singlet state, with a potential range of ˜ 12–21. These values are in qualitative agreement with the results of semiempirical MO calculations, which indicate that the total charge density on nitrogen is greater in the first excited singlet than in the ground state ( Q N * > Q N ). However, pertinent to vision, CNDO/2 calculations on all‐trans and 11‐ cis Schiff base and SB‐H + indicate that, for torsional angles of approximately 80–100° around the 11–12 double bond, Q N * < Q N . This result suggests that it may be possible for the proton to come off the imine nitrogen during isomerization from 11 ‐ cis SB‐H + to all‐trans SB‐H + . The potential consequence of this during isomerization of rhodopsin is the initiation of unfolding of the protein opsin.