How do I smoothly rotate the GameObejcts with negative 90 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!