How do i make a smooth rotation using negative degrees?

Quaternions do not take negative values - that is the problem… :^(
And also i could have just used transform.rotate but i need my rotation to be smooth…

float rotationSpeed = 40F;
transform.rotation = Quaternion.RotateTowards(transform.rotation, Quaternion.Euler(0, (-90), 0), rotationSpeed * Time.deltaTime);

Thank you in advance!
And have a lovely day!

A Unity Quaternion defines an orientation, not an amount of rotation. (despite the name of the member “rotation”, in Transform. It’s just a poor name choice.)
Therefore:

Quaternion.Euler(0, -90, 0);

is equivilent to

Quaternion.Euler(0, 360-90, 0)