Conformational and electronic effects in dehydroaminoacid derivatives

Journal

of Molecular Structure,

141 (1986)

415

415-418

Elsevier Science Publishers B.V.. Amsterdam -Printed in TheNetherlands

CONFORMATIONAL AND ELECTRONICEFFECTSIN DEHYDROAMINOACID DERIVATIVES , H.C.J.01TENHEIJM3, R.PLATE3and A.VIl-IADINIl. D.AJO'l, V.BUSETT12,M.CASARIN' 'I.C.T.R.,C.N.R.,Corso StatiUniti 4, 35100PADOVA (Italy) 2 Dip. di ChimicaOrganica,Universita, Via Marzolo 1, 35100 PAIKWA (Italy) 3Dept.OrganicChemistry,University,Toernooiveld, 6525 ED NIJMEGEN(The Netherlands) ABSTRACT

The electronicstructureof the cyclicdipeptideof dehydrophenylalanine has been investigated by vapour-phase He(I) and He(I1)UV photoelectron spectroscopy. Assignmentsare proposedby ccmparisonwith relatedmoleculesand supported by INEO/Scalculations. Analogiesand differences with respectto the photoelectron resultsof the cyclicdipeptideof dehydroalanine and corresponding acyclic compoundsare discussed. INTRODUCTION In many biologically active peptides a,fl-unsaturated amino acid derivatives are present (refs. 1,2). They are particularly interesting because of their unusual conformation and electronic structure.

Fig.1:Generalstructural formulaof the cyclic dehydroalanine derivativesquoted in the presentpaper.

0022-2860/86/$03.50

0 1986

Elsevier Science

Publishers

B.V.

416 We reported

previously

UV photoelectron of

(ref.3)

spectroscopy

state

and

in

the and

(depending

(which

gas-phase), several

molecular

between

conformational

different Therefore,

(MOs)

electronic

cycle

confined

2 and

-Ala) the

within giving

the rise

in

results

droalanine

molecule

(refs.

3,4). by

2 (Fig.

cycle

gative addition

on the

to

corresponding factors,

those

aN distinguish

rigid

by

systems

the

the

This

(planar)

is

cyclo(dehydrois

in

of

N-acetyldehy

tune

conformation

and

in

of

cyclo(dehydro-Phe)2

related

molecules, by

in

studies

phase

supported

n interaction

similar

with and

(Fig. 1: Rl = Ri =

main

compounds. same

of (ref.

arN - ?rCC interaction

MOs is

vapour

dipeptides

=RZ = Ri = H) 2

the

experimental

the

the

cyclic

INDO/S

the

for

crystal has

with the

state been

in

particular

cycle

(refs.

6,7)

semiempiri

from

delocali-

calculations. shoulder

while phenyl

(dehydro-Phe) interactions

is

produced

more

spectrum 2)

C and

MOs ( flHN,PH),

localized of

to

in

mechanical

zed

to

effects of

of

fragment;

and

pointed both

comparison

A and

(which

“semilocalized”

(dehydro-Phe)

corresponding

cal

Bands

cycle

enamido

(dehydro-Ala) quantum

crystal

interaction

Ala) Z (Fig. 1: RlsR’l

to nEN enamidic

The photoelectron assigned

the

In order

electronic

investigated:

theoretical

which

latter

shown.

structure

been

acyclic

of

in

relevant

investigation

(dehydro-

R2 =R;1 =H) have

extent

calculations)

both

between

was

and

the

5) and dehydrophenylalanine C6HS;

The

(by

suited.

the

dehydroalanine

esters.

(ref.4)

particularly

structure

mechanical

planar

conformation)

orbitals

/?-substituents,

appears

is

electronic

N-acetyldehydrophenylalanine

ethyl

on molecular

andnCC

the

and quantum

N-acetyldehydroalanine

non-planar)

upon

S include

band rings;

2 are whitin

molecules

the

to the

therefore,

to

between

ionizations

B corresponds be

interpreted

phenyl enamino

a relevant

and

is

fully

photoelectron on

the

basis

piperazinedione

moiety, role

MOs ( npH)

while played

results of

conju -

rings, in by

the

acyclic

conformational

in

417

D

Fig . 2:

Correlation

8 8 8 \

diagram

cyclo(dehydro-Phe)2

for based

cyclo(dehydro-Ala)2 on

INDO/S

results.

and

418

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and R.Plate, Acta Cryst., C40

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